SEISMIC TRAINING MODULE

OBSERVER

Table of Contents

MODULE 1 - SOURCES 3

MODULE 2 - STREAMERS 3

MODULE 3 - RECORDING SYSTEM 3

MODULE 4 - OBSERVER'S AND NAVIGATOR'S LOG 3

MODULE 5 - 2D AND 3D VESSEL OPERATIONS 3

MODULE 6 - MAIN SEISMIC INSTRUMENTS 3

MODULE 7 - CONTROL MECHANISM 3

MODULE 8 - INSTRUMENTAL AND SEISMIC TESTS 3

MODULE 9 - CORE TRAINING 3

MODULE 10 - NOISE 3

MODULE 11 - INFORMATION TECHNOLOGY 3

MODULE 1 - SOURCES

Aim:

To introduce the trainee observer to seismic sources. On completion the trainee should have a reasonable knowledge of how air guns are constructed, how they operate, how they are towed as arrays and the basic reasons for using arrays.

Summary:

 Review those parts of the Introduction Document that refer to the seismic input energy.

 Be aware of the basic requirements of a seismic marine source.

 Look at single guns and in particular learn the basics of how Bolt, Sleeve and G guns operate.

 Learn what a gun signature is. Know the principles of measuring it and learn to recognise its main component parts. Study the bubble pulse. Be aware of how it arises. Know what the bubble period is and know the main gun parameters that affect it and how they do. Learn how bubble effects are controlled by using groups of guns operating together.

 Understand cluster arrays and how they are assembled into total arrays. Have a basic understanding of compressor capacity versus array demand versus seismic recording cycle. Review the material on seismic noise in the Introduction Document and read the few notes on noise in the study sheets. Be aware of the link between noise reduction and the source array dimensions. Know the simple basics of near and far field signatures.

 Get a reasonable physical idea of what makes up a typical cluster array. Know the basic elements and how they are linked. Know how large arrays are created from clusters. Know the essentials of source towing, position control, depth and pressure monitoring at the array.

 Observe how seismic source system is deployed and retrieved

Materials and Methods:

This module is covered by self study combined with on-the-job observation and discussion. There is no good compact descriptive material available which covers the background of source systems in an effective and efficient way for trainee observers.

The on-the-job training is largely by controlled observation but it is very necessary and highly important here as there is also little written material available on the construction of source arrays and each company tends to develop their own techniques based on the experience of their senior personnel.

Exercises:

There are no hands-on practical tasks associated with this module but some practical observation ideas are specified at the end of this Module.

Source Requirements

The type of pulse we generate with our gun system and send into the earth is very important to the final result of the seismic survey. In fact if we don't get it right and keep it right the survey could end up almost worthless. In simplistic terms we want two main things from our source pulse.

Firstly, we want it to be nice and sharp so we can get a good accurate picture of the seismic interface. If the pulse is broad it can be very difficult for the seismic interpreter to decide exactly what the reflection time is on the seismic record. Also if two earth boundaries are close together, as you get with thin rock layers, then the broad pulse from the first boundary will get mixed up with that from the second and the interpreter may not know there are two pulses at all and consequently might easily miss the existence of the second boundary. An example of this is shown alongside where seismic trace A is produced with a good sharp pulse and trace B with a broad pulse. Clearly with B the narrow rock layer is not at all well defined. So sharp pulses help us to minimize such problems, they help us to resolve the reflections from the various layers, so one of the requirements of a seismic source is that it should provide good Resolution.

Secondly, we want our source to be able to generate a pulse with enough energy to penetrate far into the earth and still return to the cable with reasonable amplitude. If we send in a small low energy pulse to the earth we won't lose it. It will go far into the earth and back, and the returning energy will be detected, but the problem is that its return energy will be so small that it may be impossible to see it among all the other noises from ship, wind, waves etc. So we say that another requirement of our source system is that it should have good Penetration.

One of the problems that the source designer has is that these two requirements are a somewhat contradictory. As the source pulse passes through the earth it is attacked by mechanisms that reduce its amplitude. Losses come largely from the wave fronts getting bigger and bigger, from friction and from scattering from discontinuities in the earth. We find in general that sharp pulses are more affected by these effects than broad ones. So sharp pulses give good resolution but poor penetration and broad pulses have good penetration and rotten resolution. In creating a source system, the designer works hard at achieving a good compromise between these situations. He tries to create an output pulse that has a broad Frequency Range in that it contains both sharp and broad components. We will come back to this in later training when we talk of source signatures in detail but for the moment we look at the other important requirements of a seismic source. These are Safety, Reliability, Repeatability and Environmental Acceptability

Safety: This is obvious. We cannot have a source which cannot be handled with safety. This is not to say that sources are inherently safe. Any sharp release of energy such as we have with source guns is potentially dangerous and safety precautions must be rigidly observed when handling and testing such devices.

Reliability: This is fairly obvious. You will appreciate it even more as you gain more experience in seismic work. Great effort is put into achieving such a system. Delays due to failing sources often represent hard, time consuming, expensive work.

Repeatability: This is not so immediately obvious but it is also very important. The source system must be able to generate, with each shot, an output pulse which is very similar to the previous ones. If we have lots of variation it can cause problems when we stack the seismic data and it can even lead to false interpretations.

Environmental Acceptability: By this we mean that damage to the environment by the source system must be minimized: marine creatures must not be harmed and no pollution should result from the system.

Now that we have looked at the requirements of a seismic source we look at one of the most commonly used sources and consider briefly how it operates. In the practical part of the module you will learn about the guns on your vessel in much more detail.

Note: In this module description, we will use the generic term 'airgun'. On your vessel, Bolt, Sleeve or G-guns may be in use. All are variants on the same theme and serve the same ultimate purpose.

Types of air guns

PAR Air Guns

Below we show a schematic diagram of a Bolt PAR air gun. Bolt Technology were the company who first produced air guns of this type and as they had them well patented they dominated the air gun market for many years.

At this stage all you need to know is the basics of how the gun works. What happens is that the gun is charged by feeding it with high pressure air from the compressor through an air hose as shown at the top left of the diagram. This air fills the upper chamber and pushes the triggering piston down until it closes off the upper chamber, and at the same time seals the lower chamber by means of the firing piston to which it is connected by a shaft. As there is a hole in this shaft the lower chamber is also charged with the high pressure air. The assembly of the two pistons and shaft is called the shuttle. A gun in this state is said to be armed and ready for firing.

The second stage is when the gun is fired. This is done by sending an electrical signal to the solenoid valve shown at the top of the diagram. The valve opens and causes high pressure air to be injected between the base of the upper chamber and the triggering piston via the route shown on the right of the diagram. This causes the shuttle to move upwards very quickly. Round the circumference of the gun there are four large ports and as the firing piston moves past these the high pressure air from the lower chamber explodes out into the water. This provides the impulsive seismic energy.

Sleeve Guns

Sleeve guns are also a common seismic source. These are also air guns but operate differently from the Bolt PAR guns. They were introduced around 1984 by Texas Instruments as an alternative to the PAR guns.

To arm a sleeve gun air enters the fill passage and spring chamber areas and forces the external sleeve shuttle closed. At the same time the fill passage provides air to the sleeve chamber.

When the sleeve chamber and spring chamber pressures reach supply pressure the gun is armed and ready for firing.

The gun is fired by a trigger pulse sent to the solenoid valve. On receipt of this pulse the solenoid plunger rises and permits air to flow into the firing chamber. At this point the sleeve now has the same pressure acting on both sides of it. Because the firing chamber sleeve area is greater than the spring chamber sleeve area the sleeve starts to open.

With the full chamber pressure acting on the sleeve in combination with the firing chamber force, a very large opening force is created which quickly moves the sleeve to its full open position. The air in the chamber exhausts rapidly into the surrounding water and generates the acoustic pulse.

As the sleeve moves into the full opening position, the firing chamber exhaust ports are uncovered and this allows the firing chamber air to vent and cause the pressure to drop to the surrounding, ambient pressure.

The sleeve gun is now in the exhausted position. Both the main chamber and firing chamber are near ambient pressure. Now the full supply pressure acts on the spring chamber and moves the sleeve back to its closed position.

The cycle is now complete and the gun recharges.

G Guns

A third type of air gun now in use is the G Gun. These guns are manufactured and marketed by the French company Sodera and Seismic Systems Inc. of Houston. In operation it is not vastly different from the previously described guns, but it does have positive advantages over them. The main ones are that it is smaller and lighter in weight with fewer parts, the gun volume is easily adjustable with simple inserts, it can be operated continuously at high pressure, it has excellent repeatability and reliability and it is designed to have no recoil so that harness wear is minimized.

Alongside is a diagram of a G Gun. This has been simplified to show the opera¬ting principles.

Note that the solenoid sealing valve is sealing the airway leading towards the shuttle.

As the gun is charged with high pressure air the return chamber fills up and forces the hollow shuttle to close and seal the main chamber. At this point the main chamber located between the casing and the shuttle gradually pressurizes until equilibrium is achieved and the gun is then in the armed position

As for the other guns, it is fired by actuating the solenoid valve.

When this is energized high pressure air can flow from the main chamber to the triggering chamber and so unseal the shuttle. This in turn allows the larger area of the shuttle to be pressurized from the main chamber.

The lightweight shuttle now quickly acquires a high velocity before reaching the main air outlet ports of the gun. When these are uncovered the high pressure air is fired explosively into the surrounding water.

After firing the gun cycle is complete. Air can be fed again to the triggering return chamber and the solenoid valve can reseat and seal entry to the triggering area.

Source Signatures

Having now looked at the basic operation of the seismic air gun source we go on to look at the output of the source in more detail. This means we must gain a better understanding of the source output pulse. This pulse is more commonly called the Source Signature. At this stage we will consider only the output from a single gun. We will consider the meaning of Source Signature in regard to multiple guns at a later stage.

To study source signatures we use hydrophones similar to those used in the seismic streamer to detect the returning seismic signals. The source monitoring hydrophones are placed close to the gun(s), so they have to be made stronger than streamer hydrophones. The active part of a hydrophone is a crystal of the piezoelectric type. These crystals have the property of producing an electrical output signal which varies according to the pressure on the crystal. So if we fire a gun and study the electrical voltage from a hydrophone near to it we get a picture which shows how the gun is performing.

Alongside we show a graph of how the hydrophone voltage varies with time for a single air gun. At the start of recording we have a flat line which indicates no signal from the hydrophone, and then when the gun is fired by activating the solenoid we see that the explosive rush of air causes a very rapid rise in signal due to the pressure building up to the explosion peak. After this the signal decreases rapidly as the pressure falls and even goes to negative values as we get a small pressure rarefaction after the peak. Following this the pressure settles down to give low signal for a little while. Next we see a fairly quick increase in signal to a small peak which is called the bubble peak. This is caused by secondary release of energy which we will discuss shortly. Finally the signal settles down again and only moves a little around the zero value, showing that the pressure variation due to the gun firing has effectively finished. Note that in this example the total time from energy release until there is no further significant gun output is around three tenths of a second or 300 milliseconds as it is more commonly expressed. This time is fairly typical for air guns but as we will see later it does vary depending on a number of factors.

Bubbles

Now we return to the bubble energy mentioned above. This arises as follows. When the air gun is fired the initial explosion of air into the water gives us our main energy pulse. This large bubble of air then continues to expand outward until finally the outward pressure of the air bubble is balanced by pressure from the surrounding water. After a very brief period of equilibrium the water presses on the bubble forcing it to collapse inwards. The next stage arises because the water effectively over compresses the bubble beyond the pressure equilibrium point and the air bubble expands quickly out again giving a second burst of input energy. This is what we see on our signature above. The whole cycle can repeat several times more until eventually the air ceases to exist as a discrete bubble and breaks up to disperse to the surface. Usually any bubble energy after the first bubble peak is very low and can be ignored.

The bubble pulse is not very desirable to the seismologists. They like a nice clean short single pulse of energy so that each geological boundary can be clearly seen. If we have bubble pulses being produced, then each reflecting boundary reflects the main or primary pulse and then a little bit later the bubble pulse. This can confuse the interpretation of the seismic data. We will see later how to minimize the bubble energy.

Before leaving this area note two points about bubble pulses. These concerns the time between the main energy peak and the bubble energy peak; this time is usually called the bubble period. This period is closely linked to two source parameters - the source energy and the water depth at which it is operating. What we find is that powerful guns produce longer bubble periods than weaker ones. Also if we take a particular gun and operate it shallow and then deep we find that the bubble period for shallow operation is longer than for deep operation. For this reason it is important to know the operating depth of each gun, and depth transducers are fitted on the sub-arrays to supply this information. The air supply pressure to the guns also affects their energy output. This parameter is closely monitored by Mechanics and Observers. On some vessels pressure transducers are installed out on the sub-arrays to indicate the air pressure close to the guns. This depth and pressure data is generally recorded on the headers of the seismic data files.

Minimizing bubble effects

The diagram alongside shows the basis of the method. Look at the lower signatures which are from seven individual airguns of different sizes. The figures at the beginning of each signature represent the size of the gun in cubic inches.

Note that the bubble period is different for each gun. This is in agreement with what we said previously in regard to period and gun size.

If we assemble such a group of guns and if we fire them simultaneously then the result we will get will be as shown by the top signature of the diagram. All the primary pulses add up constructively and give a nice big output.

The big difference is that with a single large gun we would have a meaty bubble pulse, but we do not get that with the group firing together. As the individual bubble pulses come at different times they cancel out fairly effectively.

This is a prime reason for operating arrays of guns as sources and not using individual large elements.

Arrays

Fig. 1 - 920 cu in subarray

Above is a typical example of a group of guns. Here you see an assembly of guns grouped together in what is called a sub-array. This group works very effectively as described above. The reason the term sub is used in the description of this array is that for any seismic system there are normally a limited number of such sub-arrays used together to make up the total source system. We refer to the assembly of sub-arrays as the total or complete array (Figure 3 below shows a typical complete array situation). Sub-array groups as shown there are towed behind the vessel and when they are fired the energy from the many different guns combines and travels into the earth to give us our total seismic input pulse. For this reason the complete array is usually simply referred to as the Source.

Note that Gun 4 is used as a spare if Gun 1 or Gun 2 or Gun 3 fails. Gun 9 is a spare for Gun 8 or 10 or 11.

The guns are placed at carefully calculated positions within the sub-array to make sure the bubbles are effectively canceled. In the sub-array shown the gun center's are spaced at 3m, 2m, 2m, 2m, 2m, 3m giving 14m length overall.

Within the sub array we usually have small linked groups of guns which are called clusters. In Figure 1 for instance, guns 1 and 2, 3 and 4, 8 and 9, 10 and 11 are set up as two-gun clusters. In this case the guns in the clusters are 0.8m apart. We can have vertical clusters where one gun is mounted above the other or horizontal clusters where each gun is at the same level but held apart with a strong frame.

Fig. 2 - Two Gun Cluster

Source Towing

Adjacent Figure 3 is a typical example of a vessel towing two such Sources.

Each of these sources is made up of three sub-arrays separated by 12.5m so that the overall source dimensions are 14m inline by 25m crossline.

With some towing systems it is difficult to maintain this separation accurately, especially in strong cross currents.

The lengths of the two towing wires on each subarray are carefully adjusted to give it the correct position in the water.

Note that some sub-array strings have a laser target on their top float and this is used by the navigation system to supply information as to the sub-array position at shot time.

Fig. 3 - Complete Arrays

Noise

In seismic operations, we were constantly battling against the effects of seismic noise. In Module 10, we will discuss this noise in some detail. For the moment we need to say a little about noise in order that you can understand why we use large arrays.

Noise can be thought of as roughly falling into two main categories; random and coherent. Random noise has no pattern to it. It appears on our seismic data at times and levels which are totally unpredictable. The other type, coherent noise, does have patterns to it. By this we mean that it doesn't just come and go randomly. It can last for an appreciable time and it usually has spatial characteristics in that it will not just be seen on one trace of a seismic record but across many or all of them. The noise may also exist on many successive seismic records.

Both types of noise can originate from many different sources and these are discussed in module 10 where examples may also be found. Note also that the coherent noise we have mentioned can be inline or crossline noise. Inline means the noise travels along the length of the streamer in the direction of the ship's motion and crossline means it travels at right angles to the streamer. Often we don't just get simple inline or crossline noise but something that comes at an angle to the system. We can however think of such noise as having both inline and crossline components.

The reason we summarize on noise here is that it is extremely important in seismic acquisition that we get rid of as much noise as possible before we record the signals in the instrument room. One of the important ways of doing this is to have an energy source which has spatial dimensions related to the dominant wavelengths of the inline and crossline coherent noise in the survey area. The relationship which links the reduction in noise to the geometry of the source arrays is not simple and we will not go into it further until a later training level.

Source Signatures

Earlier in this module we deliberately talked about source signatures from single guns. This was for simplicity and to help understanding. Now think a little bit more about what we have said about arrays and you will see that with an array the source signature is a more complicated beast than the single gun signature. Putting a hydrophone near a single gun now will not give you a simple picture, for the effects of all the adjacent guns will also be recorded, and the signal will be confused. To measure the "total" signature of a complete array we need to be far enough away from the array that all the local gun interactions are complete and we are seeing the compound energy pulse which travels into the earth. We are then measuring in what we call the far field of the source array and we get a far field signature.

We do in fact still measure close into the guns using near field hydrophones and these are used to monitor local responses and can be fed to computer programs to estimate total response in the far field. We will come back to this area in later training levels.

Practical Training

1. Under guidance from senior colleagues examine typical sub-arrays. See how they differ from the ones in the diagrams.

2. Examine closely the various components and how they are assembled, tested and stored onboard.

3. Identify the positions of the depth transducers and the near field hydrophones on the sub-arrays.

4. Find out how the gun depth data is sent to the instrument room and how it is monitored while shooting a line.

5. Locate the gun air pressure gauges in the gun-shack and in the instrument room.

6. Find out if this data is recorded to tape on your vessel.

7. With senior observers or mechanics discuss the rigging and towing equipment that allows the mechanic to deploy the sub-arrays into a large array pattern. Find out how the position is monitored.

8. If possible be present when sources are deployed and recovered. Observe the procedures carefully.

9. Ask mechanic colleagues what typical problems they find with the guns and with arrays or the whole source system in general.

MODULE 2 - STREAMERS

Aim:

To introduce the trainee to seismic streamers so that on completion, the trainee should have a reasonable knowledge of how streamers are constructed, operated and repaired

Summary:

 Note that this module deals with a single streamer. Multiple streamers complicate the towing systems but do not otherwise affect the subjects discussed here.

 Become aware of how streamers are constructed. Learn about the major streamer component parts and their functions. Learn about flotation control, balancing and streamer fluid. Know how buoyancy can vary with sea temperature and the components used for acquiring a streamer's position data.

 Acquire an appreciation of the factors that are involved when deploying and recovering streamers and become very aware of the potential hazards when undertaking these tasks.

 Gain an understanding of the functions of the buoys that you will find attached to the streamer

Materials and Methods:

This module has some self-study and on-the-job observation, practice and discussion. Your colleagues will have gained a great deal of very valuable experience and knowledge and so we ask you to question them as and when appropriate. Information and guidance for learning is given in this module's study sheets

Exercises:

If at all possible, during your trainee period ensure you are present at both a deployment and recovery session for streamers. If for any reason you have to miss out on these be certain to discuss the operations with observing personnel and your mentor. Make sure you know how to perform the practical tasks detailed in the study sheets.

Streamer Components

Streamers are formed from a number of elements which, when towed astern of a seismic vessel detect the reflected seismic signals and returns this data to the recording system. In this module we shall discuss the components that make up a 'streamer' (or 'cable', as you may hear it referred to).

The components of a streamer that we will consider will be:-

1. Lead-in, to tow it all and make the electrical connection back to the ship.

2. Dilt clamp, float and bend restrictor, which attaches the deflector to the streamer and protects the streamer from the deflector's lateral forces.

3. Passive modules, that amplify all the signals being passed to and from the streamer as they travel through the lead in.

4. Head stretches, to keep shocks generated by the vessel from corrupting the reflected seismic signals.

5. Active sections, where hydrophones are held and combined into groups.

6. Active modules Takes the output from 12 hydrophone groups and converts the analogue voltages into digital signals. The digital signals are then sent back to the onboard recording system.

7. Tail stretches, to keep shocks generated by the tailbuoy from corrupting the reflected seismic signals.

8. Tailbuoy power module and adapter section, to supply power to the devices on the tailbuoy

9. Tail Buoy, helps keep the streamer under tension as it is being towed through the water. Carries equipment to determine its position and relay that information to the vessel.

We will consider each of these items individually, explaining the functions and some of their characteristics. The diagram, over, shows where the major streamer components are located in the water and how they relate to one another for normal operations.

Lead-in

The lead-in is a long length of armored cable. It is used to connect the remainder of the streamer to the vessel, both physically and electrically. It is made from a heavily armored cable which surrounds a bundle of electric wires. These wires carry all the necessary seismic signals to the onboard recording system and all the commands from the onboard equipment to the streamer components. The Lead in has to be highly robust as it is subject to great amounts of forces from the drag of the streamer combined with the sideways lift from the deflector. You may find that the lead-in is covered with a 'hairy' fairing, this is to reduce 'strumming' which occurs when it is towed through the water and can be detected by the hydrophones where it will mask the seismic data.

Lead-ins come in a variety of lengths, from 300m to 900m.

Some vessels are now using lead-ins utilizing fiber optic technology which are able to send a signal from greater distances in smaller diameter cables and so the drag is reduced.

The Boot, Dilt Clamp and Bend Restrictor

To join the heavy, rigid lead-in to the rest of the streamer, there is a short section called the Boot. It is 2- 3 metres long and consists of a flexible skin surrounding the electrical wiring for the streamer. It achieves its mechanical strength from stress ropes and a number of stainless steel spacers that are used to hold and protect the electrical wiring bundle. Its main function is to provide some flexibility to the lead-in when it is attached to the rest of the streamer.

The Dilt clamp is attached around the boot. It is at the dilt clamp that the spreader ropes and hence the deflector is attached to the streamer, so all the sideways lift of the deflector acts upon the streamer at this point. The dilt clamp has to be very robust, as you can imagine the forces exerted on it by the deflector are very high. If the deflector attempted to pull the streamer wide without a Dilt clamp it would damage the boot irreparably in a very short period of time. As the dilt clamp has to have a high degree of mechanical strength, it is made from solid metal. This renders it rather heavy and so a dilt float has to be attached to it to prevent the whole arrangement from sinking too deep. We shall discuss floats in more detail later in the module.

The Bend Restrictor is a series of plastic rings placed around the lead in, prior to the boot. As the name suggests, it reduces the bend radius that occurs on the lead in when it is subjected to the sideways pull of the door in conjunction with the towing force.

The diagram above shows the dilt clamp attached around the boot, with the dilt float attached to the dilt clamp. The bend restrictor is shown forward of the dilt assembly.

Passive Module

This is a module physically the same as any other that you will encounter on the vessel. Its only function is to amplify all the electrical signals, so that they pass through the lead in with sufficiently large amplitude to be of use.

Please note that the Passive modules look identical to the Active modules. The only way to differentiate between the two is to look closely at the body of the module to see whether the words '24 BIT PASSIVE' have been stamped on the body or the serial number starts with a letter 'P'.

Head Stretch sections

A stretch section consists of a number of elastic, nylon ropes which allows the section to alter its 50 meter length by approximately 10%. The stretch also holds the bundle of electrical wires running through the streamer. The whole arrangement is covered in extra thick skin and filled with streamer fluid. The stretch section also houses a number of communication coils for use by the bird/compass and acoustic units.

Stretch section(s) are placed at the head and the tail of the streamer. They are used as 'shock absorbers' to ensure that all the active sections and modules are towed through the water with as few disturbances as possible. As the vessel sails down a line it will roll, pitch or yaw depending upon the sea conditions. These movements will be transmitted from the vessel, down to the lead-in, indeed they are usually amplified by the towing arrangement. If these movements were not checked they would be detected by the hydrophones, where they would mask the required seismic data. Stretches are placed at the head of the streamer to damp out these movements, before any of the active sections.

Active Sections

This is the point at which data acquisition begins.

Hydrophones are the basic element in the seismic recording system. To be able to use them in the 'real world' we have to mount them into what are referred to as 'Active Sections'.

The first type of section is the Teledyne series 4000. Each hydrophone is arranged within the streamer as shown on the right, it is mounted between the plastic spacers that protect it from large mechanical shocks. These spacers are also used to protect the electrical wiring that runs through the section. The spacers are joined together using stress members. These are made from either stainless steel or synthetic rope. The entire arrangement is encased in a polyurethane skin, filled with a special type of fluid.

The second type of active section is the RDH (Reduced Diameter Hydrostreamer), this is constructed in a similar manner to the series 4000 section, but has two major differences. Firstly, the hydrophones are mounted inside their own spacers and the diameter of the entire assembly greatly reduced (hence the name!).

We shall discuss how the hydrophones work, how they are connected within an active section, and how these active sections are combined to form the data acquisition portion of a streamer in later section.

Active Modules

Each active module collects the electrical signals from 12 hydrophone groups (we shall define what a hydrophone group is in section 2). Once collected, the electronic circuitry within the module converts the continuous (analogue), electrical voltages produced by each hydrophone group into a series of measured values - the digital signal. These digital signals are then sent to the onboard recording system via the wiring that runs the length of the streamer. Digital signals are used as opposed to analogue ones because; they can be sent to the vessel quicker, are less prone to corruption and do not require as many wires to carry the signals the length of the streamer. Some of the electronics inside an active module are shown in the picture above.

The subject of how the necessary seismic data is acquired and transmitted back to the vessel will be discussed more fully in later modules.

Tail Stretch sections

As with head stretches, stretches at the rear of the streamer are also required to damp out any sudden movements. For tail stretches, these shocks are generated by the movement of the tailbuoy through the water, as opposed to head stretches where shocks are generated by the vessel. Unchecked these movements will also be transmitted to the hydrophones, where they will be detected and mask the seismic data. As with head stretches, tail stretches are 50m long, consist of elastic, nylon ropes which allow the section to alter its length and two communication coils.

So it can be seen that a combination of head and tail stretches should ensure that the Active sections which hold the hydrophones are towed smoothly through the water.

Tailbuoy Power Adapter Module and Adapter section

The tailbuoy power adapter module is a standard sized module (i.e. it looks identical to both Active and Passive modules). The only function that it has is to lower the streamer voltage to a level that is suitable to power the devices on the tailbuoy (These devices will be described later).

The tailbuoy adapter section is a short section (typically 3.5m) which is used to attach the streamer to the tailbuoy and carry power to the tailbuoy. At the tailbuoy end of the adapter section are two eyes which the chains to the tailbuoy are attached and an electrical socket to take the power up to the devices on the tailbuoy.

Tailbuoys

The tailbuoy is the last component in the streamer. On it are mounted a variety of devices, these may include a combination of the following:-

• Warning light.

• GPS positioning devices

• Radio positioning devices

• Backup batteries

• Emergency generators

• Voltage regulator.

Physically, there are two types of tailbuoys:-

a. The older, floating platform type.

b. The new low-drag, easy storage buoy.

Streamer Theory

Hydrophones and Active sections

Hydrophones are the basic receiving element in the seismic recording system. They are detectors that are sensitive to very slight variations in pressure, producing an electrical output that is proportional to the applied pressure.

These hydrophones are piezoelectric devices. Piezoelectric devices have the property that they will develop an electrical voltage when subjected to a pressure. The best example of this is the cigarette lighter - you push the button, which applies pressure to the piezoelectric material. The piezoelectric material converts the applied pressure to a voltage which is used to provide the spark that ignites the gas.

In the seismic case, the pressure applied to the hydrophone (and hence the piezoelectric material) comes from the reflected seismic signals. The voltage that the hydrophone then produces is directly proportional to the pressure of the seismic reflection.

Hydrophones are manufactured so that two plates of piezoelectric ceramic are mounted at either end of a brass cylinder. Electrical connections are made so that if both plates bend inwards, as they would in response to an increase in pressure outside the unit, the induced voltages add. If the plates bend in the same direction, as they would in response to an accelerating force, they cancel. This feature is called acceleration cancelling.

Each hydrophone is manufactured to receive the most desirable frequency range required for seismic work. The terminals are colour coded so that an increase in pressure results in a negative output voltage at the white terminal.

Whilst the hydrophone is ideal as a detector of seismic signals, on its own it does not provide a strong enough signal to be of use to us. To produce a usable output voltage a number of hydrophones have to be electrically connected in parallel, or 'grouped'. There are normally 16 hydrophones in one group and these will be spread out along a 12.5 meter length of the active section. The manner in which they are electrically connected is shown on the right. In this context you will probably hear the hydrophone groups referred to as channels or Traces.

It is possible to alter the hydrophone group length using either programming plugs within the section or the onboard recording system. For both RDH and Teledyne 41250 sections the hydrophone group is altered by multiples of 12.5 meters, i.e. 12.5m, 25m, 37.5m etc.

If you look closely at an active streamer section you will see that every 12.5 meter there is a large white spacer with a number of metal bands around it and four brass screws. These 'oil blocks' are used to physically isolate each 12.5 meter hydrophone group from the neighbouring groups. Again, by carefully looking at an active streamer section you will notice that there are 5 oil blocks, dividing the section into 6 compartments. So by calculation you will see that an active section is 6 x 12.5 meters = 75 meters in length.

The arrangement of the 6 compartments in a 75 meter long active section is shown below. The exploded view shows the hydrophones in a single compartment.

Practical Training 1

Using an active section that can be partially decked, identify the internal components.

1. Point out the + and - power wires,

2. The 'Active' and 'Passive' telemetry wires,

3. A hydrophone.

4. Notice the different colour spacers around the Bird coil. Give your colleague your reason for the change in spacer colour at the coil.

5. Discuss the purpose of using different types of ropes as the stress members of a section.

6. Find one of the "Oil Fill Blocks" and ask what their function is.

7. Decide whether the "head" end connector came off the reel first and if that is the way a spare section should be stored.

8. Finally, make sure that you can demonstrate to your senior colleague how the two ends of a streamer section should be joined before being deployed in the water.

Active Sections, Modules and Streamers

As you will have noticed, a streamer consists of a number of Active sections and modules. The numbers and arrangement of these devices depends upon the client specification. Normally a streamer will be between 1500 and 6000 meters long, depending upon the geological target area beneath the seabed.

If we take the case where a client specifies that the 'active' portion of the streamer (i.e. the portion with the active sections and modules) is to be 3000 meters, and that a 12.5 meter hydrophone group length is required. Then there will be 240 hydrophone groups (3000 meters / 12.5 meters = 240).

Practical Training 2

Calculate the length of the 'active' portion of the streamer if the client specifies that he requires 324 hydrophone groups to be recorded with a group length of 12.5 metres.

Secondly, calculate the number of hydrophone groups (or channels) that are recorded when using a 6000 metre streamer, where the hydrophone group length is 25 metres.

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As we mentioned earlier, each active module takes the output from 12 hydrophone groups. If the length of each hydrophone group is 12.5 metres long, then this means that you will normally encounter an active module every 150 metres (12.5metres x 12 groups) along the streamer. Similarly, if the hydrophone group length is 25m, then a module will be placed every 300 metres along the streamer (25 metres x 12 groups).

So in the case where we have a 3000 metre streamer, with a hydrophone group length of 12.5 metres, we will have 3000 / 150 = 20 active modules. We also stated that each active section was 75 metres long, and so in the same streamer we will have 3000 / 75 = 40 active sections.

Practical Training 3

Find out the length of streamers and the hydrophone group length that you are currently using onboard from an experienced Observer. Once you have this information, calculate the number of active sections and active modules that each streamer will be made up from. Have the same Observer verify your answers.

Streamer Depth Controller

To successfully record seismic data, it is necessary to keep the streamer at a known and constant depth. This depth is specified by the client, but is usually between 6 and 9 metres, with a tolerance of +/- 1metre. To enable us to achieve this we use a combination of depth controlling devices (or 'Birds' and to a lesser extent 'Retrievers') and streamer ballasting techniques. We shall begin by considering the Birds.

The Cable Levelling units (Birds)

Picture beside is showing a DigiCOURSE compass birds attached to an active streamer section. High-energy Lithium batteries power these birds.

Birds are easy to identify by their wings. These devices are attached to the streamer by bird collars and they keep the streamer at a specified depth during seismic data recording. When data is not being recorded, birds are used to control the depth of the streamer for any number of reasons.

The operator onboard commands the bird to maintain a certain depth (thus keeping that portion of the streamer to which the bird is attached at the specified depth). The bird accomplishes this by measuring the depth it is currently at and driving the streamer up or down by moving its wings accordingly until the required depth is reached. The bird reports its depth and the angle of its wings back to the instrument room, where they are displayed and logged automatically.

As you may have guessed, for the birds to be able to operate they must be able to communicate in both directions, with the onboard systems. Communication coils are embedded in the front of each active and stretch sections. When a bird is placed next to one of these coils an electromagnetic communications link is made between this coil and one which is within the bird. Signals are sent from the vessel, through the streamer, via the electrical wiring bundle, to the relevant streamer communication coil. Here the message is passed to the bird using electromagnetic induction techniques between the two coils. Data is passed from the bird to the onboard systems in a reverse manner. So all communications take place without any direct electrical contact between the streamer and bird.

Retrievers

Mounted on top of most birds you will normally find Retrievers. These are also "depth control devices" but only in disaster situations. They normally have a pre-set activation depth of 50 meters. If a streamer sinks to this depth, the Retriever will activate, inflating a bag with carbon dioxide gas. The inflated bag remains attached to the Retriever which is attached by bird collars to the streamer. The buoyancy of the inflated bag lifts the streamer to the surface where it can be recovered

Commonly used are the SRD-300 retrievers made by Concord Technologies, Inc. An SRD-300 is green and black; it has about 6.5 pounds of in-water lift, before activation. One of these is normally mounted on the same collar set as a bird. In normal operations they are designed to hold the devices vertically in the water, so keeping them stable.

The engineering drawing for the SRD-300 is shown below.

Practical Training 4

1. Find out what Bird collars are and how to attach a device to them.

2. What dictates the position of Bird collars on a section?

3. Identify for your Chief or Mentor a communication coil in an active streamer section.

4. Learn how Birds and Retrievers are properly attached to the streamer.

5. Finally, have a colleague show you where the storage area is for the Birds and the Retrievers.

Streamer Buoyancy and Ballasting

As mentioned earlier, it is important that the entire length of the streamer runs at a constant, uniform depth. The first method that is used to keep the streamer at the required depth is by using birds evenly spaced along the streamer, as discussed earlier. The second is subtler and involves altering the buoyancy of the streamer sections to suit the water temperature and salinity. If the streamer has the correct buoyancy it will run true, without too much help from the birds. If the streamer has the wrong buoyancy, then the birds will have a lot of work to do to keep the streamer at the required depth (if they are able too at all) and so their batteries will become depleted very quickly. At best this will mean that the workboat will have to be deployed more often to change the birds, at worst it could mean operational downtime to retrieve the streamers and change all the batteries whilst onboard.

In a perfect world we aim to have very slight positive buoyancy along the streamer. This is in case the streamer is severed at some point; the loose part will float gently to the surface making location and recovery easier. On the other hand we do not want the streamer to be too buoyant as this will make it unstable in marginal weather conditions and make the birds work harder, thus decreasing battery life.

It is the aim of ballasting to achieve this uniform, very slight positive buoyancy along the entire length of the streamer.

The buoyancy of the streamer is an ever changing entity. As stated earlier, it is affected by both the temperature of the water and its salinity. So even if the vessel remains in the same area, the changing seasons will alter the water temperature, and to a lesser extent the salinity. This means that some 'fine tuning' of the ballasting will be necessary during most deployments. If the vessel changes areas in the world, some fairly drastic re-ballasting will have to take place.

We have two methods of ballasting the streamer: -

 The addition / removal of lead

 The addition / removal of streamer fluid.

The Addition / Removal of Lead

The most common and easiest way to alter the balance of the streamers is to add and remove lead. For the Teledyne 41250 Active and stretch sections pieces of lead sheet are taped onto the section using electrical and friction tape to achieve the necessary ballast load.

When adding leads in this manner, we do not use more than 1 Kilogram sheets at any single point on the section and we also aim to attach all the leads with an equal distance between them. The effects of these last two points will be to avoid dips or bows in the streamer profile.

When taping leads to a section, everyone will tell you something different as to how many layers of tape, direction of taping, knots and glues used, etc.

There are just a number of basic, common sense rules to be adhered to when attaching leads:

• The leads must be SECURE. Unsecured leads will cause noise on the streamers, with the leads knocking against the skin.

• Do not place a lead over a hydrophone as it will effectively mask that hydrophone.

• Do not place a lead over a communication coil as it will be rendered unusable.

• Always wrap a lead around a spacer, making it more secure on the section.

• Do not place leads next to birds just to keep the bird at depth, as this will not keep the whole section at depth.

For those people using the RDH streamer, life is slightly easier. Specially designed 0.75kg leads are bolted over the oil blocks.

This allows a precise amount of lead to be added to a section simply and uniformly, Plastic 'dummy' weights can be added to the other side of the lead weight to ensure a smooth flow of water over the weight and oil block and also to protect against barnacle invasion.

The Addition / Removal of Streamer Fluid

We can also alter the ballasting of the streamer by adding to and removing the streamer fluid within each section. This is not as easy as adding and removing leads as it is far harder to quantify, takes longer to perform and is very messy.

You will find that the Teledyne 41250 sections are filled with Kerosene ('Kerosene' is in fact a misnomer, the fluid within the section, to give it its correct name, is ISOPAR-M. The most important property of this material to be aware of is the low flash point temperature of 75 degrees Celsius.), where as the RDH streamers are filled with the more benign 'Streamer IX' fluid.

Note: No open flames should ever be used on the back deck while streamer sections are onboard, leaky or otherwise.

Practical Training 5

During the next deployment of the streamers identify the type of lead weights attached to the sections and assist in adding or removing the leads as necessary. Make sure an experienced Observer explains the hazards of exposure to the streamer fluid.

Streamer Positioning Devices

There are very strong geophysical reasons why we have to be able to determine where all the hydrophone groups on the streamer are located for every shot on the survey. To be able to do this successfully we use a wide range of devices. As this module is concerned with streamers we shall just look at the two devices that you will find attached to the streamers that help us to collect all the relevant data.

Acoustic Positioning Devices

Acoustic Positioning Devices (or Pingers, or XSRSs as you may hear them referred to as) are attached to the streamers using bird collars. There are usually two units mounted at the front of the streamer and three at the rear. In some circumstances units may be mounted in the centre of the streamer.

The picture on the left shows a pinger used in the DigiRANGE system.

Compass units mounted in the Birds

The second function that the bird has is to acquire heading data. Each bird has a very accurate heading sensor (or compass) which measures the magnetic heading that the unit is currently pointing at. This information is reported back to the instrument room where it is combined with heading data from all the other compasses on the streamer to build up a precise estimate of the entire shape of the streamer. This information is required for cable positioning, when the Navigators 'Bin' the recorded data.

Streamer Deployment and Recovery

This part of your on-the-job training may already have been a large part of your life by the time you read this although you may not have considered some points yet.

Weather dictates what procedures can be used and when. A recovery operation requires good heading and speed control of the vessel. Streamer section skin can be damaged by movement of one section that is going on the reel, sliding over one already on the reel. Poor heading control can cause this to happen and high winds can cause poor heading control. Pulling the streamer onto the reel with too much tension, as happens when the vessel's speed through the water is too great, can also cause skin damage. More subtle damage to the stress members is caused by excess tension. When a module goes on the reel there maybe a very large force on one of the stress members in each of the sections connected to it. This large force is created by the module's length acting as a lever with the rotation of the reel. Fatigue breaking of the stress member at the connector is the ultimate result of this abuse. Birds and Retrievers can also be damaged by tension as they slide over the back deck rail or roller.

During a recovery operation, assist your senior colleague in taking the door off the dilt. Have your colleague show you what water in a section compartment looks like. Remove from the streamer, then store both a bird and retriever.

Deployment has fewer problems or, better put, a different set. Putting the Tail Buoy out and getting it moving away from the stern is the first problem. After the Tail Buoy is away, it will tend to pull the streamer out as it spools off the reel. Because deployment usually requires less muscle, this is a good time to check for faults: Stress members with broken strands; low cable oil; bad or sticking bird collars; loose leads; wire bundles caught between the skin and stress wire, etc. This last example happens because the wire bundle is about 82 metres long which is inside a 75 metre section.

During a deployment assist your senior colleague in getting the Tail Buoy checked out and launched. Take a bad bird collar off the streamer and replace it with a good one. Remove a lead from a section that is heavy. Show how to push a wire bundle loop back into the centre of a section. Tape patch a hole in a section with your colleague's guidance. Demonstrate the correct way to attach a bird and test it on the streamer. Now get it over the stern and into the water properly. As needed, change the O-ring on a section or module connector. Clean any debris out of the connectors before reassembling them. Assist in the connection of the door to the dilt and putting an outer streamer out wide. Make notes about any procedures that you feel can be made more efficient or safer. Share your notes with your senior colleague and discuss the merit of each change.

Streamer Repair

As with all equipment, streamer components sometimes fail and so need to be replaced and repaired. Malfunctioning streamer components can sometimes be replaced whilst they are deployed using the workboat, otherwise the streamer has to be brought onboard. Once the faulty component has been recovered it may be repaired onboard, or if the fault requires testing or work that is beyond the facilities onboard it will be sent onshore to specialist facilities.

As mentioned above, streamer components may be replaced whilst still deployed in the water using the workboat. This is an operation that can only take place in good weather using highly trained and knowledgeable people. The advantage of using the workboat to replace malfunctioning equipment is that we save ourselves the time it would take to recover and re-deploy the equipment. If you consider that at an absolute minimum we have to recover all of the guns, some of the lead-ins, dilt floats and clamps and some of the streamer. So by replacing equipment in situ we can save ourselves a great deal of time that would be better spent recording seismic data.

Using the workboat it is possible to replace malfunctioning birds, pingers, modules and streamer sections. By far and away the most complex of these operations is to replace an Active section. This is due to the length and bulk of the replacement section and the need to take part of the streamer under tow. As a Trainee you are not permitted to participate in any small boat operation prior to completing the relevant training. This training forms part of the Common section at Observer level. However you may be taken along during one of these operations to provide you with experience of the techniques that the crew use to carry out these tasks. To help you in understanding these techniques please watch the video Workboat Training - Change of Streamer sections. We will ask you to watch this video again later on in your training so that you understand the procedures fully. At this point in your career we ask that you pay particular attention to the safety aspects that the video highlights, especially the sequences which show where not to stand in the workboat.

If the malfunctioning streamer component cannot be replaced using the workboat it will have to be recovered by bringing the streamer onboard. If this occurs, you will normally find that other remedial faults will be repaired and every effort made to ensure that the streamers are deployed in the highest possible condition that time allows.

There are two levels at which streamer components can be repaired. Onboard there are extensive repair facilities that allow us to repair most of the common faults that occur in the streamer components. From your vessel familiarisation tour you will have seen that the vessel has a large area designated to streamer section repair and workshops for repairing birds and pingers. The repair of streamer components is a large part of the Observer's job and so we will discuss this topic in great detail at the next level of your training. As a Trainee, it is highly likely that you will be asked to assist senior colleagues in these areas.

Floats and Buoys

Structure

There are two types of materials currently used to construct buoys and floats. You will find that the older buoys and floats are made from Aluminium, which tends to produce a high maintenance, heavy and expensive buoy. The current trend is for plastic to be used to construct modern buoys and floats. Plastic is less expensive, lighter and so easier to handle and can be easily repaired, so it is rapidly becoming the logical choice for offshore applications.

Dilt Floats

As explained earlier, those vessels that use a system involving dilt clamps and spread ropes to achieve the necessary separations for a large number of streamers need to use Dilt floats.

With this type of towing arrangement the lead-in has a tendency to hang rather slack in the water. Because the weight of the dilt clamp, the whole arrangement has to be supported to keep the front of the streamer at the required depth. The dilt float and the length of stainless steel chain perform this function. A black, rubber 'Bettis' decoupler is included along the length of the chain to damp out the swell movements of the float.

These types of floats are constructed from rigid plastic, filled with foam to provide extra buoyancy. They are compartmentalised to provide some protection to the entire buoy if a leak occurs. At present two sizes of dilt floats are used, 1500 litre floats are generally used on the outer streamers (where the heavier dilt clamp is used) and 1000 litre floats used on the head of streamers which have the lighter clamps).

Tailbuoys

The main function of the tailbuoys is for positioning of the streamers. You will notice that the buoy is equipped with both Radar reflectors and Global Positioning Systems (commonly referred to as GPS). This allows the vessels to locate and find positions for all the buoys and for other vessels to see (with Radar) the tail end of all our streamers. Another function of the tail buoy is to add just enough drag to the tail end of the streamers, to add stability, which helps to prevent the tail of the streamer from whip-lashing.

There are two types of tailbuoy commonly used:-

Firstly, floating platform type (shown on the left), which allows a person to stand on the decking, whilst working on devices held onboard. These buoys are made from aluminium.

A stainless steel lifting rope is anchored at the centre of the buoy and attached to the streamer, using a modified bird collar. The lifting rope, as the name implies, is used to lift the buoy onboard using either a crane or Baro boom, depending upon the vessel's setup.

Secondly, the new low drag, easy storage buoy is used on vessels having a high number of streamers deployed and where deck storage space is at a premium. These items are made from plastic and are constructed in a similar manner to the Dilt floats. An example is shown on the right.

The main advantages of the plastic buoy are that it is considerably lighter and easier to handle, and so is a logical choice for off-shore applications.

Power to the devices onboard either variety of tailbuoy is via the streamer and through the tailbuoy power module and adapter section.

A bank of batteries are also incorporated on the tailbuoy to provide operation of the positioning devices for approximately 24 to 36 hours if a problem occurs within the streamer power supply to the tailbuoy. If it is not possible to repair the power supply to the tailbuoy, generators may be installed on the buoy without recovery taking place. Propellers dragging through the wake are used to turn the generators and hence provide enough electricity to power the buoy.

Your vessel may be using any of these types of buoy described. The mechanics will be knowledgeable of the rigging and maintenance of the buoys.

Practical Training 6

Read the vessel's procedures for the launching and recovery of the tail buoys. You may have to assist the Navigators when they encounter difficulties due to weather or broken equipment. Discuss with your Chief and Mentor those problems that they regularly encounter and the solutions.

MODULE 3 - RECORDING SYSTEM

Aim:

To familiarise the Trainee Observer with the general structure of the overall seismic source and recording system and to introduce how it is set up and operated while running seismic lines. On completion the Trainee should have a good feel for the core elements of an observer's job function.

Summary:

 Study the SEAL operator's manual, relate to it by observation of the onboard system.

 Observe how the recording instruments, streamers and sources are initialised for shooting.

 Observe the operations, monitoring and output during the running of a line.

Materials and Methods:

This module is covered by a mixture of self study closely linked to on-the-job observation and discussion and explanation.

Exercises:

If possible, operate the instruments under close supervision of an experienced person. Produce your own summary of what you learn or deduce from this module.

Introduction

Before you move into this module, you will need to find out from a senior colleague which recording system you are using onboard.

At the start of each line the observers have to set up the recording system and the gun controller for operation. Other systems such as the bird controller need to be checked. Each crew tends to develop its own procedures for starting lines, some of which are dependent on the survey specification and setup. Another set of tasks have to be carried out at the end of the line.

It is recommended that vessels to use a formal checklist, however, sometimes these procedures are not written down and it is difficult for the Trainee to grasp what is being done. Below is an example based on Line Parameter Sheet used on the Nordic Venturer. However, some details may not be exactly the same as on your vessel.

Tasks & Exercises

1. Compare the procedures used on your vessel with those outlined above. Make a note of any additional checks carried out.

2. Draw up your own complete check list and discuss this with your mentor.

3. During the actual running of a line the observers carry out a series of tasks associated with system monitoring and output. Summarise these on paper in sequential order and comment on the significance or importance of each task.

4. List all the output that is produced in the recording process and say how you think each is later used

5. There are obviously strong links between the recording and navigation sections during data acquisition. Write down the key contact points and their importance to the operation.

MODULE 4 - OBSERVER'S AND NAVIGATOR'S LOG

Aim:

To make the trainee observer fully aware of the contents and importance of the observer's and navigator's log. On completion the observer trainee should be able to interpret and complete the observer log and be able to understand the important sections of the navigator's log.

Summary:

 Study the observer's log format in detail. Examine previously completed logs. Make sure you understand all sections. Be certain that you know the meaning of all the abbreviations used. Know the importance of the log in later data processing.

 Study the navigator's log to understand its format and key areas. Examine completed logs and be sure that you can interpret them.

 Understand why it is essential to compare the Observer's Log with the Navigator's Log before making the final printout

Materials and Methods:

This module largely involves on-the-job training. There is little or no literature that describes observers or navigator's logs so discussion and questioning are the main training tools here. The Field Geophysicist will explain the use of the logs after they leave their producers.

Exercises:

While a line is being run, keep your own version of the observer's log. Compare it with the actual. Explain any differences to yourself and your mentor.

Introduction

Although Observer Log and Navigation Log differ from vessel to vessel, the information required will be similar.

On a prospect, the client may request different or more information through the Party Chief.

Study the following examples of an Observer & Navigation Log. Compare this with the ones produced on your vessel.

Below is a sample of Daily Observer Log, to keep track of the acquisition activities:

Observer's Line Log

Line: is an identifier usually a number for the survey line being travelled by the vessel.

Sequence: the order that line data is collected, starting from #1 at the beginning of the job.

Heading: line heading or bearing in degrees.

Streamer Depth: indicate the depth of streamer at the Start of Line

Gun Deployed: indicate the number of Gun deployed simultaneously to obtain the Gun volume

The Comment column is where the explanation of anything that could affect recorded data is noted for the processors. Some common remarks is to indicate wind and sea state and noise record.

Navigation Daily Log

Navigation Line Log

Tasks & Exercises

1. Ask your mentor any questions you have about the Observer Log.

2. Find out who assign both the Line identifier and the Sequence number? Who assign the line heading?

3. Discuss with your mentor what information is duplicated on the Navigation Log and why.

4. Discuss with your mentor other abbreviations used when you review an Observer Log from your vessel.

5. With examples of both Observer and Navigation Logs from your vessel, go over the information required on both. Discuss with your mentor the sources of this information. If possible a Field Geophysicist should join this discussion to help explain what information on the logs is critical to good processing.

MODULE 5 - 2D AND 3D VESSEL OPERATIONS

Aim:

To ensure that the trainee is clear on all the main aspects of single boat operation in both 2D and 3D operational modes and also to provide the basics of multi-boat operation. On completion, the trainee should understand and be able to describe all common shooting techniques and be able to discuss difficulties and advantages.

Summary:

 Read the material provided in this study guide, discuss it with your colleagues and make sure you understand.

 With what you have read in mind, consider and discuss the capabilities of your own vessel. Think hard about the limits of the overall recording system. Make sure you get a clear picture of the relationship between source groups, streamers and CMP lines. Be aware of the importance of streamer and source towing techniques.

 Consider shooting speed carefully. Be sure you clearly understand its significance. Know the normal shooting speed for a job and the factors limiting it.

Materials and Methods:

This module has a mixture of self study and on-the-job observation and discussion.

Exercises:

Some exercises are provided at the end of the study sheets

Single vessel, One source, One streamer

The most basic arrangement for seismic surveying is a single vessel towing one source and one streamer. 2D surveys use this technique and so did the first 3D surveys. The streamer and source can be put in place easily, and the recovery operation can be very fast. This is particularly important if bad weather is coming. The vessel moves astern as the streamer comes on board, reducing the strain on the sections and making it easier to pack the streamer neatly on the reel. Spooling devices are rare on single streamer boats, and the observers have to use brute force to guide the streamer into place. The single source can be towed directly from the stern or booms can be used to keep the subarrays apart. The navigators position the vessel, and simple geometry is used to locate the source and receiver groups. The tailbuoy is fitted with a radar reflector and this is tracked manually on the bridge radar to monitor streamer feather. For a 3D survey, streamer compasses are added to allow basic calculation of receiver group positions, but fixed offsets are still used at the front end.

Multiple Sources

The first variation on this theme is to use two sources instead of one. Each sail line now provides two CMP lines. The sources fire alternately so the effective shot point interval is doubled. 25m shotpoints become 50m shotpoints if the vessel keeps the same speed. The CMP coverage is halved. 60 fold becomes 30 fold. So why not slow down ? The depth controllers on the streamer act like wings and need to be moving through the water to produce a deflection. Speeds of less than 4 knots make it difficult to maintain the streamer at a steady depth. On rare occasions surveys are carried out in areas where there are strong tidal currents. If the tide cycle is regular and predictable it may be possible to plan the survey so that all the lines are shot while heading into the current. In this case the speed through the water stays high but the speed over the ground is reduced, so the distance between the shots can stay small. Obviously the problem gets worse if we try to use three sources or four. One seismic contractor carried out some big surveys using four streamers and four sources. The effective shotpoint interval was 100m and the data was 15 fold. This meant that the CMP stack could only cope with low levels of noise and the operation was limited to good conditions.

Multiple Streamers

The alternative approach is to increase the number of streamers. This immediately creates a lot more problems but it does mean that the fold of coverage is maintained. Each time the source fires data is acquired by each streamer at the same time. Combination of One source and two streamers gives two CMP lines with no loss of fold. At present, most surveys are carried out with two sources and multiple streamers, but in problem areas where high fold data is needed a single source with multiple streamers may be preferred.

Towing two streamers behind the ship means that we must provide some way of keeping them apart. By now you have probably seen the diverters which are used for this purpose, plus all the associated chains and wires. The diverters must be able to maintain a steady streamer separation while shooting a line so that the relative position of each receiver group is maintained. Deployment and recovery now become more complex operations. On vessels with small back decks the streamers must be laid out one at a time, and they may also be brought in separately. It is no longer possible to back down. The first streamer is brought in while moving ahead at 2 knots to maintain control of the other streamer in the water. At this point rough seas can cause serious problems. This first streamer comes in under some tension and quite often the first few sections will be damaged. Great care is needed to keep the reel turning very slowly while the first half of the streamer comes on board. Spooling devices are rare on two streamer boats.

With two streamers we need a better way of positioning the receiver groups. Acoustic measurements can be made between the streamers and the precise geometry computed for every shot. Unfortunately this means that more pods have to be clipped onto the streamers during deployment and removed again during recovery. It also becomes important to know exactly where the units are on the streamers, so tape measuring may be needed to check their positions. The simple tailbuoy with its radar reflector is no longer adequate. Active tailbuoys are used, with GPS receivers to fix their positions and radio links to send this data back to the vessel. Acoustics may also be used at the tail of the streamers to tie the buoy positions with the last receiver groups.

More streamers

Adding a third streamer down the centre of the spread provides six CMP lines if two sources are used. This sounds easy but there are problems. The outer streamers will have to run wider to give the same spacing between the streamers so bigger diverters will be needed. The third streamer is towed directly off the stern which means that it behaves differently from the other two towed on the diverters, particularly when the vessel is crabbing in strong winds or currents. In these conditions it is difficult to keep the front of the streamers at the correct spacing and the centre streamer easily gets caught in the gun arrays. It is much easier to go straight from two streamers to four. In this case the whole spread is towed off the diverters and the streamers are affected equally when the ship crabs.

With a modern ship and a wide back deck handling four streamers is not too difficult. The streamers can be deployed and recovered two at a time, and there are usually automatic spooling devices to steer the cables onto the drums. Care is needed with depth control to avoid tangling streamers together, and this raises another issue. Balancing one cable was simple enough if you had time to get it right. If the balance was not perfect it just made the birds work harder to maintain a steady depth. With multiple streamers it is a different story. If the ballasting is not correct it may be difficult to keep the streamers at precise depths during deployment and recovery and tangles will result. With four streamers more use is made of acoustics to position the sources and receiver groups, and often a front buoy will be used to give a GPS position right at the front of the spread. A fan-beam laser may be mounted at the stern to give range and bearing data for the gun arrays and the front buoy.

After some time in production the bird batteries need replacing. With one streamer you just pull it in, change the batteries and put it out again. A few hours production has been lost. Multiple streamers take much more time to bring in and out, so bird batteries are usually changed with the work boat. On many vessels the boat goes out several times a week, servicing four or five birds at a time on a line change. Any bad birds or acoustics units can be changed in the same way. Some crews have even changed bad streamer sections or modules from the work boat, but this is only recommended for experienced crews and calm conditions. All this is necessary to keep the vessel in production and avoid long periods of downtime.

A fifth streamer down the middle behaves just like number three, and once again it is easier to go all the way to six. A common configuration for six streamers is 100m separation, so big diverters are needed and they have to run a long way behind the vessel to get the overall spread. This means long lead-in cables running back to the diverters. Long lead-in cables are heavy and droop in the water. Where water depth is limited or there are wrecks on the sea bed it is wise to attach floats to support the lead-ins. Some crews have found this out the hard way. Another factor to consider is drag. The outer lead-ins are towed sideways and present a large surface area to the water. If the fairing on the lead-in is not in good condition it may be impossible to achieve the required separations.

Lately, the use of eight and more streamers has required a completely new approach to vessel design. Measuring approximately 40m across the stern, these vessels provide a vast amount of space for streamer handling, and the streamer winches can be synchronized to allow all the streamers to be brought in together. Storage of spares has also been carefully worked out for extra streamers, birds, tailbuoys and acoustics units to maintain.

The quantity of equipment is not the only problem. Based on an 8 streamer vessel, the accurate positioning of the sources and receiver groups requires some 194 acoustic ranges to be read each shot, which takes 5.5 seconds. A similar length of time is needed to collect data from the 136 streamer compasses. Apart from this the vessel itself cannot turn quickly on line changes. The spread is 700m wide so in a turn the outer diverter is traveling much faster than the inner one. If the turn is not done correctly the inner diverter may stall or the outer one may run inwards.

Having said all this, once the vessels are in production they gather data at an amazing rate and sizeable surveys can be acquired in a short space of time. Crew changes can be done by helicopter and refueling carried out at sea, so the equipment can remain in the water for months at a time. This is important in areas where the shooting season is short.

Multiple vessels

The other way of shooting with six streamers is to use two smaller vessels with three streamers each. This method has been successfully used for a number of years by GeoExplorer and Malene Ostervold. This approach has a number of advantages, not least of which is the speed of deployment and recovery. This allows good use to be made of short periods of good weather and can be a significant advantage over the larger high-tech vessels. Two vessels running close together demand good liaison between the crews, and sophisticated steering techniques. The system which links the navigation computer to the auto-pilot on the bridge of the "master vessel" is in turn linked also to the auto-pilot on the "slave vessel". In production this means that both vessels are steered down the line by the navigator on the master vessel. On line changes the link can be retained so that the helmsman on the master vessel steers the two ships together.

Tasks & Exercises

1. Check that you understand why using two sources instead of one halves the fold of coverage and using four sources reduces the fold to a quarter. Draw a diagram to help you explain this to your mentor.

2. Why can we not use several sources firing in sequence and reduce the time between shots? What other factors need to be considered?

3. Slow shooting speeds cause loss of streamer control. What limits our maximum shooting speed?

4. For 5 second records what is the minimum time between shots on your vessel? What is the limiting factor? What vessel speed does this require

5. On your present survey what is the maximum number of files per tape?

6. Draw up a table showing the maximum capabilities of your vessel. Include number of sources, number of subarrays, number of streamers, length of streamers, groups per streamer.

MODULE 6 - MAIN SEISMIC INSTRUMENTS

Aim:

To introduce the new observer to the main seismic instruments and to ensure that he or she gains a good basic understanding of their physical and electronic structure. On completion the trainee should clearly understand the elements and layout of the main recording system and its related QC systems.

Summary:

 This module provides a more detailed feel for the main instrument modules, their functions and how they are linked to the navigation system and the external seismic devices. At the conclusion of this training module you should be completely familiar with the major components of the recording system, how they interconnect, and how the system "talks" to the other equipment in the recording room.

 As mass storage is a key element of the recording system, the new observer must be aware of all aspects of this and should develop competency in handling removable media. In particular be aware of regular maintenance procedures such as cleaning.

 The Trainee should carefully study the physical layout of the instrument room and be aware of all recording components and how they are linked and inter-relate both physically and electrically. A clear understanding of data flow from Streamer through to Tape must be achieved.

 Understand the meaning of Real Time signals in the Instrument Room. Understand what is meant by "Closure"; be sure you know where it comes from and what it is used for.

Materials and Methods:

Material for this module is largely the instrument room manuals and study guide linked to an enquiring tongue. Because of their specialist and changing nature, the new observer will not find detailed written material on the instrument room configuration. This is therefore very much a self-study linked to on-the-job questions. The Trainee must follow the guide in reading the base material but inevitably will require asking sensible questions.

Exercises:

Questions are provided to ensure that the Trainee understands key areas of the subject matter.

Introduction

The seismic industry is unique. The electronic equipment, and the procedures in operating them are always similar between ships and crews, but they're never exactly the same. You will learn how to operate the SEAL recording system but the procedures involved in starting and ending lines, how you may word different events in the observer log, what type of acoustic system is installed, and how the recording system interfaces to all of the other recording room electronics, may all be different when you transfer to other ships.

The Module 6 to Module 8 provides the Trainee's introduction to marine seismic instrumentation. To keep our training units to a workable size we use three modules here rather than one large one. This training also locks tightly to Module 9 - Core training. This group of four should be seen as forming a cohesive subset within the Trainee program.

At the time of writing, Nordic uses the SEAL System to record its seismic data. However, the first thing you have to find out is what recording system is used on your vessel.

Ultimately, as you progress through the observer ranks your knowledge will enable you to troubleshoot and repair the recording system and the other equipment that is the observer's responsibility. The more you know, the more complete and detailed your knowledge is of the equipment, the easier and faster you'll be able to fix it. That is the name of the game, "keep it working". Try to be aware of this as you learn the equipment.

SEAL System

SEAL system is categorised in two main broad areas; the Onboard Equipment and the In-sea Equipment.

ONBOARD EQUIPMENT

The LCI Board

The Line Control Interface (LCI) collects the data on both LEFT and RIGHT Transverses and from the Auxiliary channels connected on the Aux Line. The data from the streamer is uncompressed by a single DSP prior to any additional processing. LCI generates the clock at 16.384 MHz on the lines, manages the input and output synchronization signals to and from the two rear Blaster connectors and interfaces the unit with other slave control modules. Finally, the seismic data is converted to IEEE 32-bit format and passed to the LMP board. The power for the auxiliary channels is supplied by a daughter board called PLCI.

The LMP_S board

The Line Main Processor receives the seismic data from the LCI board and performs de-multiplexing. It also manages the Ethernet links and the serial communication links from the two Blaster connectors. After adding or replacing the LMP_ S board in a Control Module, you must enter its licence code, using the HCI Intall menu on the workstation, and reboot the workstation.

The PRM Processor

The PRM interfaces the control module with the storage devices. It receives de-multiplexed IEEE 32-bit format data from the LMP_S board, converts it to SEGD format and prepares it so that it can be sent to the export devices after temporary storage in the SEGD repository, and also to the QC workstation and the plotter.

If monitoring or plotting of a trace on a plotter is requested, rasterization is performed.

If the data is recorded to tape, the Trace Blocking mode can be used.

In Tape Bypass acquisition mode, only the outputs to the export devices are disabled. The links to the seismic QC and plotters remain.

The APPS4 board

The Acquisition Processor Power Supply generates low DC voltages for the electronics and fans in the Control Module.

IN-SEA EQUIPMENT

LAUM

The Line Acquisition Unit Marine (LAUM) is the visible part of the in-sea electronics. It is housed in an in-line canister. It performs the following functions:

• Management of 60 seismic channels.

• Power supply of the FDUs in the streamer (+/- 24 VDC).

• Frame control.

• Data routing.

• Data filtering and compression.

Each LAUM in a streamer can act as a master and a slave unit. As a master, the LAUM transmits a series of empty cells over the line, relayed from one FDU to the next in the head-to-tail direction. FDUs fill the cells in succession with the data already acquired in real time, four samples at a time. After the last FDU, the data is processed by the tail

LAUM used as a slave, and fed back to the on-board controller, while another part of the electronics in that LAUM forwards the empty cells to the following FDUs too.

Thus, each LAUM acts as a Master for the next downstream LAUM and a slave for the preceding upstream LAUM. It also replicates the data it receives from its tail. A special case arises for the first two LAUMs: they are both Slaves for the LAUXM. The telemetry lines are swapped in the LAUM prior to being forwarded to the tail.

FDU2M

The Field Digitising Unit (FDU) is the electronic device that acquires the analogue signal from the sensor and converts it into digital samples. An FDU2M inside an ALS processes two channels.

The acquisition is started and synchronised by the LAUM or LAUXM that drives the line. The FDU stores four samples at 0.25-ms Sample Rate and sends them all at a time when requested. The FDU performs both A/D or D/A conversion, CRC and parity checks and even looping of the telemetry lines. The A/D conversion is performed by a Sigma Delta converter delivering a bit stream at 256 kb per second. After decimation, 24-bit samples are available at a 4 kHz rate.

Each input can be connected to the sensor or to an internal reference voltage or an R/C network for instrument tests. The D/A conversion is used for tests too, where a digital signal is generated from the LAUM and applied to the input.

The identification of the unit is stored in an internal non-volatile memory. It consists of the following parameters:

- The type of the Assembly (ALS, HAU, HAPU or AXCU) in which the unit is mounted,

- The assembly serial number (xxxxxx),

- The Unit type (FDU2M; FDU1M used in HAU and HAPU),

- The unit serial number (xxxxxx),

- The channel type (e. g. Hydrophone),

- The position in the assembly, starting from the head,

- The channel number (1 for a FDU1M; 1 and 2 for each FDU2M).

HLFOI AND TLFOI

The Head Lead-In Fiber Optic Interface (HLFOI) converts line signals from electrical to optical and vice-versa, at the head point of the Lead-In. It is composed of two main boards or converters, each driving a telemetry line. The electronics is powered with a +/- 24 VDC voltage supplied by the LAUXM or the DCXU.

The two converter boards are fully interchangeable. They can drive optical signals up to a distance of 1500 m and electric signals up to 100 m.

The Tail Lead-In Fiber Optic Interface (TLFOI) converts line signals from electrical to optical and vice-versa, at the tail point of the Lead-In. It is composed of two main boards or converters, each driving a telemetry line. The electronics is powered with a 0/12 VDC voltage supplied by the HAU or HAPU.

The two converter boards can drive optical signals up to a distance of 1500 m and electric signals up to 100 m. They are not interchangeable.

HAU AND HAPU

The Head Auxiliary Unit performs three functions:

• It measures the tensile strength on the cable. It is equipped with a set of strain gauges mounted on the inner surface of the canister; the analogue signal is converted to digital samples by a one-channel digitising unit and relayed to the Control Module, with all other seismic samples;

• It supplies a +12 VDC power to the tail optical converter (TLFOI);

• It supplies a +/- 24 VDC power to the digitising units, on both telemetry lines, up to the next LAUM.

The Head Auxiliary and Power Unit performs four functions:

• It measures the tensile strength on the cable. It is equipped with a set of strain gauges mounted on the inner surface of the canister; the analogue signal is converted to digital samples by a one-channel digitising unit and relayed to the Control Module, with all other seismic samples;

• It supplies a +12 VDC power to the tail optic converter or TLFOI;

• It supplies a +/- 24 VDC power to the digitising units, on both telemetry lines, up to the next LAUM.

• It supplies a 40 VDC, 30 W (0.75 A max.) power intended for a head buoy. In addition, it duplicates auxiliary lines, like Acoustic or Modem, towards the head buoy.

TAPU

The Tail Acquisition and Power Unit is mainly composed of the same electronics as an LAUM. That is, it acts as a slave LAUM during normal acquisition. But, in case of a disruption in one telemetry link, anywhere in the streamer, the data paths can be looped here between the two lines. It performs another function: supplying a 40 VDC, 30 W (0.75 A max.)

power to a tail buoy. Also, the auxiliary lines (Bird, Acoustic and Modem) are forwarded by the TAPU.

TBC BOARD

The TBC (Tail Buoy Control) board supplies regulated power to the tail buoy and allows communications between the central unit and the RGPS beacon. Two types of TBC board are available: one supplying a 14 VDC voltage (Part No. 1E41079076) and the other suplying 28 VDC (Part No.1E41083006).

The power to the TBC board is supplied by the TAPU. Also, diodes performing an "OR" function allow the voltage from an alternator (connected to P3) to be used as a power source.

Important points

Operation manuals have a tendency to glaze your eyes over with the obvious information and leave out the technical details about what is really going on. The required reading list is therefore carefully limited to the daily use of the system; you are however encouraged to read more. The best way of learning the system is by using it.

Plan your explorations of the software during extended downtime, have an experienced observer look over your shoulder as you are browsing. This is the time to ask questions and refer to the manual for insights into unfamiliar aspects of the system. At any time if you think you might have changed something you should not have, confess to your shift leader or chief well before the start of line. No learning experience is worth the price of a scratched line due to operator error. If you are a linear-type thinker, sit down in front of the system with the manual open to the first page of this section and plough ahead. This could take weeks depending on your shooting schedule but by the end of it you'll be one enlightened observer, well on your way to "mastering" the system.

Instrument room mass storage

SOME HISTORY OF DATA STORAGE

Nothing has revolutionized the seismic industry more than the advances made in the storage capabilities of tape drives and tape media. These advances continue today.

Not too long ago seismic data was recorded on reel to reel 9 track tapes. Bit density was 800 bits/inch in NRZI format, so you can imagine how many hundreds of boxes of tapes were required for each trip. Tape loading/unloading was a long and arduous job. Tape drive and tape media technology soon advanced, doubling the density, then doubling it again, still using the round 9 track tapes and what is known as GCR recording with bit densities in the 3200 bits/inch range.

In the mid 1980's IBM developed the 3480 tape drives and square cartridge tape media. It was a large step forward. The same amount of data could be stored in about a third less physical space and weight; premium commodities on board ships. Concurrently, however, developments in streamer and seismic system technology increased rapidly, meaning more and more data being recorded for a typical prospect. Clearly with 8 streamers in the water the 3480 tape drives were reaching the end of their service life.

So the IBM 3590 was introduced in 1996. These data cartridges are the same physically as the 3480 cartridges, but can hold 10 GBytes of data as opposed to the 220 Mbytes that is all that can be stored on a 3480 cartridge.

When selecting a new recording media for use in the seismic industry there are a number of factors that have to be considered. Firstly, it is not only the amount of data that a recording medium can hold, but also the speed at which the data can be written to tape. As you are aware, all seismic data is recorded in real time. There are many recording media that can hold larger amounts of data in more compact forms, but they have longer write times which renders them unusable in our industry.

Secondly, it is not good recording data onto a media that cannot be read by the processing centre. So there is a problem in getting the entire industry to adopt a certain recording medium as standard. This is a chicken and egg situation, with no obvious leader dictating matters.

THE 3590 DATA CARTRIDGE

The 3590 data cartridge is shown on the left. As mentioned above, each cartridge is capable of holding 10 GBytes of data recorded on the ½ inch tape contained within the cartridge.

The tape remains within the cartridge during storage and handling to help protect the tape from external contaminants and damage. When the cartridge is inserted into a drive, the drive's mechanism unlatches the tape leader block, pulls the tape leader out of the cartridge and threads the tape onto a reel in the drive.

Each cartridge includes a file-protect selector that, when set, prevents data from being written on or erased from the tape by the cartridge drive.

Each tape can be identified by the labels attached to the side of the cartridge, opposite the file-protector selector, and the top of the cartridge. It is normal procedure to attach a numerical identifier to each tape before it is recorded on, and a description of the data recorded onto the cartridge immediately after it is ejected from a drive.

CARE AND HANDLING OF THE 3590 DATA CARTRIDGES

The 3590 tape cartridges are virtually sealed from dust contamination and are very robust. Bit density is extremely high however, and it literally takes only a speck of dust or a smoke particle to cause a short record.

The physical length of the tape changes slightly due to temperature. The tapes should be thermally stabilized about 24 hours before their use. In other words if the tapes are stored in a non air-conditioned area they should be brought into the instrument room about 24 hours before they are used. Condensation on the surface of the tape is also a possibility if a "hot" tape is suddenly exposed to the cold recording room environment.

Most, if not all, tape manufacturers take extraordinary measures to ensure the quality of their tapes. The media is manufactured in a clean room environment and QC is performed in a studious fashion and a loose or damaged tape does not happen very often. It is for this reason that the vast majority of tapes will work just fine straight out of the box.

THE 3590 CARTRIDGE DRIVES

The IBM 3590 cartridge drives are compact, high capacity, high performance tape drives. They are attached to their host processors using the Small Computer System Interface (SCSI).

The drive data rate of 9Mbyte per second is three times faster than the 3480 drives so allowing applications to read or write data more quickly. The 3590 drive incorporates a number of features to ensure the integrity of all the data written to tape.

However, each drive requires some care to ensure that it works when called upon to do so. Without occasional cleaning the interior of the drive will be caked in dust. The Tape paths are cleaned every time a cleaning cartridge is run though the drive while dust will accumulate everywhere else.

A cleaning cartridge should be run through each drive when the operator is instructed. During a line, if a tape drive indicates an excessive number of retries manually change the tape and run a cleaning tape through it.

Repair of these units is very limited. Troubleshooting is typically limited to the mechanical tape loader. An electronic failure is "fixed" by replacing the unit. The recording head wears out over time necessitating the eventual return of the drive to a re-manufacturing company.

Tasks & Exercises:

1. Have a senior colleague to explain the procedures onboard for the cycling of data tapes onboard, from the reception of blank tapes to the final dispatch of the recorded tapes.

2. Make sure that you locate the storage areas for both blank and recorded tapes.

3. Understand the onboard procedure for cycling blank data tapes into the instrument room. Locate the numbering labels that are used to identify each individual tape and the storage areas for these tapes.

4. Once data has been recorded, understand where the label for each cartridge is produced and where they are stored.

MODULE 7 - CONTROL MECHANISM

Aim:

To establish a clear understanding of the instrument control mechanisms that are available for source and streamer and how this control is achieved or monitored. On completion the trainee should be able to clearly describe the what, why and how of source and streamer control. The understanding should be of the basic elements of the systems and not at this stage involve complex detail.

Summary:

 Streamer control, commands and data: - study the material listed in the study guide. Observe very carefully how streamers are assembled. Understand clearly the general physical structure of a section but also be clear on the functions of the different elements that make up the total streamer. Know where commands to the streamer are generated and data is returned from the streamer. Observe where and how this two way communication is monitored.

 Streamer control, depth: - Study the overall streamer from a buoyancy point of view. Be sure that you understand how cable operating depth is achieved and then maintained in operation. Look at the depth controllers in detail and be clear how they work physically and electrically. Know where cable depth and position are monitored in the instrument room

 Source control: - The study guide references the material on gun control. In this section the trainee should be clear on how the guns fire and on the need for monitoring and control of the firing of each gun system. Study the gun controller manual and observe where and how the firing is monitored. Learn how the gun firing system is tested.

Materials and Methods:

The material for this module is onboard manuals and data sheets.

Exercises:

Practical tasks are detailed in the study sheets.

Streamer Control, Commands and Data

Streamer seismic acquisition control through digital telemetry revolutionized this industry. Previous to this design, each seismic channel was interfaced to the recording system via its own pair of wires. These systems were known as analog cables since the hydrophone data remained in analog form until it reached the recording room equipment. A 120 channel cable meant that there were 240 hydrophone wires running into the back of the recording system not to mention the lines required for cable depth indicators and bird control. Tracking down leakage and cross-feed problems on all these wires could take days or even weeks. Anybody who worked on these cables will tell you it was no fun.

Eventually, digital cables were introduced. The hydrophone data is converted into digital words by electronics within the modules of the streamer cable. Cross-feed and leakage problems are thus isolated to the short length of line between the hydrophone array and the module. The data from all the seismic channels is broadcast to the boat as digital words over a common transmission line.

There are two common telemetry techniques, time division multiplex (TDM) means that each module has its own time slot to output data. Another telemetry technique common to us all is frequency division multiplex (FDM).

The TV and Radio we are entertained by, have channels that broadcast simultaneously but are separated by frequency. We tune our receivers to the carrier frequency of our desired channel in order to acquire the data on that channel. In TDM, the data from all of the channels appears on the same carrier but at different times. In TDM, channels are separated by time rather than by frequency.

The Streamer Interface Board in the SEAL System generates the commands that control the acquisition of data within the modules and receives the seismic data generated by the modules. There are two telemetry lines. One is called the passive rail. The command signals that originate in the Streamer Interface Board travel to the modules along this path. The second telemetry line is called the active rail. The seismic data that the modules have acquired is sent to the Streamer Interface Board along this path. The telemetry lines are twin-axial cables with data rates in the 10 megabit/second range.

The commands on the passive rail are first received by the active module that is closest to the ship. The module decodes and stores this command but does not initially act on it. This module repeats the command on the passive rail so that the next module downstream will receive the same command. In this way the commands trickle down to the last module near the tail of the cable.

This active module at the tail of the cable, "module 1", is configured by the system to wrap around the commands received on the passive rail onto the active rail. When the command appears on the active rail it is acted on. If the command was "Extract Data", this module at the tail will repeat the command and then output its acquired seismic data onto the active rail. The next module upstream, "module 2", will then receive on the active rail, the command and the data generated by module 1, will rebroadcast module 1's data, then insert its own data onto the active rail. In this way each module will insert its data onto the active rail "behind" the data generated from the previous, downstream module. At the Streamer Interface Board, the data from module 1 is received first, then in sequential order towards the head of the cable.

This is the basic nature of the data telemetry. In detail, it is more complex and you will be expected to know more as your career advances. Ask questions when telemetry problems are being investigated on your ship. You will find that you can't help but learn things when the equipment fails.

Streamer Control, Depth

Review the information about streamer ballasting and depth control in Module 2. Ballast is the physical adjustment of the streamer for neutral buoyancy. Once a streamer is neutral, the birds, depth control devices, have less work to do to keep the streamer at the commanded depth of operation. In the Reference manual for your vessel's system, find the section which describes the bird's "depth keeping mode". All electronic birds have this function in common and it is what automates streamer depth control. Also read the procedures for using the onboard controller to send commands to the in-water birds.

Find the bird controller in the instrument room. With your senior colleague, go over the controller functions. Learn how to change the information displayed on the controller and whether there is any change to the data output IF the display is changed. Don't make any changes to the controller while ON-LINE without prior OK from at least a Shift Leader! Get familiar with the procedure to get a bird ready to go on the streamer. This is usually the last step in the onboard repair process for birds and, as such, is required learning.

Familiarize yourself with all the major parts of a bird. Find out who is the main repair man for birds on your vessel and spend some time learning this trade. Getting some experience working on birds prior to need, will improve your work when the pressure is on. Even the steps for check out and preparation to go on the streamer can be difficult when EVERYONE is in a hurry.

Source Control / Understanding the Basics

Have the Gun controller manual with you when you and a senior colleague go over the various screens used for air gun operation. It is the Observer's responsibility to drive the source controller while in production. You must become completely familiar with the operation of your ship's particular source controller. You should be able to quickly and correctly disable guns that have failed. A clumsy source controller operator can cause the loss of a line. You need to carefully read the prospect specifications for gun drop outs. The contract itself or some kind of cheat sheet should be easily available so that it is completely clear which guns can be disabled.

When a gun misfires or autofires and it is apparent that it may need to be disabled, it is a good idea to inform the gunners of what you're about to do. They may have some insight into the history of the particular gun in question and may be able to force it into working a while longer by reversing the polarity of the solenoid pulse. Autofires especially should be dealt with immediately. An autofire on a record is certain to trash the record and sometimes it is difficult to stop a gun from autofiring once it starts. Disabling the gun occasionally has no affect. Some ships have gun arrays that are equipped with remote controlled air shutoff valves for every gun. These will prevent the loss of a line for a persistent autofire or a bad air leak. Different ships have different methods for monitoring the air pressure to the arrays. On your ship it may be part of the observer responsibility to keep track of the air pressure. Usually it is a combined effort between the gunners and observers.

Gunners are often busy with other things while shooting and may not be aware of the situation when a gun starts to go. It will help them during the line change when they are fixing the failed gun to have actually seen the problem for themselves. A considerable amount of troubleshooting can be done just by looking at the signal return display. In any case it may be a good idea to printout the line statistics for the guns at the end of every line so you can document the bad guns and have hard proof to hand to the gunners before they start to drag the arrays in.

Occasionally, a gun that was disabled during the line will seem to work fine during the line change. Gunners may blame the source controller itself for problems that they are finding difficult to track down. Cooperation is the key. The work done by the ship is a team effort. Lending a hand to the gunners with troubleshooting their flaky gun problems will go a long way to maintaining a cooperative atmosphere.

MODULE 8 - INSTRUMENTAL AND SEISMIC TESTS

Aim:

To introduce the trainee to the various types of instrumental and seismic tests, which are created by the seismic observer in the course of his job. On completion the trainee should clearly understand the reasons for the tests and be familiar with the output from the tests and with the essentials of test interpretation.

Summary:

 Understand the purpose and output for each test.

 Discuss with senior colleagues how the tests are done on your vessel and read any available vendor literature on testing.

 Look at current vessel tests and make sure you understand them in sensible detail.

 Discuss the reasons for the tests from an internal and from a client point of view.

Materials and Methods:

Test descriptions are provided and these give the basis of some self study. As some testing is complex the trainee may need to discuss such with his mentor or other senior observers, however reasons and broad understanding are required rather than detailed knowledge.

Exercises:

None are provided for this module

Daily Tests/ Understand Reasons and Output

Recording quality seismic data is the prime objective of our company. The seismic data is only as good as the acquisition equipment. By this we mean that if we have any faulty elements in our equipment these may affect our seismic data and produce a contaminated output. There have been stories of crews shooting days of data only to find out weeks later, after processing, that the data was trash due to a streamer, instrumentation, or configuration problem. It is the Observing Department's responsibility to ensure that this does not occur.

With a large portion of the seismic processing now taking place on the actual ship, turnaround time to the final product is much faster than a few years ago. Problems with the seismic acquisition are therefore usually caught before any major disaster has unfolded. However, on board processing is a "backstop" for acquisition problems that will only come into play if the Observer Department fails in its responsibility.

Running Daily Tests is one way that the observer insures that the streamer and instrumentation is working within manufacturer and contract specification. With older seismic equipment, system tests were a lengthy tedious process. Tests are automated with the SEAL system, thus requiring a minimum of effort and time. There is therefore no excuse for not running a daily test.

Printouts and/or tapes from the daily tests are kept as proof that the equipment is working properly at that time. If a streamer section or a module should fail, the time of the last successful daily test is used as starting point in time for when the quality of the seismic data is in question. Production records from that point on should be viewed with suspicion and processing should be immediately informed of the problem area.

Since the daily tests are so quickly and easily run, some crews generate a daily test at the beginning and end of every line thereby insuring the quality of each and every line. Daily tests are also used to maintain a daily progressive log of bad or failing channels. Printouts from the most recent daily tests are consulted when the streamers are deployed so that the bad channels can be repaired.

The actual set of individual tests that are run for the daily tests is selected by the Chief Observer, typically with approval by the client. They aren't usually written to tape and at no time should they be written to a data tape of a line since the recording parameters are changed. There are seven types of tests that can be run on the streamer. They are listed here with a brief explanation. Note that for the results of each test to be valid, error free telemetry from the streamer must be established.

1) RMS NOISE AND DC OFFSET CONFIGURATION

This test looks at the electronic noise and the residual dc offset of the electronic components in the module.

2) IMPULSE RESPONSE

This test confirms the integrity of the high and low cut filters within the analog section of the modules as well as the digital filter on the Digital Signal Processing Board.

3) CHANNEL GAIN ACCURACY

This test measures the percentage of gain error of each channel. For example a gain error value of -0.14% for channel 1 indicates that channels output is 0.14% lower than the input signal. Acceptable limits are +/-0.5%.

4) AMPLIFIER HARMONIC DISTORTION

The frequency and amplitude of the sine wave is selected by the operator. Harmonic distortion is measured for each channel and the results given as the value of the signal at the 2nd and 3rd harmonic and noise relative to the fundamental frequency

5) DYNAMIC RESOLUTION

Same test as the Amplifier Harmonic Distortion test except that the amplitude of the sine wave is not selected by the operator. The test automatically runs through a series of sine wave amplitudes.

6) STREAMER RMS NOISE

The RMS noise on the seismic channels are measured for a period of time and expressed graphically in both µV and µB.

7) HYDROPHONE ARRAY LEAKAGE

This test measures the discharge rate and hence the amount of leakage on the hydrophone line by making measurements of the voltage on the hydrophones after a pulse was applied.

Monthly Tests / Understand Reasons and Output

Monthly tests are usually defined and run by the Chief Observer. They are normally written to tape and stored on board the ship. Monthly tests are more detailed than the daily tests. Tests are run with both the low cut in and with the low cut out. Monthly tests are used to confirm beyond any doubt that the system is operating properly and to show the client that every effort is being made to maintain the integrity of the system. Test tapes are kept to document this effort and can be used in situations where the proper operation of the instruments in past lines is in question.

Start Up Tests / Understand Reasons and Output

At the start of a new prospect it is important that the proper operation of all the equipment in the recording room is confirmed. A monthly test is usually run on the instruments and it is also possible that the client will request special tests. It is important to obtain the confidence of the client and the way the first line is shot on a new prospect is often a defining factor. Professionalism must be maintained.

After all the equipment is deployed, it is a good idea to have navigation start up a test line so that everything will truly be ready at the start of the first line. Header transfers and header lengths should be confirmed and the system parameters should be checked. All the tape drives should be cycled through and the integrity of the data written to tape should be confirmed. The plotters and printers should be started up. All of the auxiliary equipment, fathometer, tension meter, two boat equipment, cable depth controllers, acoustic equipment, gun controller; everything should be closely scrutinized so there are no surprises on the first shotpoint.

MODULE 9 - CORE TRAINING

Aim:

To ensure that the Trainee Observer has a basic understanding of those fields of electronics and AC power systems that are commonly encountered in the marine seismic environment. To also ensure competency in the basic skills of soldering and multimeter use. On completion trainees must be able to demonstrate that they have the necessary knowledge and abilities to lead them forward to the full observer position

Summary:

 Using the references and the study guide, cover the analogue and digital electronic areas defined. The categories have been carefully selected as representative of the type of electronics particularly relevant to seismic recording.

 Discuss with senior colleagues the AC/DC power system of the vessel especially in regard to how it links to the instrument room. Be sure you appreciate the safety aspects.

 The trainee should become completely familiar with the use of all types of multimeter test instruments.

 Soldering is an important skill required by observers, these are important especially in relation to working with delicate devices. Be very aware that competency comes very much by practice.

Materials and Methods:

This module is a mix of self study and practical on-the-job training. It is also a module which will vary in its application to the Trainee depending on previous background. Mentors or Chiefs should exercise discretion to allow trainees to bypass areas with which they are already acceptably knowledgeable or competent.

Exercises:

A few specific exercises are provided in the electronics section of the study guide to ensure that trainees really do have reasonable understanding of the essential areas. Thereafter tasks are associated with normal on-the-job activities.

Introduction

Electronics forms the heart of the seismic acquisition industry. The systems used to acquire and record seismic data have rapidly advanced. With these advancements the equipment has become more and more reliable and system hardware failures are now few and far between. However this increase in reliability has been offset by the dramatic increase in the amount of equipment that the Observer's are responsible for.

With these advancements in technology and the amounts of equipment, it means that field personnel do not need to troubleshoot the systems down to component level. Previous generations of seismic systems required that the Observer was able to troubleshoot the system down to component level using oscilloscopes, logic analyzers and other instruments. Today, troubleshooting is typically limited to close observation of system displays, running diagnostic software and replacing the most likely board. It sounds easy and sometimes it is. However, as with any complex system, faults can occur that produce confusing symptoms in unlikely places.

Software, hardware, firmware, signal processors and micro-controllers all interact in intricate ways with blinding speed. Occasionally with massive or multiple failures, diagnostic software cannot be run or the operator display and interface may be dead. Occasionally, seemingly random intermittent faults occur with few clues as to their origin. In these instances, troubleshooting the problem becomes very difficult especially if the field personnel are completely ignorant of the inner workings of the equipment.

Troubleshooting is still an acquired skill and the knowledge of basic electronics is essential. You will find that the majority of the field people who ride the boats have had some kind of electronic training. You'll also find that the few experienced observers who have had no formal electronic training have learned some basic electronics, can operate a multimeter, and have developed some troubleshooting skills. It's part of the job.

Electronic Theory For Seismic

CLOSURES AND REAL TIME SIGNALS

The seismic record cycle begins with a signal from navigation. This signal is called Nav Start and it is a signal often called a closure. A closure is basically an electronically controlled switch. A relay could be used to generate a closure. Investigating a closure with a multimeter set on resistance you would find an infinite resistance, an "open", until the signal occurred at which point the "switch" would close momentarily and you would briefly measure zero resistance, a "short". The word "Closure" is often used to describe any Real Time Signal but that isn't strictly accurate.

Nav Start is considered a Real Time Signal because its occurrence defines the actual moment of an event. It is used by the receiving equipment, at that moment, to begin the sequence of events that make up a record cycle. Other Real Time Signals would be "System Start In", "Time Break In", and "System Ready Out". These signals, however, are technically not closures. In these cases the time of the event is defined by a +5VDC pulse.

AC POWER

Alternating Current power onboard a seismic vessel is essential to the operation of all systems and functions. Sufficient power is necessary to run the hydraulic systems, air conditioning and both Navigation and recording systems. Without stable AC power to the instrument room the monitors, tape drives and computers would malfunction. This is the reason for two distinct AC power systems on seismic vessels.

'Ship's power' is unfiltered AC from the ship's generators. It is stepped down through transformers from the high voltage output of the generators to the normally used 230 / 240 VAC used by the equipment. Hydraulic equipment is an example of high power (or load) equipment that uses the voltage output by the generators directly. The main advantage of this is that the high voltage output from the generators means that the current required by the system is low. Ohm's law can be used to roughly show this relation, even though it has to be modified to give a completely accurate answer with three phase AC power. Air Conditioning is another big user of ship's power on a seismic vessel. Without it the instrument room and accommodations would be either unbearably hot or cold for both people and equipment.

'Uninterruptable Power System' or UPS power is the second AC power system onboard a seismic vessel. A UPS is chosen for the load it must supply if ship's power fails for any reason. UPS power is normally confined to the instrument room. It has three purposes -

• Isolating the instruments from surges and sags in the generator supplied AC power.

• Keeping the navigation and recording computers running during a brief ship's power outage.

• If the ship's power is lost for an extended time the UPS allows enough time to shut down the instrument room systems in an orderly way. This saves the data and programs from loss. Reloading some of these programs takes hours; all lost production time. Even with extended ship's power loss the GPS primary navigation system can usually be kept operating. The load of this system is small compared to the load capacity of the UPS.

Practical Electronics

MULTIMETERS

If you know little about electronics, have your Chief Observer or one of the more experienced Observers shows you how to operate a multimeter. A basic multimeter measures resistance, voltage and current. You will see the multimeter frequently used to check the continuity of a line and to measure voltage of a power supply or battery. Ask questions and become familiar with their use. Read the manual supplied with the multimeter. These show safe operation and proper set up for various types of readings.

There are two types of multimeter, Analog and Digital. Analog multimeters such as the Simpson Meter, are best used to measure leakage and fast, intermittent open or short circuits. Gunners often use analog meters when they're chasing down electrical problems in their firing line bundles. Analog meters are sometimes harder to read as the needle deflection can indicate different values depending on the setting of the scale selector switch. In other words, analog meters require some interpretation.

Digital meters are usually easier to operate since they give a numerical display of the measured value. Interpretation however is still necessary if the multimeter has a scale selector switch. Some digital multimeter are "auto-ranging" and can also measure capacitance and frequency. Digital meters typically sample about twice a second making them unsuitable to measure intermittent short and open circuits or rapidly changing leakage.

Have your mentor give you some tasks to accomplish using a multimeter. Choose the correct meter to use for the task. Report your results and save them in your training notebook. What type of meter did you choose? Give your reasons. Did you get the results you expected? What safety precautions did you take? Discuss the accuracy of your results with your mentor.

SOLDERING

Soldering is a skill which is being used less often in the seismic industry. The complexity of circuits makes troubleshooting down to component level difficult if not impossible without very specialised equipment. This was stated back in the Introduction section. Reduced size of discreet components and density of contacts in surface mount technology all make unsoldering a very time consuming process. Often this will also destroy the very board that needs to get fixed. This type of soldering is best left to the manufacturer's specialised equipment. With all that said, soldering is still a necessary skill.

Communications between different instrument room systems is over wires. These wires must attach to the proper connectors to get the data into a system. This is where soldering is required of an Observer. Ask your mentor to show you where the soldering equipment is kept. If you have no experience soldering, get your mentor or someone acknowledged as good at soldering to show you how. Pay attention, this is a learned skill. No reading can teach the skill. Practice by making cables for new system connections when needed.

There are many radio links used in seismic these days. Putting RF connectors on different types of Coax properly will be the difference between a good radio link and an intermittent link. Many of these RF connectors require soldering. Have your mentor show you the different types of RF connectors used. Discuss the equipment needed for good solder joints on larger and smaller wire sizes.

Ask your Chief or Mentor for a task that will test your soldering skills. A good task could be replacing some of the serial cables that have become crushed or spliced with reuse and age.

MODULE 10 - NOISE

Aim:

To lead the new observer to a clear appreciation of the noise components of the marine seismic system. On completion the Trainee should be aware of the origin and impact of the common noise systems and be able to specify how they can best be avoided or alleviated

Summary:

 The first section of the module starts with a brief description of what is noise and what is the required signal. Ensure that you understand how the wanted signal and noise components are brought together as the Signal to Noise ratio. Following on from this there are discussions on the different characteristics of noise.

 The second section deals with noise that is generated by the seismic system. At this level we need to look briefly at instrument noise, streamer noise and noise generated by the towing system and the vessel.

 The third section describes in broad detail noise that is generated externally to the seismic system. Here we look at the effects of the weather and other seismic vessels operating in the near vicinity.

 The final section briefly describes how noise levels are measured and displayed by the Field Geophysicist

Materials and Methods:

This module largely involves self study based on the study guide but as some of the issues are fairly complicated the trainee may well have to approach a mentor for help in understanding these.

Exercises:

Practical tasks for this module involve streamer tasks and analysis and are detailed in the study sheets.

Introduction

At this stage we are trying to build up an appreciation of the classes of seismic noise, how it arises and what it looks like.

Observers by the nature of their job must develop a keen appreciation of the whole noise range and its expressions on seismic records. Only by having a good understanding can an observer assess whether he has a problem in his recording process. The experienced observer must be able to differentiate between noise over which he has no control and that about which he can do something and which may indeed show him that he has problems with some of his equipment.

Seismic Noise

SIGNAL VERSUS NOISE

The reliability of the seismic method is strongly dependent on the quality of the recorded data. The quality of seismic data varies greatly from area to area

We use the term signal to mean any event on the seismic record from which we wish to obtain information (this usually means primary reflections from the subsurface). Everything else is noise. The signal to noise ratio (S/N) is the ratio of the signal in a specific portion of the record to the total noise in the same portion. Poor seismic records result when the signal to noise ratio is small.

RANDOM AND COHERENT

Seismic noise may be either random or coherent. Random noise has no pattern to it. It appears on our seismic data at times and levels which are totally unpredictable. It could be caused by electrical unrest in the instruments, a fish having a go at the cable, signals from the restless earth itself ... all are unpredictable and hence the name random.

The other type of noise, coherent noise, does have patterns to it. By this we mean that it doesn't just come and go randomly. It can last for an appreciable time and exhibits a behavior which is consistent in the seismic data. So we might see it on an individual seismic trace from shot time until the end of the listening time.

A good example might be what the seismic trace shows when the vessel is recording near an active drilling rig. A repetitive signal from the rotating drill bit is picked up by the streamer hydrophones which provide the seismic signal and this will certainly last for the whole listening time of the seismic record. Such noise has spatial characteristics too. By this we mean it will not just be seen on one trace of a seismic record but across many or all of them. The noise may also exist on many successive seismic records.

In some areas, we can also get coherent noise which is actually also seismic signal. It comes from our source energy and is a form of seismic signal which we do not want. This type of noise can be highly coherent and show strong alignments which cross our seismic traces and interfere with the actual data we want to get. Such noise is often said to be predictable because if we repeated the seismic shot in identical conditions the coherent noise would repeat itself exactly.

INLINE AND CROSSLINE

Note clearly that noise can be inline or crossline noise. Inline means the noise travels along the length of the streamer in the direction of the ship's motion. This means that successive hydrophone groups in the cable see the noise at ever increasing times. This tends to be the dominant type of coherent noise on seismic data. Crossline means it travels at right angles to the streamer, so that the hydrophone groups see the noise coming in at the same time. Of course life being what it is, we often don't just get simple inline or crossline noise but something that comes at an angle to the system. We can however think of such noise as having both inline and crossline components.

Self Generated Noise

INSTRUMENT NOISE

No instrument system is perfect. All have an underlying noise component which must be monitored and measured to ensure that it does not exceed acceptable levels. In a properly working system, thermal and switching noises are the major components of instrument noise.

Thermal noise is generated by every electronic component of a circuit. It is the random change of the input signal by the temperature of the component. Switching noise is caused by electronic gates changing state, power supplies creating different DC voltage levels and by gain ranging amplifiers, such as that used by the Analogue to Digital converter. Good components are chosen to minimise thermal noise and power filtering is used to minimise switching noise. By design, the sum of these is below a threshold level. The RMS noise test described in Module 8 is used to determine that the noise is below this threshold.

Harmonic Distortion is another instrument problem. Harmonic Distortion is caused by wrong measurement or digitising of the analogue signal. When reproduced, this error in amplitude causes a step or straight edge in the otherwise smooth amplitude change of the signal. A step creates Harmonics of the original signal sampled, thus, the name. The A/D circuit is carefully constructed, using precision components, to minimise Harmonic Distortion.

STREAMER NOISE

The most likely source of recording noise is the seismic streamers. As you have learned from Module 2, the seismic cable is a complex structure and it is subject to considerable stresses in normal use. Just as there is an inherent noise level in the instruments so there is one in the streamer itself. We have a lot of instrumentation built into it and many attached pieces of equipment buoys, birds, etc. which change the water flow creating noise. Module 8 gives the specifics of the 'Streamer RMS Noise' test.

Major areas of noise generation are where the flow pattern of the water changes near a hydrophone. The hydrophone detects the pressure changes caused by the alteration of the flow and inputs this signal to the module to be digitised. Any component that changes the flow is therefore a potential noise source. This includes lead weights, retrievers, birds, acoustics, any buoy attached to the streamer and the modules themselves.

TOWING SYSTEM NOISE

The system used to tow a large number of streamers in a wide configuration can add significantly to the noise detected by the hydrophones at the front of the streamer.

As you may have noticed the equipment that is needed to achieve wide spreads is very large, very complicated and under considerable amounts of force when in the water. These factors can greatly increase noise levels.

The major component of noise generated by the towing system is tugging noise. This is caused when the vessel moves rapidly in a different direction to the movement of the towing system (These movements usually occur when a sea swell is present and increases as the weather worsens). This movement by the vessel causes the towing system to be 'tugged' in the direction that the vessel is moving. This tugging motion on the towing system is transmitted to the streamers where it is detected by the hydrophones.

These tugging motions are amplified on the outer streamers by the nature of the towing arrangement.

Tugging noise is usually seen as low frequency noise trains on the near traces.

In configurations where a vessel is towing a large number of streamers in a wide spread, this problem is increased as the towing system can amplify the noise as it travels to the outer streamers. Indeed the towing system itself can be a major contributor of noise. Large deflectors, dilt floats and spreader ropes all cause disruption to the flow of water around the seismic system. With the wide spreads and large numbers of streamers that are towed by a single vessel today, the noise caused by the towing system has become a big issue.

VESSEL GENERATED NOISE

For seismic vessels, we aim to use an acoustically quiet hull and propulsion system. The design of the vessel's hull is responsible for the amount of noise it generates. For purpose built vessels this means that the hull generated noise can be significantly reduced by careful design and simulation.

The vessel's propulsion system can be another source of noise. In virtually all cases, seismic vessels use diesel electrical drive systems. The beauty of these systems is that the main engines are decoupled from the drive systems, this means that vibrations from the main engines are not transmitted through the shafts and propellers and so through the water and onto the streamers.

External Noise

SEISMIC INTERFERENCE

The acoustic nature of seismic source signals means that, if two seismic operations work within "hearing" distance of each another then each will, to an extent, dependent upon just how close they are, record the source signal from the other. This signal from the other source is "noise" which detracts from the required source signal. The avoidance of Seismic Interference or SI is a major problem in survey planning and execution.

Seismic acquisition is driven by the demand for surveys from our clients and our desire to shoot areas with high sales potential. Unfortunately, this is the same for the other seismic companies too. In the summer months when the weather is good, the most desirable areas tend to be a magnet for seismic operations. In view of the SI problem, it is in the interests of each seismic operation to co-operate with the others. This involves the different operations liaising with one another about their line lengths and line orientation in order to work out a schedule of alternating time slots such that each operation has an exclusive acquisition period without interference. This process of co-operation is known as time-sharing.

However, it is not an automatic right to time-share with other vessels. Unfortunately, the fact that you are recording an unacceptably high level of interference does not always mean that the operation causing the interference is suffering the same level of interference from you. This can come about for a number of reasons...

1. The geology between your vessel and the other vessel can attenuate interference in one direction only.

2. The relative orientation of the two survey areas can be important as the angle of incidence of interference at the streamers will have a bearing on the impact that the interference will have on the data.

3. You may find that your operation is in between two other surveys causing you to have to timeshare with two operations which are not interfering with each other, but they are both affected by and interfering with you.

A more recent aspect of seismic interference is the development of software to remove it from seismic records. This has created situations where rival seismic operators will no longer time-share with one another at all. The economics of the situation are such that it is viable simply to shoot with other vessels, rather than waiting for a quiet period to acquire perfect data, then to reshoot those areas where the data is not usable, even after post processing treatment.

Elsewhere though, crews will liaise to minimise the interference and consequently the lost acquisition time, by careful choice of shooting location, shooting direction and time relative to one another. Time-sharing does not always require that no one be shooting other than you. Careful analysis of factors 1 to 3 mentioned above may lead to situations where more than one vessel can shoot at one time or part of the time, optimising time efficiency for everyone.

THE WEATHER

The weather can be a crucial factor in stopping acquisition of data due to noise. Although you will have seen that we usually operate the streamers at depths between 6 and 10 metres, sometimes the weather can affect them drastically. In these cases the weather deteriorates to a point where the wave movement on the surface results in pressure variations in the water that the hydrophone groups detect.

Sometimes vessels operate in areas where the weather is almost always calm and never stops production. In other parts of the world only brief intervals of good weather are available to enable us to acquire good data. In these areas we are said to be looking for a weather window. As you can imagine, if the weather is poor so that we cannot acquire good data, there is nothing we can do about it, except wait and carry out any essential maintenance in the meantime.

Tasks & Exercises:

1. Ask a senior colleague to show you how to measure the noise on the streamers using the plots and QC instruments.

2. Observe noise levels on the front and tail end of the streamer. Comment on how the noise in these areas compares with other parts of the streamer.

3. Observe noise levels in the vicinity of modules and birds or acoustic units. Comment on how the noise in these areas compares with other parts of the streamer

4. The next time you are working on the streamers, learn to install the bird/acoustic safety straps and leads properly.

Processing QC

Most large marine seismic contracts now involve some degree of onboard processing of the seismic field data. This may be limited to simple Quality Control checks on selected data or it can extend to advance processing which provides onboard stacked seismic sections.

Common routines include the calculation of mean RMS values for a 500 ms window at the end of each record. This is carried out for each trace being recorded. By the end of the record most of the seismic signals have died away and we are left with the background noise. The data is presented as an areal display with trace number on the vertical axis, shot number on the horizontal axis and a color code to show the noise level. The geophysicists monitor the display during production and can draw the observer's attention to any unusual features. Problems like towing noise, tailbuoy tugging, individual noisy traces, seismic interference, and ship noise can all be monitored, quantified and quickly evaluated.

A second display can be produced showing the mean RMS value for a streamer, both raw and with a selected frequency filter applied. This is often used when the weather is deteriorating and low frequency noise is increasing. The lowest frequencies are outside the frequency range of normal seismic data, so removing these gives a clearer picture of noise which might actually damage the data. This type of display gives a clear picture of worsening conditions and allows timely decisions to be made about continuing production or recovering the gear.

MODULE 11 - INFORMATION TECHNOLOGY

Aim:

It is the aim of this module to provide the Trainee Observer with a brief introduction to the subject of Information Technology. As you are no doubt aware, this is a massive subject.

Summary:

 An introduction to I.T and a brief explanation of the history and development of the subject.

 A brief discussion of the various physical elements required to construct a network.

 A brief introduction to the workings of some of the communication systems that you will find onboard the vessels. This section is concerned with the communication of digital data and voice data via satellites.

Materials and Methods:

Study sheets are provided which should contain all the information necessary to complete the module.

Exercises:

A short exercise is included at the end of the module to ensure that all the relevant points have been understood.

Networks

INTRODUCTION

'Networking' is the sharing of information and services. It is only possible when individuals or groups have information that they wish to share with others. Computer networking provides the communication tools to allow computers to share information.

Since the 1950s, people and organisations have used computers to manage information at a rapidly increasing rate. Historically, technology required that these computers be very large. These Mainframes were used to store and organise data. Data was entered using local terminals. A single mainframe was capable of servicing requests from multiple remote units. Mainframes provided all the data storage and computational abilities, while the terminal was simply a remote input / output device. Computer networks were created when organisations began to require that mainframes share information and services with other mainframes.

As the computer industry developed, smaller personal computers allowed individuals total control over their own system. Instead of centralising all computers processing into a single mainframe, 'distributed' computing uses multiple smaller computers to achieve the same goals. Separate computers work on a subset of tasks without relying on a single central computer for processing. To compete with mainframes, a computer network is required to share the vast amounts of information and services available from each computer.

A new model called 'collaborative' computing is now becoming important. Collaborative computing is where networked computers actually share processing abilities, with two or more computers used to achieve the same processing task.

Today, a typical computer network will include a mixture of mainframes, personal computers and a variety of other computers and communication devices.

Computer networks are often classified by size, distance covered or structure. Even though these distinctions are fading rapidly, the following classifications are commonly used:

1. Local Area Network (LAN):- this refers to a combination of computer hardware and transmission media that is relatively small. LANs normally do not exceed distances of 10 kilometres. Typically they are contained entirely on one site and tend to use only one type of transmission medium.

2. Metropolitan Area Network (MAN):- is a network that is larger than a LAN. It normally covers the area of a city, hence the name. Different hardware and transmission media may be used in MANs because they must efficiently cover the larger distances or because they do not require complete access to locations between the networked sites.

3. Wide Area Network (WAN):- a WAN includes all networks larger than a MAN. WANs interconnect LANs which may be at opposite sides of a country, or located around the world.

Many people believe that eventually these classifications will disappear. Ultimately all computer networks will inter link to form a single computer communication infrastructure. However, many issues need to be resolved before this is fully implemented.

NETWORK ELEMENTS

All networks require the following 3 elements:

1. At least two individuals having something to share - Network Services.

2. A method, or pathway for contacting each other - Transmission Media

3. Rules so that the two or more individuals can communicate - Protocols.

Network Services

Network services are the capabilities that networked computers share. They are provided by numerous combinations of hardware and software. Depending upon the task, network services require data, input/output resources and processing power. Computer networks are valuable because of the services that they provide or manage. Among the many possibilities are the following:

1. File Services - includes file transfer, data storage and migration, file update synchronisation and archiving.

2. Print Services - provides increased access to printers, eliminates distance constraints, handles simultaneous requests and shares specialised equipment.

3. Message Services - facilitates electronic mail, object orientated applications, work group applications and uses directory services.

4. Application Services - allows specialisation of services as well as improved scalability.

5. Data Base services - involves the co-ordination of distributed data and replication.

When an organisation implements a computer network, decisions must be made whether to centralise or distribute network services. Among the many different factors for consideration are:

* control of resources

* Server specialisation

* choice of Network Operating Systems

Transmission Media

Before a computer network can be implemented, a physical path must be created for the computers to contact one another. The path may be composed of one or more of the following cable and wireless media types:

* Unshielded twisted pair wires

* Coaxial cable

* Fibre optic cable

* High or low power, single frequency radio.

* Spread spectrum radio

* Terrestrial microwaves

* Satellite microwaves

* Point to Point infra-red

* Broadcast infra-red

Some organisations may use public or private transmission media to supplement media that they own.

Transmission Media Connectors

Connectivity hardware completes the path created by transmission media. It performs the following functions:

* connects computers to the raw media

* connects pieces or lengths of media together

* uses the media's capacity effectively.

* connects logically separate networks.

The hardware and software combinations that serve these purposes are classified as network or inter network connectivity devices. Network devices include transmission media, connectors, network interface boards, modems, repeaters, hubs, bridges and multiplexers because each is used to form a single network. Inter network connectivity devices include routers, brouters and Channel Service Units/ Digital Service Units because they interconnect separate networks.

Communication Systems

In this final section of this module we shall introduce the communication systems that you will find onboard the vessels. At this stage, these subjects will be covered in a very general way so as to provide a foundation for more comprehensive training at later stages.

INTRODUCTION

In the past, ships at sea were equipped with High Frequency (HF) radio systems using telephony and TELEX transmission services. The earlier systems also relied on Morse code. While HF systems are still in use, the majority of Maritime applications now use satellite systems for the transmission and reception of data and voice signals.

SATELLITE COMMUNICATIONS

Satellite communications use microwave frequencies to receive radio signals from transmitting stations on earth and to relay signals back down to earth stations. So it can be seem that the satellite serves as an electronic relay.

In the example on the left, earth station A transmits signals of a specific frequency ('uplink') to the satellite, these signals maybe data transmissions, television signals, voice images, or a combination of these three.

The satellite receives the signals and retransmits them back down to earth station B at the 'down-link' frequency. The down link can be received by any earth station that falls within the radiated signal.

The signal from the earth station is sent at a different frequency from the one used to re-transmit the signal from the satellite. This prevents the up and down signals from interfering with each other.

Today's satellites are normally in a geostationary orbit. The satellites are positioned 22,300 miles above the earth in a plane perpendicular to the equator. Since the satellite's motion is fixed relative to the earth, the earth station's antennae can remain in a relatively fixed position. This obviously only applies to land based earth stations, those attached to vessels will have to track the satellite as the vessel moves.

Communication using satellites provide several attractive features:

• Each satellite has a large transmission capacity. Since the satellites are operating in the broad band-width range of the Giga Hertz level, a satellite can support several voice grade channels.

• Communication satellites have the capability of providing a broad range of coverage. Some satellites can cover an area the size of the United States. This is an attractive feature for organisations having widely dispersed personnel and offshoots, as the cost of transmitting signals is independent of the distances between the earth sites. It is immaterial if the two earth sites are 5 or 1,000 miles apart, furthermore stations in between can also pick up the same signal.

• When used as the transmission media for a communications network, the broadcast capability of satellites can result in reduced costs compared with land based network. Since the earth stations communicating with satellite are sending and receiving on the same two channels, they need only to listen to the down link frequency to determine whether the transmission is destined for them. If it is not, they simply ignore the signal. If it is their data, they copy the signal and present it to the end user.

Unfortunately, satellite communications are not without some problems, the most obvious of these are:-

• Poor weather conditions can interfere with the signal as it travels through the atmosphere, heavy rainstorms being the worst culprit.

• Since the signals to and from the satellites cover such large distances a delay occurs in the reception of the signal at the earth station. This may cause problems with line protocols and response times.

• Periodically, the sun, satellite and earth station are directly aligned. The sun's rays travel directly into the earth stations antennae creating excessive thermal noise in relation to the received signal.

• Finally, a finite number of frequencies exist which satellites can use, and a finite number of satellites can be placed in orbit. This frequency range and orbit space has not been a hindrance in the past. It is becoming a problem which will require increased co-operation by the many nations using satellite technology.

The INMARSAT System

In 1979, the International Maritime Satellite Organisation (INMARSAT) was founded to foster standards and operational facilities for communications at sea. It became operational in 1982 and now provides continuous communications services to vessels worldwide.

A vessel's communications process is controlled by an onboard 'earth' station (obviously a misnomer in this case). The vessel's antenna dish is quite small (usually less than 1 metre in diameter) and is mounted high on the super-structure of the ship, so avoiding interference with the signal. A stabilisation system is required to keep the dish pointing directly at the satellite.

The structure of the INMARSAT service is shown on the left. The maritime satellite circuit is the satellite channel between the vessel's 'earth' station and the land based earth station. The maritime terrestrial circuit is the circuit connecting the coastal earth station to the maritime satellite data switching exchange (MSDSE). This exchange provides the internetworking between the satellite system and the public data network. It handles routing and call controls for ships at sea. It is also responsible for managing the charging of calls. The maritime local circuit is between the vessel's computer network and the vessel's 'earth' station.

The NORSAT System

On newer vessels, a second satellite communication system may be installed in parallel with the INMARSAT system. In this system a portion of a satellite's bandwidth is leased by the company. This bandwidth is converted into a number of communication 'lines' (both data and voice) that are routed from the onshore earth station to the local office. This system proves to be more convenient and cheaper than using the INMARSAT for lengthy calls.

In this section we will consider a very basic system for communicating voice and data between a vessel and office via satellite. This example is based more on the NORSAT system more than the INMARSAT system as it consists of fewer components. This subject will be considered in greater detail later on, at present we ask only ask that you appreciate the basic layout and equipment required to transmit and receive data.

Multiplexers

Multiplexer is a device which allows multiple data lines to share one communications channel. For example, we have 4 data lines and 4 telephone lines sharing the one satellite line. So the use of multiplexers substantially reduces the number of communication channels required.

Routers

A Router is defined as a device that is used to connect two or more logically separate sub-networks. It takes data from logically separate sub-network elements and forwards it to the correct sub-network element.

When data is being sent from one sub-network data channel to another at the same location, the data is sent to the Router which dispatches it to the correct receiving sub-network communication channel.

Communication Channels

As discussed earlier, the communication channel (or Transmission Media) is the physical interconnection between the differing elements on the network. This may be twisted pair wires, coaxial cable etc.

The Private Branch Exchange (PBX)

The PBX is the telephone switching network. It is used in a similar manner to a router, in that it takes voice data and forwards it to the correct receiving party. In more recent systems, the PBX can also be used for switching computer data as well as voice data. In this way it may be used as a powerful tool at the heart of a PC network.

Satellite Receiving and Transmitting Equipment

The equipment required to send and receive data over a satellite link usually consists of:

1. An interfacing unit which connects the user lines to the receiving and transmitting equipment.

2. A Communications controller which consolidates the functions of timing, switching and processing of data.

3. A modem which converts and sends out the signals to the antenna

4. Transmit / receive antenna, which send and receive the signals to and from the satellite.

Tasks & Exercises:-

1. Networking can be defined as?

2. What are the 3 basic elements required to make up a network?

3. Describe the purpose of rules in computer networking?

4. In which frequency band does a satellite communication take place?

5. What does it mean when a satellite is described as being in 'Geostationary' orbit?

6. Describe 4 factors that disrupt or delay communications to and from a satellite.

7. Have your Chief or a Shift Leader show you the major components that make up the satellite communications system(s) that are found onboard your vessel.