Most Offshore seismic exploration does not have the logistical problems associated with Onshore exploration, but the
operational difficulties more than compensate for this!
All of the necessary shooting and recording equipment must be deployed from the rear of the vessel, and towed continuously.
Of necessity, offshore seismic exploration vessels have become larger over the years to cope with the increasing workload, and are expected to operate in all but the very worst weather conditions.
In many areas of the world 24-hour operations are conducted, requiring sufficient personnel to operate both the vessel and the seismic equipment round-the-clock. Multiple seismic sources and receiver cables are used to provide the high densities of data now required.
Even in the simplest 2D work, such a vessel can easily obtain 10 times the amount of data as the best (Vibroseis) land crew in any given time.
Although explosives were once used as an energy source for offshore exploration, the environmental repercussions, and the need for rapid firing and repeatability have brought about the design and construction of new sources.
The most common offshore (or Marine) source in use today is a variety of Air-Gun, first produced in the 1960's. These guns use compressed air (at typically 2,000 to 5,000 psi) to produce an explosive blast of air into the water surrounding the gun. The latest of these, with the movable shuttle that releases the air on the outside, is the Sleeve-Gun.
Air enters through the pipe (A) and is fed into the main chamber (D) and the air-spring return chamber (C).
44
Once the Solenoid valve (B) opens, air is allowed into the firing chamber (E), and the pressure differential forces the outside sleeve to the left with great force, releasing the air from the main chamber. The resultant air-bubble produces a shock-wave in the
surrounding water.
A single air-gun produces a pulse of energy (or signature) that looks something like this (the upper plot shows the time-function, and the lower shows the frequency content of the signature):-
Hardly a perfect energy source!
Although the initial energy burst is reasonable, a complex pressure
interaction between the air "bubble" and the water causes the bubble to oscillate as it floats towards the surface - this
produces the extraneous bursts of energy following the initial burst.
The amplitude and period (time
difference) between these bubble pulses depend on the depth of the gun and the size of the main chamber in the gun.
We can't do much about the depth (see Ghosts in the next Chapter), but, if we build an array of guns, made of different chamber sizes, and fire these simultaneously, we gain several advantages.
1. We obviously increase the total amount of energy being directed into the ground for one "shot".
2. The different chamber sizes will produce different bubble responses, and these will tend to cancel out.
3. We improve the directivity of the source. Other than directly below the source array, some frequencies will be attenuated by the spatial design of the array.
45
Once again, here's a plot showing the time and frequency response of the entire array.
When the entire array is fired, the bubbles "cancel-out" (more or less), and the frequency content is much flatter over the range of typical seismic frequencies.
This is now close to an ideal source, and is very repeatable.
It has to also be very reliable as shots are normally fired roughly every 5-10 seconds - possibly up to 10,000 shots in 24 hours!
We have mentioned before the concept that an array or group of shots or geophones can improve the spatial response of our source and/or receivers.
This plot shows the hemisphere of energy emanating from the source array shown above (viewed from below).
The grey line shows the direction of the seismic line (the direction the boat is moving in) with the arrow showing the vertical output from the array. The colours show the total energy, red being the highest.
Change the frequency of interest and see how, although the downward energy is always maximum, the energy at other angles is attenuated by the array design.
46
Although the actual "shooting" of Marine data is made simple by modern Air-gun arrays, there are still some operational difficulties associated with these sources. The high
operating pressures are very dangerous. Air at 80-100 psi is used in industry, with an appropriate abrasive, to remove paint from metal. Air at 2000-5000 psi will remove almost anything (including skin) without any abrasive.
The guns must be properly maintained - any gun failure will damage the desired array output response and re-introduce bubbles into the signature. Deployment of the arrays is made relatively simple by the use of floats etc., but the position of each array (multiple arrays may be used for alternate shooting etc.) must be carefully monitored in all three dimensions. We'll discuss the positioning of arrays in more detail when we discuss shooting geometry.
Again, just like on land, many different types of seismic source have been used in the marine environment. However, as almost all modern data is acquired with variations of air-guns, we will restrict our discussion to those and move on to the recording instruments.
47