Applied Seismology: Introduction and Principles
4.5 Seismic energy sources
4.5.4 Non-explosive sources
All the non-explosive marine sources (cf. Table 4.4) give rise to varying degrees of a bubble pulse. When the source is discharged, cavitation occurs when the surrounding water is displaced by the formation of a gas bubble, usually of air. Hydrostatic pressure causes the bubbles to collapse, so pressurising the gas which heats up and expands to reform smaller bubbles, which too, in turn, collapse and reform (Figure 4.19A), until such time that the hydrostatic pressure is too great and the remaining gas dissolves in the water or
Bubble
Bubble collapses
RADIUS A1
A2
A3
T1 T2 T3
Duration depends on energy in bubble Depends on initial conditions
Time after shot P0
P1
P2
P3
T1 T2 T3 T4
100ms
Time (A)
(B)
Figure 4.19 (A) Archetypal marine source showing the response of the bubble from the point of forming at the firing of the source through to venting at the water surface. The respective bubble pulse periods (T) and amplitudes (P) are indicated as a function of time. Courtesy of BP. (B) The corresponding bubble pulse train.
finally vents to the surface. The acoustic noise from the successive collapses of the bubbles produces noise which is undesirable (Figure 4.19B) and which can either be removed by later processing or is minimised using a tuned array of sources. Airguns (see next section) with different volume capacities are fired simultaneously, but the respective bubble pulses occur at different times and with varying amplitudes. By summing all the individual signatures of an array, a much more acceptable overall source signature can be produced (Figure 4.20). Bubble pulses can also be reduced by using a waveshape kit within each airgun, for example. Air is bled under pressure into the expanding bubble, thereby reducing the rate at
which it collapses, with a consequent reduction in the overall length and severity of the bubble train. The effectiveness of a marine source can be measured in terms of the ratio of the amplitudes of the primary to first bubble signal. A summary of marine airgun arrays has been provided by Amundsen and Landrø (2010a,b).
4.5.4.1 Airguns and sleeve guns
An airgun is used to inject a bubble of highly compressed air into the surrounding water and is the most commonly used of all seismic sources (Figure 4.21). Airguns, although usually deployed in marine
100 Gun volume (inches3) 40
90
140 50
200 40
30 140 60
70
70 140
100 200 300
Time (ms)
400 500
Figure 4.20 Individual airgun signatures and the simulated array signature produced by summing them. The asterisk indicates a malfunction. Courtesy of BP.
surveys, can also be used in modified form in marshlands, in a land airgun, and as a borehole source. Compressed air is fed through a control chamber (Figure 4.22A) into a lower main chamber and in so doing a shuttle is depressed, closing off the external ports. Opening a solenoid valve (Figure 4.22B) releases the sealing pressure in the upper control chamber and the shuttle moves rapidly upwards, so releasing the high-pressure air in the lower chamber to vent explosively through the ports (Figure 4.22C and D). The shape of
High pressure air
Upper chamber
Shuttle
Lower chamber
Port
Position of shuttle when open Solenoid
valve
Figure 4.21 Schematic cross-section through an airgun.
High-pressure air flows continuously into the upper chamber and through the shuttle into the lower chamber. Opening the solenoid valve puts high-pressure air under the upper shuttle seat, causing the shuttle to move upward, opening the lower chamber and allowing its air to discharge through the ports to form a bubble of high-pressure air in the surrounding water. The size of an airgun is indicated by the volume in cubic inches of its lower chamber (1991). Courtesy of Bolt Associates.
the source pulse is dependent upon the volume of air discharged, its pressure, and the depth at which the gun is discharged. It has been found (Langhammer and Landrø, 1993) that airgun signatures are also affected by the temperature of the surrounding water. The primary-to-bubble ratio and the bubble time period both increase with increasing water temperature.
Airguns range in volume capacity (lower chamber) from a few cubic inches to around 2000 cubic inches (0.033 m3), and pressures from 2000 to 4000 pounds per square inch (1400–2800 Mg/m2).
Older airguns had 4 or 6 cylindrical holes through which air was expelled. More recently, sleeve guns have been developed which allow the air to escape through an annulus as a doughnut-shaped bubble (Figure 4.23) and the effect is to reduce the bubble pulse.
When the gun is fired, instead of an internal shuttle moving, an external sleeve moves.
4.5.4.2 Water guns
Instead of expelling compressed air into the surrounding water, which often has had deleterious effects on local fish stocks, air is used to force a piston forward which discharges a slug of water.
When the piston stops, cavitation occurs behind the expelled water, resulting in an implosion which creates the seismic pulse. The firing sequence is outlined in Figure 4.24. The major advantage of a water gun is that no real bubble pulse is produced.
(A) CHARGED
(B) FIRED
(C) EXHAUSTING
(D) EXHAUSTED SOLENOID VALVE
AIR INTAKE
SOLENOID VALVE
RING CHAMBER EXHAUST PORTS
FIRING CHAMBER
AIR INTAKE
FIRING CHAMBER FIRING PASSAGE
SOLENOID VALVE
AIR INTAKE
SPRING CHAMBER CHAMBER SOLENOID
VALVE
AIR INTAKE
FILL PASSAGE
CHAMBER FILL ORIFICE
Figure 4.22 The operation of a sleeve gun is very similar to that of an airgun. Instead of a shuttle moving to release the
compressed air, an outer sleeve is used instead. (A) The sleeve gun charged ready to fire; (B) fired; (C) the lower chamber discharges its compressed air into the surrounding water, until (D) the lower chamber is exhausted of air.
4.5.4.3 Gas guns/sleeve exploders
A mixture of butane or propane with oxygen or air is exploded under water using a gas gun. Alternatively, the mixture is detonated inside a tough rubber bag that expands to contain the explosion, and then collapses creating an implosion which is the main source pulse. The exhaust gases are vented from the sleeve, up hosing to the surface, so that there is no bubble pulse. A major trade name is AquapulseTM(Esso Production Research) and the system is used under licence by several seismic contractors.
4.5.4.4 Steam gun and Starjet
Compagnie G´en´erale de G´eophysique (CGG) has developed a steam gun under the name VaporchocTM. Steam generated on board ship is fed into the water through a remotely controlled valve (Figure 4.25A). This releases a bubble of steam, causing a small amount
(A)
(B)
Figure 4.23 The exhaust bubbles from (A) a conventional airgun, and (B) a sleeve gun. Courtesy of Texas Instruments.
of acoustic noise which precedes the main seismic pulse. This pre- emptive noise is known as the source precursor. On closing the valve, steam in the bubble condenses and the internal gas pressure drops to less than hydrostatic pressure, so causing the bubble to implode, radiating acoustic energy with negligible bubble pulse.
However, to overcome the problems over the precursor signal, CGG has developed the Vaporchoc principle in a system known as Starjet, which uses four systems, each of which has four tunable guns.
By varying the timing of the discharge of each of these systems, the precursors can be manipulated to become self-cancelling. The overall source pulse shape is far cleaner with minimum ringing, as can be seen in Figure 4.25B.