GB2204129A

GB2204129A - Method for improving the performance of a water gun seismic source array including guns of different volumes

Info

Publication number: GB2204129A
Application number: GB08710210A
Authority: GB
Inventors: Dr Mo Hussain Safar
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-04-29
Filing date: 1987-04-29
Publication date: 1988-11-02
Also published as: GB8710210D0

Abstract

The performance of the far field pressure signature radiated by the water gun is improved by implementing either one of the following methods. The first method involves firing a water gun sub-array, which consists of a single P400 water gun and three S80 water guns. The second method involves firing a water gun sub-array, which consists of two water guns, a P400 and an S80, sequentially three times once with P400 and S80 water guns simultaneously and then twice with S80 water gun only. The effective far field pressure signature radiated by the second sub-array is obtained by summing the three far field pressure signatures obtained as outlined above. A P400 water gun has a firing chamber of 400 cu in and two ports and an S80 water gun has a chamber of 80 cu in and four ports.

Description

MEH10DS FOR IMPROVING T9E PESFOEFANCE OF THE WRITER GUN SEISMIC SOURCE This invention relates to methods for improving the performance of the water gun seismic source.

The water gun, which was developed by Sodera (Societe pour le Development de la Researche Appliquee), has been used as marine seismic source since 1977.

The first version of the water gun is the S80. In 1983 Sodera introduced the P400 hydropneumatic water gun for the purpose of improving the level of the signal radiated by the water gun in the band 10-20 Hz.

The machanical construction of the P400 and S80 water guns is essentially the same except that the P400 water gun has a firing chamber volume of 400 cu.in. which is five times that of the S80 water gun and the P400 water gun has two ports instead of four as in the case of the S80 water gun.

Figure 1 shows a schematic diagram of the S80 water gun at the beginning of the firing cycle. The water gun consists of an upper chamber referred to as a firing chamber which is filled with compressed air, a lower chamber is sealed by the lower piston of the shuttle which is held in this position partly by the force produced by the hydrostatic pressure on the lower face of the lower piston and partly by the difference between the force acting on the lower face of the upper piston and that acting on part of the upper face of the lower piston When the solenoid valve is energized, the shuttle is forced to move downwards allowing the full pressure in the firing chamber to be applied at the rear face of the shuttle lower piston.This makes the shuttle move downwards at a very high speed resulting in the expulsion of the water from the lower chamber through the gun ports. The flow of the water through the gun ports at a very high speed generates a member of empty cavities.

The empty cavities reach a maximum volume when the kinetic energy of the displaced water is partly radiated as acoustic energy and partly stored by the cavities radiation mass as potential energy. This phase of cavity's motion will be referred to as the 'growth phase'. Because the hydrostatic pressure just outside the cavities is considerably larger than the pressure inside the cavities, the cavities begin to collapse and consequently the potential energy stored by the cavities radiation mass is transformed intc acoustic energy, heat and potential energy stored by the air inside the cavities. This phase of the cavity's motion will be referred to as the 'collapse phase'.

The difference between near-and far-field pressure signatures radiated by a single water gun can be explained by considering Figure 2 which shows a water gun placed at a depth D below the sea surface. When the distance r between the observation point Q and the water gun is considerably larger than the water gun depth D, so that the amplitudes of the direct and surface-reflected pressure pulses are approximately the same, then the pressure pulse measured at the observation point Q is referred to as the far-field pressure signature of the water gun.However, when the observation point Q is near the water gun, so that the amplitude of the direct pressure pulse is considerably larger than that of the surface reflected pressure pulse, then the measured pressure pulse at the observation point is referred to as the near-field pressure signature of the water gun In other words, the water gun near-field pressure signature is the direct pressure pulse radiated by the water gun.

Figure 3 shows a near-field pressure signature radiated by the S80 water gun at a depth of 6m and with chamber pressure of 2000 psi. It can be from Figure 3 that during the cavities growth phase and pressure radiated by the water gun increases from zero to a peak value of P1 then begins to decrease and reaches a negative peak value of Pns The negative peak value of occurs when the cavities reach a maximum volume. During the cavities collapse phase the radiated pressure increases and reaches a peak value of P2. The presence of the air due to the rectified diffusion into the cavities causes the collapsed cavities to rebound, resulting in radiation of decaying pressure oscillation following the pressure peak P2.

Figures 4 and 5 show the calibrated amplitude spectra and the energy per unit area spectral density radiated at an angle of 00 from the vertical by a single P400 water gun and a single S80 water gun fired simultaneously at a depth of 5m.

The amplitude spectrum values shown in figures 4 and 5 are expressed in dB relative to Pa/Hz at Im whereas the energy per unit area spectral density values are expressed in dB relative to J/m2/Hz at Im.

It can be seen from Figure 4 that the single P400 water gun amplitude spectrum has two peak values of 6dB and 7dB at 29Hz and 53Hz. Between these two peak values the amplitude spectrum has a notch at 39Hz with a value of ldB. It can also be seen from Figure 4 that the amplitude spectrum after reaching the second peak value begins to decrease at a rate of 12dB/oct.

The second notch at about 150Hz is caused by the destructive interference between the direct pressure pulse radiated by the water gun and its surface reflection.

It becomes clear when comparing Figures 4 and 5 that the amplitude spectrum of the far-field pressure signature radiated by a single S80 water gun is similar to that of the far-field pressure signature radiated by a single P400 water gun except that the values of the peaks and notch occur at higher frequencies. In other words, the amplitude spectrum of a single P400 farfield pressure signature is shifted towards the low frequency end relative to that of the far-field pressure signature radiated by a single S80 water gun.

One major shortcoming of the pressure signature radiated by the P400 water gun is the presence of the notch which occurs in its amplitude spectrum at a frequency equal to 39Hz. To overcome this shortcoming Racal proposed the use of a two element water gun sub-array consisting of a P400 water gun and an S80 water gun. Figure 6 shows the amplitude spectrum of the far field pressure signature radiated by Racal two element water gun sub-array. it is clear from Figure 6 that the notch has not been completely eliminated.

It is an object of the present invention to improve the amplitude spectrum of the far field pressure signature radiated by the P400 water gun by eliminating ocopletely the low frequency notch in its amplitude spectrum.

According to the present invention two methods for improving the performance of the P400 water gun involving the use of water gun sub-arrays are proposed.

In method 1 a water gun sub-array consists of a single P400 water gun and at least three S80 water guns. In method 2 a water gun sub-array consists of two water guns a P400 water gun and an S80 water gun. In method 2 the subarray is fired sequentially three times once with P400 and S80 water guns simultaneously and then twice with S80 water gun only.

The effective far field pressure signature generated by method 2 is obtained by summing the three far field pressure signatures obtained when firing the second sub-array three times.

Preferably all the water guns in method 1 are fired simultaneously or substantially so.

Preferably the water guns in method 1 and method 2 are arranged so that there is a negligible interaction between the water guns.

Preferably the water guns in method 1 and method 2 are located at substantially the same depth.

Preferably the water guns in mehtod 1 and method 2 are operated at a chamber pressure of 2000 psi (137 bar).

The invention is illustrated by the following example. The water gun is a knawn article and forms no part of the present invention.

EIMPLE Figure 7 shows the arrangement of the water guns forming a short sub-array.

The water guns are spaced so that there is negligible interaction between the water guns. Interaction between the water guns is said to be negligible if the far field pressure signature radiated by the sub-array when firing all the water guns simultaneously is the same as that obtained by firing the water guns separately.

Figure 8 shows the amplitude spectra of the far-field pressure signatures radiated by Racal water gun sub-array and by the new proposal sub-arrays consisting of a single P400 water gun and three, and four S80 water guns.

It is clear from Figure 8 that unlike the Racal water gun sub-array, the new proposed water gun sub-arrays are capable of eliminating completely the notch of the P400 water gun amplitude spectrum.

Claims

What we claim:

1. A system for use as a seismic source comprising at least three'S80 water guns and a single P400 water gun arranged so that when all the water guns are fired there will be negligible interaction between the water guns.

2. A method of operating a seismic source as claimed in claim 1 wherein the water guns are fired substantially simultaneously and the water gun are all operated at substantially the same chamber pressure.

3. A method of operating a seismic source as claimed in either claim 1 or claim 2 wherein the water guns are all located at substantially the same depth below water surface.

4. A system for use as a seismic source comprising a single P400 water gun and an S80 water gun arranged so that there is negligible interaction between the two water guns and fired once with both water guns and then twice with S80 water gun only.

5. A method of operating a seismic source as claimed in claim 4 wherein the two water guns are operated at substantially the same chamber pressure.

6. A method of operating a seismic source as claimed either in claim 4 or claim 5 wherein the two water guns are located at substantially the same depth below the water surface.

7. A method of operating a seismic source substantially as hereinbefore described with reference to the Example.