GB2203545A

GB2203545A - A method of operating a seismic air gun array

Info

Publication number: GB2203545A
Application number: GB08708992A
Authority: GB
Inventors: Mo Hussain Safar
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-04-15
Filing date: 1987-04-15
Publication date: 1988-10-19
Also published as: GB8708992D0

Abstract

A compact airgun sub-array is formed by a number of airguns having different or the same chamber sizes. The airguns are spaced so that there is sufficient interaction between the airguns when fired simultaneously. The presence of interaction between the airguns makes it possible to generate a desired pressure pulse using fewer airguns. The characteristic of the pressure pulse is further improved by firing the airgun sub-array several times with some airguns being switched off. The period of the combined pressure pulse is different from that of the individual pressure pulses and the combined bubble pulse is more strongly damped.

Description

ME FOR FOR DMPROVDNG THE OPERATIN OF A SEISMIC SCURCE The invention relates to a method of operating a marine seismic source.

The marine seismic source is an essential part of the seismic reflection method which has been proved to be a very successful hydrocarbon exploration tool. The most popular marine seismic source is the airgun.

Figures 1 and 2 show cross sections of the conventional airgun and sleeve airgun respectively. The airgun consists essentially of a shuttle and a gun chamber in which a compressed air is stored. When the gun shuttle is released, the stored high pressure air escapes through the gun ports into the surrounding water forming a high pressure air bubble.

Immediately after the airgun ports start to open the airgun commences to radiate an acoustic pressure pulse which is shown in Figure 3. The initial phase of the radiated pressure, which lasts from the moment the ports begin to open until the air bubble formed by the expanding air reaches its first maximum volume, we call the initial pulse. When the air bubble formed by the expanding air reaches its maximum volume, the pressure inside the bubble is much less than the hydrostatic pressure just outside the bubble and all the energy is stored as potential energy in the acoustic radiation mass. At this moment the bubble starts to collapse and the bubble wall begins to accelerate towards the centre of the bubble. During the collapse stage the air inside the bubble is compressed until the bubble reaches a minimum volume and the velocity of the bubble wall is zero.This means that when the buble is compressed, the energy is stored as potential energy in the air. The process of the bubble expansion and compression, i.e. the oscillation of the energy between the radiation mass and the air compliance, goes on until all this initial energy is dissipated. Part of this energy is radiated into the far field giving rise to what is widely known as the bubble pulse, and the remaining part is dissipated as heat.

A disadvantage of the airgun as a seismic source is its fairly long pressure bubble pulse. This means that most of the energy radiated by the airgun is concentrated in a narrow band. To overcome this disadvantage a technique known as a tuned airgun array system has been previously proposed.

This technique involves the use of a linear array of airguns of different sizes which are fired simultaneously. The duration of the pressure bubble pulse is reduced by destructive interference between the individual pressure bubble pulses, while maintaining constructive interference between the initial pulses.

It is an object of the present invention to improve the tuned airgun array system by exploiting interaction between the airguns forming the array and by firing a single sub-array several times with a number of airguns being switched off.

A method has now been invented which uses the effect of interaction between air bubbles, in which the damping and the periods of the individual pressure bubble pulses are modified so that an improvement of the tuned airgun array system is achieved. Further improvement of the turned airgun array system is achived by summing a number of traces obtained when the airgun sub-array is fired several times with some airguns being switched off.

According to the present invention a method for improving the operation of a seismic source at a location submerged underwater involving the use of airguns comprises:- Operating at least two airguns at a chamber presure in the range 20 to 150 bar, the guns being of either substantially equal chamber size or different chamber size and arranged so that there is an interaction as hereinafter defined between the air guns, which interaction has the effect that the period of the pressure pulse produced by the guns is different from the period of the individual pulses in the absence of interaction and has the effect that the damping of the pressure bubble pulse is increased as compared with the individual pressure bubble pulse.

The term "period" is defined as the duration between consecutive peaks of the pressure pulse and the term interaction refers to the effect obtained with a plurality of guns when the effective pressure acting on the surface of the air bubbles proeduced by the guns is different from that with only one gun.

Preferably the reflected pressure pulses are detected and recorded.

Preferably the guns are all fired simultaneously or substantially so.

Preferably the guns all have either substantially the same chamber size or different chamber size are operated at substantially the same chamber pressure.

Accordfflng to another aspect of the present invention there is provided a system for use as a seismic source comprising at least two air guns of either substtIaIly equal chamber size or differnt chamber size arranged so that when fired at an underwater location interaction between the guns is effected which interaction has the effect that the period of the pressure pulse produced by the guns is different from the period of the individual pulses in the absence of interaction and has the effect that the damping of the pressure bubble pulse is increased as compared with the individual pressure bubble pulses.

Preferably the airguns are arranged so that the minimum distance between any pair of guns is equal to the sum of the equilibrium radii of the air bubbles produced by the guns and the maximum distance between any pair of air guns is less than 1/10 of the dominant wavelength The term "equilibrium radius" is defined as the radius of the bubble when the pressure inside the bubble is equal to the hydrostatic pressure immediately outside the bubble.

Preferably the guns are located at substantially the same depth, although the actual depth at which they are located is not critical.

Generally, the guns are from 1 to 10m below the water surface. The deeper the guns the greater will be the frequency of the pressure bubble (assuming other variables are constant).

Preferably the guns are operated at a chamber pressure of from 40 to 13 bar. The chamber size of the guns can be from 10 to 540 cu ins.

The number of guns emplyed in the array can be from 2 to 7 or more. A convnient number can be from 2 to 5.

Preferably the pressure waves are detectedf by hydrophone for example at 5 to 15m belay the water surface.

the hydrophone can be operated in a known manner for example in the form of a streamer towed from a ship. In a typical instance a streamer may be about 2 miles in length and consist of 48 to 240 stations, each station consists cf 20 hydrophones, all the hydrophones being 5 to 15m below the water surface. The operation of hydrophones in this manner is well known to thcse spilled in the art.

rhe method of the present invention can be employed in a geophysical survey of subterranean strata or in a well velocity survey.

The invention is illustrated by the following example. The airgun is a known article and forms no part of the present invention EXAMPLE Figure 3 shows an airgun sub-array which consists of four conventional airguns with volumes of 10, 20, 30 and 40 cu.in. The airguns were arranged so that there was sufficient interaction between the airguns for the purpose of increasing the damping of the pressure bubble pulses radiated by the airguns.

Figure 5 shows the amplitude specturum of the far field pressure signature measured at a distance of lOOm from the centre of the airgun subarray when all the airguns were fired simultaneously at a depth of 2m.

One major shortcoming of the far field pressure signature radiated by the sub-array shown in Figure 4 is the presence of a deep notch which can be seen in the low frequency end of the amplitude specturm shown in Figure 5.

In order to overcome this shortcoming the sub-array will be fired sequentially three times once with 10, 20, 30 and 40 cu.in airguns and then with 10, 20 and 30 cu.in airguns and finally with 10 and 20 cu.in airguns.

The effective far field pressure signature will be obtained by summing the three far field pressure signatures obtained as outlined above.

Claims

What we claim is:

1. A system for use as a seismic source comprising at least two airguns of substantially equal chamber size or different chamber size, arranged so that when fired at an underwater location interaction has the effect that the period of the pressure pulse produced by the guns is different from the period of the individual pulses in the absence of interaction and has the effect that the damping of the pressure bubble pulse is increased as compared with the individual pressure bubble pulses.

2. A system as claimed in claim 1 wherein the airguns are arranged so that the minimum distance between any pair of guns is equal to the sum of the equilibrium radii of the air bubble produced by the guns and the maximum distance between any pair of airguns is less than 1/10 of the dominant wavelength.

3. A method of operating a seismic source at an underwater location involving the use of airguns comprising operating at least two airguns at a chamber pressure in the range 20 to 150 bar, the guns being of substantially equal chamber size or different chamber size and arranged so that there is an interaction as hereinafter defined between the airguns, which interaction has the effect that the period of the pressure pulse produced by the guns is different from the period of the individual pulses in the absence of interaction and has the effect that the damping of the pressure bubble pulse is increased as cocoarod with the individual pressure bubble pulses.

4. A method of operating a seismic source as claimed in claim 3 wherein the guns are fired substantially simultaneously several times and the guns are all operated at substantially the same chamber pressure.

5. A method of operating a seismic source as claimed in either claim 3 or claim 4 wherein the guns are all located at substantially the same depth below the water surface.

6. A method of operating a seismic source as claimed in any me of claims 3 to 5 wherein the guns are operated at a chamber pressure of 40 to 138 bar.

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