WO1998048301A1

WO1998048301A1 - A new efficient method of operating air-gun arrays

Info

Publication number: WO1998048301A1
Application number: PCT/GB1997/001097
Authority: WO
Inventors: Mo Safar
Original assignee: Mo Safar
Priority date: 1997-04-21
Filing date: 1997-04-21
Publication date: 1998-10-29
Also published as: AU2395097A

Abstract

A marine seismic source is formed by a number of sets of subarrays deployed either in line or cross line with each set of subarrays consisting of a number of air-guns having the same chamber sizes. The subarrays and the air-guns within a sub-array of at least of one set of subarrays are spaced so that the air-bubbles released by the air-guns when fired simultaneously do not coalesce but the effect of interaction between the air-bubbles on their resistive loading is significant. All the air-guns forming the subarrays are placed at a depth close to 1/8 the wavelength of the fundamental resonance frequency of the air-bubbles released by the air-guns forming one set of subarrays.

Description

A New Efficient Method of Operating Air-Gun Arrays

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 air-gun. Figures 1 and 2 show cross sections of the conventional air-gun and sleeve air-gun respectively. The air-gun 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 air-gun ports start to open the air-gun commences to radiate an acoustic pressure pulse which is shown in Figure 3. The initial phase of the radiated pressure pulse, 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 me 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 bubble is compressed, the energy is stored as potential energy in the air. The process of the bubble expansion and compression, ie. 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 air-gun as a seismic source is its fairly long pressure bubble pulse. This means that most of the acoustic energy radiated by the air-gun as can be seen in Figure 3 is concentrated in a narrow frequency band centred around a fundamental resonance frequency and the remaining part of the radiated acoustic energy is concentrated in a number of narrow frequency bands centred around the harmonics of the fundamental resonance frequency. To overcome this disadvantage a technique known as a tuned air gun array system was developed and it is currently being used in the oil and gas exploration industry. This technique, which involves the use of a linear array of air-guns of different sizes fired simultaneously, is capable of generating a seismic pulse with the characteristic that the acoustic energy is radiated uniformly over a wide frequency band. In the early 1970s when dealing with the radiation of acoustic waves from an air-gun, I recognised that because of the way the air-gun was being operated, its efficiency in generating acoustic energy in the frequency band 10-40Hz was extremely poor. This led me to develop a number of methods (Safar, 1976b. 1980, 1983) for the purpose of improving the air-gun poor efficiency. One technique (Safar, 1976b) which is based on the exploitation of interaction between the air-bubbles released by the air-guns, involves the use of a compact air-gun array consisting of a number of properly spaced identical air-guns. This technique was successfully tested (Curtis & Siddaway, 1985) in the West Shetland area which is known to exhibit poor seismic data This technique, which was proved to be well suited to the West Shetland area, has been exploited by Geco-Prakla in their standard arrays

Recently, I discovered that my works (Safar, 1980, 1983) inspired Itremer's Earth Sciences Group to develop the 'Single Bubble' pulse generating method (Avedik et al 1993) which has recently been evaluated and its performance has been compared with that of a large tuned air-gun array in the Central Mediterranean Sea when carrying out deep seismic profile However, it was found that the 'Single Bubble' pulse technique has one major shortcoming, namely the bandwidth of the radiated acoustic pulse was rather narrow. This is because the acoustic energy centred around the second harmonic frequency is radiated off the vertical axis.

The reason why the air-gun arrays, currently operated in the oil and gas exploration industry, are extremely inefficient in generating acoustic energy in the 10-40Hz frequency band is because the air-bubbles released by the air-guns forming the array together with the sea surface produce arrays of dipole sources. It is well known that a dipole source, which is formed by two monopole sources pulsating with 180° out of phase and separated by a distance considerably smaller than the wavelength, is extremely inefficient in generating low frequency acoustic energy

On the other hand, the method which I proposed (Safar, 1980, 1983) for operating a single air-gun and arrays of air-guns involves operating the air-guns at a such depth, sometimes referred to as the optimum depth, that the air-bubbles released by the air-guns together with the sea surface produce arrays of monopole sources A monopole source is considerably more efficient than a dipole source in generating low frequency acoustic energy

It is an object of the present invention to improve the efficiency of the tuned air-gun array system by exploiting interaction between the air-bubbles released by the air-guns.

A method has now been invented which uses the effect of interaction between air-bubbles in which the acoustic radiation impedances of the individual air-bubbles are modified so that an improvement in the efficiency of the tuned air-gun array system is achieved. According to the invention there is provided a method of operating efficiently a marine seismic source involving the use of air-guns comprising the steps of: a) placing at depth d at least two sets of air-gun subarrays, deployed either in line or cross line, each set of air-gun subarrays consists of at least two air-guns having substantially equal chamber volumes, the air-guns of one set of subarrays are spaced so that to prevent coalescence of the air-bubbles released by the air-guns and to obtain the desired low frequency response. b) firing all the air-guns substantially simultaneously.

Since the invention is essentially based on the exploitation of interaction (mutual coupling) between the air-bubbles released by a number of air-guns, therefore it is appropriate to give a brief review of the physics of interaction.

It has been shown (Safar, 1976a, 1976b) that the air-bubble released by an air-gun is in fact a damped nonlinear resonant system consisting of a mass, a compliance and some damping partly caused by the radiation of acoustic energy and partly caused by non acoustical losses.

The nonlinearity of the air-bubble system is due to the fact that the compliance is not constant but is a function of the air-bubble volume displacement. Because of the nonlinearity of the air-bubble system, the air-bubble radiates acoustic energy not only at its fundamental resonance frequency but also at the harmonics of the fundamental resonance frequency f₀ which is given by f₀ =(MC)A2π (1) where

C is the compliance of the air-bubble when pulsating with small volume displacement, and

M is the acoustic radiation mass which is a measure of the mass loading of the air-bubble by the surrounding water.

The width between the 3dB points of the narrow bands centred around the fundamental resonance frequency and its harmonics is determined partly by the nonacoustical damping and partly by the acoustic radiation resistance R(ω) which is a measure of the acoustic radiation resistive loading of the air-bubble by the surrounding water. This means that one possible method of broadening these narrow frequency bands ie. smoothing the amplitude spectrum of the pressure pulse radiated by an air-gun is by increasing the acoustic radiation resistance R(ω). It will be demonstrated below that this can be achieved by a properly designed compact array of a number of identical air-guns with sufficient interaction between all the air-bubbles released by the air-guns. For the case of an air-bubble released by a single air-gun operating far away from any boundary and in the absence of other air-guns, the effective pressure acting on the air- bubble is mainly due to that exerted by the surrounding water. However, if the single air- gun is operating in the presence of other air-guns, then the effective pressure acting on each air-bubble will consist of two parts: one part is exerted by the surrounding water and the other part is exerted by the other air-bubbles and therefore mutual interaction is said to occur between all the air-bubbles. The presence of interaction between the air-bubbles will result in modifying the acoustic radiation mass M and resistance R(ω) of each air- bubble.

It can be shown (Safar, 1976a, 1976b) that for the case of a compact air-gun array consisting of a number of identical interacting air-guns, the acoustic radiation mass M', and resistance R';(ω) of the ith air-gun are given by:

R'i(ω) = R,(ω) ( 1 + Σ sinkD_g/ D„) (3) where a₀₁ is the equilibrium radius of the ith air bubble

D₀ is the distance between the air-bubbles released by the ith and jth air-guns

M, and R,(ω) are the acoustic radiation mass and resistance of the ith air-bubble when the ith air gun is operating alone k is the wavenumber ω/c₀

c₀ is the velocity of sound in water, and ω is the angular frequency

It can be deduced from eqns. (1) and (2) that because of the presence of interaction, the fundamental resonance frequency will be reduced. This means that interaction can be exploited in shaping the low frequency end of the amplitude spectrum of the pressure pulse radiated by an air-gun. Since the reciprocal of the fundamental resonance frequency is equal to the period of the pressure pulse radiated by an air-gun, it follows therefore that the presence of interaction results in increasing the period. Figure (4.) shows a comparison of the measured increase (Giles & Johnson, 1973) in the period of the pressure pulse with that predicted from eqns. (1.) and (2.) for the case of two 40 cu.in. air-guns. It is clear from Figure (4.) that eqns. (1.) and (2.) predict fairly accurately the effect of interaction on the radiation mass and consequently the fundamental resonance frequency of the air-bubble released by an interacting air-gun.

It can be seen from eqn. (3.) that the presence of interaction results in the increase of the acoustic radiation resistance. This means that interaction can be exploited in broadening the narrow frequency bands centred around the fundamental resonance frequency and its harmonics.

The invention will be more clearly understood from the following theoretical example of the preferred method which consists of a number of steps with reference to Figures 5 to 9.

Fig. 5. shows the arrangement of 18x54 cu.in. air-guns forming three subarrays deployed in line.

Fig. 6. shows the amplitude spectrum of the far field pressure signature radiated by

54 cu.in. air-gun placed at a depth = 10m with chamber pressure = 2000

Fig. 7. shows the amplitude spectrum of the far field pressure signature radiated by the three subarrays shown in Figure 5 when all the air-guns are fired simultaneously.

Fig. 8. shows the arrangement of two sets of subarrays deployed in line. One set of subarrays consists of 18 x 54 cu.in. air-gun and another set of subarrays consisting of 18 x 130 cu.in. air-guns. The two sets of subarrays are placed at the same depth of 10 m.

Fig. 9. shows the amplitude spectrum of the far field pressure signature radiated by the two sets of air-gun subarrays shown in Figure 8 when all the air-guns are fired simultaneously. The first step of implementing the preferred method involves the design of one set of identical subarrays consisting of a number of identical air-guns which consists of determining the following parameters.

2. Volume of the air-gun

3. Depth of the air-gun

4. Spacing of the air-guns

Let us assume that we have 18 air-guns which can be placed at a depth of 10 m and spaced so that the air-bubbles released by any two adjacent air-guns do not coalesce The volume of the air-gun is determined by assuming that the air-gun depth is equal to Vβ of the wavelength of the fundamental resonance frequency of the air-bubble released by the air- gun. Therefore the air-bubble resonance frequency f₀ ιs given by f₀ = 1500/80 = 19Hz

It can be shown (Safar, 1976a) that the volume of the air-gun operating at a depth of 10 m, chamber pressure of 2000 psi and producing an air-bubble having a fundamental resonance frequency of 19 Hz is about 54 cu.in

The reason for placing the air-gun at a depth close to Vfe wavelength of the fundamental resonance frequency of the air-bubble is to make the air-bubble operate as a monopole source at frequencies close to and above the second harmonic frequency. This will result not only in producing a maximum enhancement of the level of the second harmonic pressure component but also a slight enhancement of the level of the fundamental pressure component

The amplitude spectra of the far field pressure signatures radiated by a single 54 cu.in air-gun and a set of subarrays consisting of 18 x 54 cu.in. air-guns deployed in line are shown in Figures 6 and 7 It is clear from Figures 6 and 7 that because of interaction between the air-guns we have achieved the following

1 Increase in the amount of acoustic energy radiated at low frequencies.

2. Increase in the amount of acoustic energy radiated at high frequencies.

3. Reduction in the sharpness of the peaks of the amplitude spectrum of the far field pressure signature radiated by a single 54 cu.in. air-gun

4. Reduction in the depth of the notches of the amplitude spectrum of the far field pressure signature radiated by a single 54 cu.in air-gun To further improve the amplitude spectrum i.e. reducing the depth of the first notch in Figure 7, we deploy another set of three subarrays consisting of 18 identical air-guns with volume such that the second harmonic frequency radiated by the air-gun is close to the notch frequency 24Hz. The volume of the air-gun which produces an air-bubble with fundamental resonance frequency of 12Hz can be shown (Safar, 1976a) to be in the region of 130 cu.in. This step can be repeated if further improvement of the amplitude spectrum is desired.

Figure 9 shows the amplitude spectrum of the far field pressure signature radiated when all the air-guns forming the two sets of subarrays are fired simultaneously. It can be seen from Figure 9 that the addition of a second set of subarrays results in a considerable improvement of the amplitude spectrum, namely a significant increase in the amount of acoustic energy radiated at low & high frequencies and a reduction in the depth of the first notch of the amplitude spectrum shown in Figure 7.

One important feature of the invention is the improvement of the acoustic efficiency of the marine seismic source when designed according to the invention. This will be demonstrated below by comparing the output of a conventional tuned air-gun array currently being operated by Western Geophysical with that produced by a moderately tuned air-gun array designed by using the invention.

The Western Geophysical's tuned air-gun array consists of two tuned 1450 cu.in. air-gun subarrays deployed in line. The new air-gun array consists of three sets of air-gun subarrays deployed cross line. One set consists of four 3 x 54 cu.in. subarrays, a second set consists of three 3 x 150 cu.in. subarrays and a Λird set consists of one 3 x 300 cu.in. subarray. When implemented in the field, me new air-gun array will consist of three 970 cu.in. subarrays deployed in line.

The arrangements of the Western Geophysical's air-gun array and the new air-gun array are shown in Figures 10 and 11. It can be seen from Figures 10 and 11 that the new air- gun array has 24 air-guns with a total volume of 2910 cu.in. whereas the Western Geophysical's air-gun array has 24 air-guns with a total volume of 2900 cu.in.

Figure 12 shows a comparison of the amplitude spectra of the far field pressure signatures generated by the Western Geophysical and the new air-gun arrays.

It is clear from Figures 10, 11 and 12 that the application of the new proposed air-gun array designed technique results in not only more efficient, but also, more reliable air-gun arrays than those currently being operated by Western Geophysical.

Claims

CLAIMS:

1. A method of operating an air-gun array comprising the deployment of a number of air-gun subarrays either in line or crossline at a depth d, each air-gun subarray consists of a number of identical air-guns, the volumes V , VM, V₃ , V_n of the air-guns forming the air-gun subarrays are such that V_] < V₂ < V₃ < ... V_n, the volume V, is chosen such that the depth d is approximately V₈ of the wavelength of the fundamental resonance frequency of the air-bubbles released by the air-guns with volume V,, the volume V₂ is chosen such that the frequency of the second harmonic pressure component generated by the air-bubbles released by the air-guns with volume VM is close to the frequency of the first notch of the amplitude spectrum generated by the air-bubbles released by the air-guns with volume V, when fired simultaneously, the volume V₃ is chosen such that the frequency of the second harmonic pressure component generated by the air-bubbles released by the air-guns with volume V₃ is close to the frequency of the first notch of the amplitude spectrum generated by the air-bubbles released by the air-guns with volumes N, and V₂ when fired simultaneously.

2. A method as claimed in Claim 1 wherein the air-guns forming the air-gun array are fired substantially simultaneously and the air-guns are all operated at substantially the same chamber pressure.

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

4. A method of operating a marine seismic source substantially as hereinbefore described with reference to the Example which demonstrates the improvement of the acoustic efficiency.