JPH0713660B2 - Ocean source array and seismic pulse generation method - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates generally to seismic exploration, and in particular to marine seismic arrays where individual hypocenters of an array are separated from one another by a critical distance and only critical distances from each other. It relates to a method of generating seismic pulses using distant oceanic sources.

(Prior Art) In ocean seismic exploration, when one or more ocean hypocenters installed in water are activated, acoustic energy is released into the water and propagates into the geological layer below the seawater. That is, a shock wave is generated. This pulse propagates through the water into the seafloor geological formations and is reflected back as a sound wave. The reflected sound waves are detected by a geophone (underground sounding device), a hydrophone (underwater sounding device), or a similar device, and converted into an electric signal. These electrical signals are recorded for subsequent analysis and clarification. By analyzing the recorded signals, it is possible to know the structure of the seafloor geological layers and the associated oil deposits.

The term "water", as used in the claims, as used herein, includes swamp water, mud water, wetland water, and all other liquids including sufficient water to enable use of the ocean epicenter according to the invention. Shall mean.

One method of land seismic exploration involves placing the ocean hypocenter in a shallow well or a mud well used for normal drilling. Acoustic pulses generated and reflected by the oceanic epicenter are detected by a geophone placed on the ground. Alternatively, geophones can be installed in the well wells nearby. As a modification of this method, the reflected acoustic pulse can be detected by a hydrophone installed in water, that is, in a muddy well.

There are many types of conventional oceanic hypocenters that generate acoustic pulses in water. For example, explosives such as dynamite can be used to drive strong pulses into the formation. Another conventional ocean seismic source uses a group of capacitor discharges by underwater electrodes to produce a burst of implosion bubbles. This acoustic pulse generation method is widely used when high resolution response is desired from a geological layer near the water surface.

Ocean seismic sources using explosive gases (eg, a mixture of propane and air or a mixture of propane and oxygen) that produce an acoustic pulse when ignited have also been widely used. The main types of explosive gas guns are (1) those that produce an effect by detonating a mixed gas behind a flexible membrane that is in contact with water, and (2) bubbles that result from gas explosion. In some cases, the effect is obtained by directly releasing it into water. An example of the former type of gas gun is U.S. Pat.
No. 658,149 (issued April 25, 1972),
An example of the latter type of gas gun is U.S. Pat.
Issue (issued March 18, 1980).

Also, acoustic energy sources using high pressure gas instead of explosive mixtures have been widely used in industry. A typical construction of a high pressure gas gun with an open port is U.S. Pat.
Seen in 653,460 (issued April 4, 1972) and No. 4,141,431 (issued February 27, 1979). A typical high pressure gas gun for ocean seismic exploration has a housing with a chamber configured to confine a filled compressed gas at high pressure. The chamber is equipped with a valve that is closed during high pressure in the chamber. When the gun “trigger” is pulled, the valve opens rapidly. This causes the high pressure gas to exit the chamber and expand through the outlet port of the housing into the ambient medium, producing an acoustic pulse.

Special high pressure gas guns, or air guns, have become widely used as ocean seismic energy sources.
A typical air gun has the structure of the high pressure gas gun described above, where the high pressure gas is air. Generally, the compressed air in the air gun is maintained at a pressure of 2,000 to 6,000 psi before being released into the water to produce the desired acoustic pulse.

The cylindrical housing of a conventional air gun typically has an exhaust port through which high pressure gas is released when the valve is opened. There are various structures for the discharge ports of these underwater air guns. In a typical construction, four exhaust ports are symmetrically arranged on the outer circumference of the cylindrical housing of the air gun. The PAR® air gun sold by Bolt Technology Inc. is an example of an air gun with four symmetrically arranged exhaust ports. In another construction, 360 around the high pressure gas gun.
Depressurized air is expelled through one extending exhaust port. The external sleeve air gun, designed by Geophysical Service Inc., is an example of an air gun with a 360 ° discharge port as described above. External sleeve air
In the case of a gun, a shuttle valve concentric with the gun housing slides along the outer surface of the housing to open and close the 360 ° discharge port.

Arrays of two or more oceanic seismic sources have long been used when conducting oceanic seismic surveys. For example, U.S. Pat. No. 3,437,170 (issued April 8, 1969) discloses a method of simultaneously activating several epicenters in an array. According to this method, the energy-frequency spectrum of the acoustic signal produced by the array is shaped by producing bubbles at each source such that some bubbles coalesce while others do not. Each coalesced bubble produces a frequency component that differs from that of the energy frequency spectrum that would occur if the coalesced individual bubbles forming them were not coalesced. The above-mentioned U.S. Pat. No. 3,437,170 does not mention the improvement of the method for suppressing the continuous vibration strength of all the generated bubbles with respect to the strength of the initial component, that is, the "first order" component of the generated acoustic signal. . In particular, said U.S. Pat.No. 3,437,170 does not teach anything about a preferred source limit interval that improves the amplitude ratio between the primary signal component of the generated signal and the strongest continuous vibration component associated with that signal, and It does not suggest. This amplitude ratio may hereinafter be referred to as the "primary to bubble ratio".

A source array designed to suppress secondary bubble vibrations to the generated primary signal has been proposed. For example,
In the case of the air gun array described in US Pat. No. 4,382,486 (issued May 10, 1983), the volume ratio of the air guns and the spacing between the guns reduces the acoustic noise due to secondary bubble vibration. At the same time as making it smaller, the array is a “point epicenter”
Is chosen to satisfy the constraint of making the overall dimensions of the array sufficiently small. The above-mentioned U.S. Pat.No. 4,382,486 teaches that the individual guns of the device should be placed far enough apart so that the generated bubbles are independent and do not interfere, and in addition, according to a special equation, each air gun It also teaches that by choosing the volumetric ratio of, the "tuning" should be made to reduce the acoustic noise due to bubble vibration.

Each gun in the county is placed close enough to the other guns in the county so that the generated bubbles coalesce.
Another array design using gun counties and single air guns has been proposed. This array produces acoustic pulses with a primary-to-bubble ratio greater than that of the acoustic signal that would be obtained by replacing each group with a single air gun of a volume equal to the sum of the gun volumes in the group. Can occur. It is stated that this single gun county and gun county combination is required to obtain an improved overall array response. For more information on the above arrays, see BF Giles et al., "System Approach to Air Gun Array Design," Geophysical Prosye.
cting, Vol 21, pp. 77-101, (1973) and June 22, 1984.
See WRCotton's paper, "Use of External Sleeve Guns for Ocean Source Arrays," presented at the 14th Congress of the European Geophysicist European Union of Exploration Geophysics held in London, 24th June. However, if the hypocenters that were constructed to generate bubbles with about the same maximum radius were placed a distance apart as described in this application, the resulting acoustic pulse would produce an unexpectedly large primary to bubble ratio (below 49: 1 with a recording bandwidth of 5 to 880 Hz under the conditions described in detail.
The conventional technology does not disclose or suggest that it will be effective for seismic exploration, especially when it is installed at a shallow water depth, for example, at a water depth of 10 feet or less. , Never before been known.

Another array design that reduces secondary bubble vibration is described in US Pat. No. 2,771,961 (issued November 27, 1956). The array consists of two explosives with a specified relative potential energy placed at a sufficiently large vertical distance to prevent the bubbles of the resulting explosion from merging. According to the above-mentioned US Pat. No. 2,771,961, the preferred vertical distance D of explosive is (A ₁ + A ₂ ) ≦ D ≦ 3/2 (A ₁ +
A ₂ ) teaches that the relationship should be fulfilled. Where A
₁ is the maximum radius of the explosive bubble caused by the first explosive, and A ₂ is the maximum radius of the explosive bubble caused by the second explosive. Further, the above-mentioned U.S. Pat.No. 2,771,961 has the following array structure.
He teaches that the energy of secondary bubble vibrations is dissipated due to the interaction between two explosive bubbles. Explosive arrays with vertical spacing taught by U.S. Pat. For work, no adequate acoustic signal will be obtained.

(Structure of the Invention) The marine seismic array of the present invention comprises three or more marine seismic sources configured to generate bubbles having a substantially equal maximum radius R when installed and operated in water. . Each epicenter is separated from the other epicenters by the critical distance D selected to maximize the primary to bubble ratio (for the signal produced by the array). In all examples, D is no less than 1.2R and never more than 2R.
Each epicenter of the array according to the invention is characterized by "interdependence" when separated by a critical distance. In a first preferred embodiment, the array has a side length of about It has four hypocenters located at the four corners of the square. In a second preferred embodiment, the array is approximately two sides long. The method according to the present invention, in which each source of the above array is operated simultaneously, which has three hypocenters arranged at approximately the vertices of the isosceles triangle of convenient.

(Example) The array of the present invention, when installed in water and operated,
It has three or more oceanic seismic sources configured to generate bubbles of approximately equal maximum radius R. Commercially available air guns of approximately equal volume and approximately equal pressure are suitable for use in the preferred embodiment of the present invention. The array embodiment of the invention shown in FIG. 1 has four approximately equal volume air guns (identified by reference numerals 1, 2, 3, and 4). Each air gun is provided with one or more exhaust ports 10 for releasing gas in a direction perpendicular to the longitudinal axis of the gun. A flange 12 extends from each gun for connecting the guns to each other and to a support structure or frame 5. Gun 1
A chain 8 or the like is mounted between the pair of flanges 12 to connect the four. A chain 6 or the like is mounted between the flange 12 and a flange 14 extending from the frame 5 to connect the guns 1 to 4 to the frame 5. Frame 5
Is a boat, barge, buoy, float, dock, wharf,
Alternatively, it can be attached to an underwater support structure such as a wharf and supported at a desired position with respect to the water surface. In the following description, the frame 5, the chains 6 and 8, and the flanges 12 and 14 may be referred to as "positioning unit". It will be appreciated that many alternative positioning units could alternatively be used to hold the array's air guns in a desired position relative to each other and to the surface of the water.

The positioning unit, when activated, causes each source of the array to generate bubbles in water that have a maximum radius of approximately R,
And each source should be appropriately dimensioned and placed against the surface of the water so that it is separated from the nearest other source in the array by a critical distance D. In FIG. 1, the embodiment has four epicenters arranged at the four corners of a square, each epicenter preferably being separated from the nearest other epicenter in the array by a critical distance D. Where D is Almost equal. In one embodiment, the squares are oriented in water to generate bubbles having an initial center at a common water depth L.

It is not absolutely necessary that each bubble produced by the epicenter of the array has its original center in the same horizontal plane (ie at a particular depth). Properly speaking, it is recommended that each hypocenter be placed in water so that, when activated, the water depth difference between the initial center positions of the individual bubbles is sufficiently small and the maximum radius of each bubble generated is approximately the same. That is, within the scope of the present invention. Therefore, the longitudinal axes of the guns 1 and 2 (the axes around which the ports 10 are concentrically arranged) are located at different water depths than the longitudinal axes of the guns 3 and 4, similar to the array of FIG. The use of an array falls within the scope of the invention if the maximum radius of each bubble generated is about the same. As another example, when operating the gun, using an array similar to the array of FIG. 1 in which each port 10 of the gun 1, 2, 3, 4 is located at approximately the same depth,
If the maximum radius of each bubble generated is approximately the same, it is within the scope of the present invention.

In the embodiment shown in FIG. 1, the acoustic pulse produced by the source array of the present invention causes the gun to move to a new position such that the initial center-to-center distance of the bubbles produced by the gun after rotation is about the same as before rotation. 1 of the array if placed
It was found to be unaffected by the rotation of one or more guns.

It will be appreciated that the size of the maximum bubble radius R will depend on a number of parameters, including the depth L at which the initial center of the bubble is L and the characteristics of the ocean source. It is, of course, conceivable that sources other than air guns may be used in the array of the present invention. In the preferred embodiment using an air gun, the magnitude of R will depend on the volume of the gas chamber and the gas pressure in the chamber just prior to gas activation, as is well known in the art. . For example,
United Geophysical Corporation (Un
Ited Geophysical Corporation) staff at the 38th Annual Meeting of the Society for Exploration Geophysicists in Denver, November 1968, on how to estimate the maximum bubble radius R from known properties of air guns. See “1968 Seismic Energy Source Handbug,” submitted for illustration.

For an outer-sleeve array with a pressurized gas chamber volume of 40 in ³ and four air guns filled at 2000 psi, at a depth of 5 feet (ie, about 5 feet to the original depth). It has been found that the optimal array configuration, when used to generate a centered bubble, is to place the epicenters at the four corners of a square with sides of about 221/2 inches. Table 1 shows the average estimated primary to bubble ratio (for a recording bandwidth of 5-880 Hz) for the above arrays operated at water depths of 20, 15, 10, and 5 feet, respectively.

The array used to create the data in Table 1 is shown in Figure 4
FIG. 3 is an array of the type shown in FIG. 1 in which the ported air gun is replaced with an external sleeve gun. The data in the leftmost column of Table 1 was obtained by operating all four guns in the array simultaneously. The data in the second column was obtained by operating only guns 1, 2, 3 (identified in FIG. 1) simultaneously. Similarly, the data in the other columns are guns 1 and 2, respectively.
It was obtained by operating guns 1 and 3 and only gun 1 at the same time. The array was placed in the water with the vertical axis of each gun being vertical and the gun's gas release ports oriented horizontally at the specified depth.

FIG. 2 shows the time domain characteristics of the acoustic pulses produced when the gun outgassing port was placed at a depth of 5 feet and the array guns were simultaneously activated to obtain the data in Table 1.
The horizontal axis is the time transition (msec) after operation, t = 100m
Indicates that the operation has occurred at sec. The curve is the time received by a detector placed 183m directly below the source array and normalized to pressure equivalent to that measured at a distance of 1m in a manner commonly used in industry. The acoustic signal pressure is shown.

FIG. 3 shows the frequency domain characteristics of acoustic pulses generated in the same way with the same array. Second acoustic pulse
This is the same as the time domain characteristics shown in the figure. Of particular interest is the smooth and wide bandwidth seen in the amplitude spectrum shown. It has been found that the air guns of the present invention can be installed and operated in water depths of 10 feet or less to obtain much better temporal resolution than that obtained with conventional air gun arrays. It was At such shallow depths, the ghost notch (ghost notch) at frequencies above 250 Hz
notch) will appear, and recording of 1 msec is the minimum allowable criterion for optimal use of the hypocenter array.

FIG. 5 shows diagrammatically four bubbles which overlap one another. Each bubble was generated by actuation of one epicenter of the array embodiment of the present invention, and each bubble was at the moment it expanded to its maximum radius R. Each epicenter of the array in Fig. 5 has a side length Are arranged in the four corners of a square. If the array 4
It is clear from the overlapping regions of the bubbles that the four bubbles produced interact significantly if the hypocenters are activated almost at the same time.

Arrays with four or more or four ocean seismic sources are also within the scope of the invention. For example, one preferred embodiment uses three hypocenters, each capable of producing bubbles of maximum radius R. In this alternative preferred embodiment, each source is held by a positioning unit to generate bubbles of maximum radius R when activated and has two sides of length. Each epicenter is located at the apex of the triangle. Also,
The three hypocenters are arranged so that the center of the bubble generated at the time of operation is initially at the apex of the triangle, so that the length D of at least two sides of the triangle is within the range of 1.2R ≦ D ≦ 2R. In some cases, it falls within the scope of the invention.

A preferred embodiment of a three-source array can be made by removing one gun from the array of Figure 1 and operating the other three simultaneously. It is within the scope of the invention that only three of the four guns in the array of FIG. 1 are activated at the same time and the fourth is not. Secondary pulse suppression is improved as long as the preferred gun spacing described above is maintained. For example, the data in the second column of Table 1 shows that the 3-gun embodiment of the present invention produces pulses with a primary to bubble ratio of 28: 1.

Also, each epicenter of the array is arranged so as to generate bubbles of the same volume (having the maximum radius R), and each epicenter of the array is separated from the nearest other epicenter by the limit distance described here. In some cases, it is within the scope of the invention to use an array with four or more epicenters. In one example, six hypocenters are arranged in two parallel rows of three, and the distance between each hypocenter and the nearest hypocenter is approximately Is.

The optimum limit interval in each embodiment is determined by the characteristics of the epicenter and the type of positioning unit used to position the epicenter as required. In use, for a given array, it is preferable to empirically determine the optimum critical source spacing by performing several measurements sequentially at different source spacings selected from the range 1.2R-2R. The maximum first-order-to-bubble ratio for a given recording bandwidth should be obtained at the optimal margin interval.

Also, the use of arrays having more than one set of focal sources is within the scope of the invention if each set itself embodies the array according to the invention. For example, the embodiment shown in FIG. 4 has four sets of hypocenters, each set having four sets shown in FIG.
The epicenter type. The first set of hypocenters is hypocenters 111, 112, 11
3 and 114, and the other three hypocenters are
It consists of hypocenters 115-118, 119-122, and 123-126. Each epicenter is located in a predetermined position with respect to the frame 110 with a chain 130 or similar, and each set of epicenters is located with respect to each other with a chain 131 or similar.

In one variation of the embodiment of FIG. 4, the array was filled with about equal pressure and had an air gun of about equal volume, and the maximum radius of bubbles produced by each source of the array was It is installed in the water so that each epicenter produces bubbles with an initial center at a depth that is sufficiently narrow to be about the same. In another variant, each set of epicenters had an approximately equal volume of air gun filled with approximately equal pressure (the volume and pressure of the air gun differed from group to group). Or it may be the same for all pairs of arrays), so that the bubbles produced by the first set of hypocenters occur at a different mean depth than the bubbles produced by the second set of hypocenters, Each set is held in place by a positioning unit of a different size than the positioning unit shown in FIG. In this latter variant, the bubbles produced by the first set of epicenters may have a different maximum radius than the bubbles produced by the second set of epicenters, or they may have the same maximum radius. It also means that each set has different types of sources (eg different volume air guns) if all the sources of each set are activated, they have almost the same maximum radius (this maximum radius is It may fall within the scope of the present invention as long as it produces different bubbles) and the sources of each set are separated by a critical distance for their maximum bubble radius. The frame members 160 connecting the individual sets of sources are preferably short enough in length so that the array has the characteristics of a point source. In some applications, the length of the frame member 160 can be chosen so that each set of hypocenters is nearly independent.

The maximum bubble radii for each set of epicenters may be the same, or alternatively may be different for each set. Several sets of air
In one gun-based embodiment, each set has a common volume of air guns, and the volume ratio of each set is determined according to known methods for adjusting an air gun array.

No matter which array embodiment described above is used, the acoustic pulse produced by operating the interdependent sources of the array at about the same time has its bubble oscillations suppressed to the amplitude of its primary pulse component, The amplitude of the energy traveling downward in the primary pulse is maximized. For example, the present invention has three sets of sources, each set at a different depth of water, and all of the sets are arranged to, when activated, generate bubbles centered at a depth of water within a range of 10 feet. For array implementations, it is preferable to operate the epicenter for a duration of about 2 msec. In this implementation, if each of the three hypocenters produces bubbles centered at depths L, L-5, and L + 5 feet,
It is preferable to operate sequentially at intervals of 1 msec.

Various modifications and alternatives will occur to those skilled in the art when implementing the present invention without departing from the scope and spirit of the invention. Also, while the invention has been described with respect to particular preferred embodiments, it should be understood that the invention as claimed is not limited to such particular embodiments.

[Brief description of drawings]

FIG. 1 is a perspective view of an ocean epicenter array embodying the present invention. The array has four air guns and a frame mounting the guns, which are held at a predetermined distance from each other. FIG. 2 is a graph showing the time domain characteristics of an acoustic pulse produced by a preferred implementation of the invention using an array of four air guns. Each air gun had a volume of 40 in ³ and was operated at 2000 psi to generate bubbles at a depth of 5 feet. FIG. 3 is a graph showing the frequency domain characteristics of an acoustic pulse obtained by operating the same array used to generate the pulse shown in FIG. FIG. 4 is a simplified perspective view of another array embodiment of the present invention having four sets of four epicenters, each set of the type shown in FIG. FIG. 5 is a schematic diagram showing four bubbles each having a maximum radius R. Each bubble has a side length D In an array embodiment of the present invention having four epicenters located at the four corners of a square, the result is the actuation of one ocean epicenter. 1, 2, 3, 4 ... Air gun, 5 ... Support structure (frame), 6, 8 ... Chain, 10 ... Discharge port, 12,
14 …… Flange, 110 …… Frame, 111-114, 115-11
8, 119-122, 123-126 …… hypocenter, 130, 131 …… chain, 160 …… frame member.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-57-148280 (JP, A) JP-A-58-32185 (JP, A)

Claims (17)

[Claims] 1. A method of generating seismic pulses in water, comprising the step of: (a) placing a set of at least three ocean sources in water, each of said ocean sources being:
It is configured to generate bubbles with a maximum radius of approximately R in water, and is installed at a water depth that produces bubbles with a maximum radius of R when activated, and of the nearest oceanic source in the set. They are arranged to be separated from each other by a critical distance D, where D is in the relationship of 1.2R ≦ D ≦ 2R and is continuous for the primary pulse of the acoustic signal produced by the operation of each ocean source. Selected to maximize the suppression of vibrations, and (b) including actuating each of the oceanic seismic sources at about the same time to generate seismic pulses in the water. Underwater earthquake pulse generation method. 2. The method of claim 1 wherein each of the oceanic epicenters is an air gun having approximately equal chamber volume and operating at approximately equal pressure. 3. The set according to claim 1, wherein the set includes four ocean seismic sources arranged so that the initial centers of the four generated bubbles are located at the four corners of the square.
Method described in section. 4. The limit distance D is The method of claim 3 which is substantially equal. 5. The set includes three ocean hypocenters arranged such that the initial centers of the three generated bubbles are located at approximately the vertices of the triangle, and the length of at least two sides of the triangle is at least two. A method according to claim 1 which is equal to. 6. The method of claim 1, wherein each of the oceanic epicenters is installed at a depth within about 10 feet below the surface of the water. 7. A method of generating seismic pulses in water, comprising: (a) placing a first set of at least three oceanic sources in water, each oceanic source in the first set comprising: , Is designed to generate bubbles with a maximum radius of approximately R in water, and is installed at a water depth that produces bubbles with a maximum radius of R when activated, and is the most deep in this first set. Critical distance D from each of the near ocean hypocenters
Are placed so that they are only separated, where D is 1.
2R ≤ D ≤ 2R and is selected to maximize the suppression of successive vibrations to the primary pulse of the acoustic signal produced by the operation of the oceanic sources in this first set, and , (B) including placing a second set of at least three ocean hypocenters in water, each of the ocean hypocenters in the second set producing bubbles in water having a maximum radius of approximately S And is installed at a water depth such that when actuated, it produces bubbles of maximum radius S, and a critical distance E from each of the nearest oceanic sources in this second set.
It is arranged so that it is only separated, where E is 1.
2S ≦ E ≦ 2S, and is selected to maximize the suppression of successive vibrations to the primary pulse of the acoustic signal produced by the operation of the oceanic hypocenters in this second set. (C) A method for generating an underwater seismic pulse, including the step of operating each of the oceanic hypocenters in each of the sets at substantially the same time so as to generate an earthquake pulse in water. 8. Each ocean source in the first set is an air gun having approximately the same chamber volume as the other ocean sources in the first set and each ocean source in the second set. 8. The method of claim 7 wherein the ocean source is an air gun that has approximately the same chamber volume as the other ocean sources in this second set. 9. The method of claim 7 wherein said maximum radius R is approximately equal to said maximum radius S. 10. The method of claim 7, wherein each of the oceanic epicenters is located at a depth within about 10 feet below the surface of the water. 11. An ocean hypocenter array comprising: (a) a first set each comprising at least three ocean hypocenters capable of producing bubbles in water having a maximum radius of approximately R; To generate bubbles of maximum radius R at water depth L when each ocean source within the first set is activated, and each ocean source within this first set is the closest ocean source within this first set. A first positioning unit for positioning the first set of oceanic hypocenters in water such that they are separated from each other by a limit distance D, wherein the limit distance D is 1.2R ≦ D. ≤2R and is selected to maximize the suppression of successive vibrations to the primary pulse of the acoustic signal produced by the operation of the oceanic source in this first set. Ocean hypocenter array. 12. The first set includes four oceanic hypocenters, and the critical distance D is approximately A hypocenter array according to claim 11, which is equal to. 13. The first set has a length of at least two sides. 12. An ocean hypocenter array as claimed in claim 11 including three ocean hypocenters located at approximately the apices of a triangle equal to. 14. A second set each comprising at least three ocean sources capable of producing bubbles in water having a maximum radius of approximately S, and actuating each ocean source within the second set. To generate bubbles of maximum radius S at depth M, and each ocean source in this second set
A second positioning unit for positioning the second set of oceanic hypocenters in the water such that they are separated from each of the nearest oceanic hypocenters in the set by a limit distance E. Distance E is selected to be 1.2S ≤ E ≤ 2S and to maximize the suppression of successive vibrations to the primary pulse of the acoustic signal produced by the operation of oceanic hypocenters in this second set. The ocean seismic source array according to claim 11. 15. The bubbles produced by oceanic sources in the first set are largely independent of the bubbles produced by oceanic sources in the second set.
Ocean hypocenter array described in paragraph 14. 16. The marine seismic array according to claim 14, wherein the maximum radius R is approximately equal to the maximum radius S. 17. The marine seismic array according to claim 14, wherein the maximum radius R is significantly different from the maximum radius S.