FR2583525A1 - PROCESS FOR PRODUCING SEISMIC PULSE IN A MASS OF WATER AND DEVICE FOR SEISMIC MARINE SOURCES - Google Patents

Abstract

THE INVENTION CONCERNS A MARINE SEISMIC SOURCE DEVICE AND ITS IMPLEMENTATION PROCESS. THE DEVICE CONTAINS THREE OR MORE SOURCES1, 2, 3, 4 EACH CONSISTING OF AN AIRGUN AND MOUNTED IN A CHASSIS5 IN A MANNER SUCH AS DISTANCE

Description

The invention relates generally to seismic prospecting. She

  relates in particular to a marine seismic source device in which individual seismic sources are spaced apart from one another by a critical distance, and

  a method of generating seismic pulses

  marine seismic sources spaced at

  others of such a critical distance.

  In marine seismic exploration, one or more marine seismic sources are deployed in a body of water and triggered to release acoustic energy into the water to produce sonic pulses or shock waves that penetrate

  propagating in the underground geological formations

  raines below the bottom of the body of water.

  These impulses propagate through the water, penetrate the geological formations below the bottom and return by reflection in the form of acoustic waves. A device of geophones, hydrophones or similar equipment detects acoustic waves

  reflected and converts them into electronic signals.

  These electronic signals are recorded for analysis and interpretation. The analysis of the recorded signals can give an indication of the structure

  underground geological formations and accumu-

  concomitant oil production in these formations.

  The term "water" as used herein includes swamp water, mud, swamp water and any other liquid containing sufficient water to permit the use of the marine seismic sources used.

in the invention.

  In a terrestrial seismic exploration, a marine seismic source is placed in a shallow well filled with water or mud and used

  otherwise for normal drilling operations.

  Reflected sound pulses, produced by the underwater source, are detected by geophones placed on the ground. Alternatively, the geophones may be arranged downhole in a neighboring well. In a variant of this technique, reflected sonic pulses can be detected by hydrophones arranged in a well containing water.

or mud.

  There are a number of types of

  classical marine seismic sources intended to produce

  re a sonic impulse in a body of water. For example, explosives such as dynamite can be used to introduce strong pulses into subterranean formations. Another classic marine seismic source uses the discharge of a capacitor bank through an electrode placed below the surface to produce a gas bubble

  implosive sagging rapidly. This general process

  sonic pulses is commonly used when

  that a high-resolution response is desired at

  from geological formations close to the surface.

  Marine seismic sources using explosive gases (such as mixtures of propane and air or mixtures of propane and oxygen) to produce a sonic pulse when

  with fire have met with a great success. The main types

  Explosive gas cannon blades include (1) those that operate by blowing a gaseous mixture back from a flexible membrane that itself is in contact with water, and (2) those that function by allowing to the resulting gas bubble

  from the gas explosion to go directly into the water.

  An example of the first type of gas cannon is described in U.S. Patent No. 3,658,149 and an example of the second type of gas cannon is described in US Pat.

No. 4,193,472.

  Sources of acoustic energy using compressed gases under high pressure instead of an explosive mixture have also been very successful in the industry. U.S. Patents Nos. 3,653,460 and 4,141,431 describe typical embodiments of open-ported compressed gas guns. A conventional compressed gas cannon for marine seismic exploration includes a body that contains a chamber designed to retain a high pressure gas charge. The chamber is equipped with a valve that is closed while

  the pressure is high inside the chamber.

  When the barrel is "fired" or fired, the valve is quickly opened. This allows the compressed gas to exit the chamber by relaxing and penetrate, through exhaust ports in the body into the environment to generate

an acoustic pulse.

  A particular compressed gas gun, the air gun, has achieved wide use as a

  source of marine seismic energy. The classic air gun

  that presents the configuration of the compressed gas gun described above, but in which the gas under high pressure is air. In general, compressed air

  in such guns is maintained at pressures

  its between 14 and 42 MPa before being released into the water

  to generate the desired acoustic pulse.

  Classic air guns include

  generally a cylindrical body presenting

  these exhaust gases through which the compressed gas is released when a valve of the barrel is opened. The configuration of the exhaust ports of these guns

  Underwater compressed air can vary. In a confi-

  current guration, four exhaust ports are symmetrically distributed on the periphery of the cylindrical body of the compressed gas gun. "PARj" airguns, available from Bolt Technology Inc., Norwalk, Connecticut, USA, are

  examples of air guns with four escape ports

  symmetrically distributed. In another configuration

  tion, the compressed air is released through a

  exhaust port that extends 360 to the periphery

  of the compressed gas gun. The outer sleeve air gun, designed by Geophysical Services Inc. of Dallas, Texas, is an example of

  air having such an exhaust port of 360.

  In an outer sleeve air gun, a reciprocating valve, concentric with the barrel body, slides along the outer surface of the body to open

  and close the exhaust port 360.

  Devices of two seismic sources

  or more have been used for a long time in

  marine seismic For example, the United States Patent

  United States of America N 3,437,170 discloses a technique

  to trigger simultaneously several sismi-

  marine problems of a device. According to this technique, the energy frequency spectrum of the acoustic signal produced by the devices is shaped by the generation, at each source of a bubble, some of these bubbles grouping by coalescence, unlike others. The clustered bubbles produce each of the frequency components, in the energy frequency spectrum,

  different from those which would have been produced indivi-

  dually by these bubbles if they were not

  in coalescence. The aforementioned patent does not disclose a method for improving the suppression of the force of successive oscillations of any of the bubbles produced with respect to the force of the initial or "primary" component of the acoustic signal produced. In particular, the aforementioned patent does not describe or suggest any critical spacing.

  preferred sources to improve the relationship of the

  of the primary component of the signal produced

  at the highest amplitude of the oscilla-

  concomitant sucessives of the signal. This report

  amplitude is sometimes referred to as "composite report"

  primary health component of bubbles ".

  Devices designed to suppress secondary bubble oscillations with respect to the primary signal produced by the bubbles have been

  proposed. For example, the United States patent of

  No. 4,382,486 discloses an airgun device in which the ratios of airgun volumes and gunslots are selected

  in order to reduce the acoustic noise due to oscilla-

  secondary bubbles while satisfying the constraint that the overall dimensions of the device are sufficiently small for the device to constitute a "point source". This patent indicates that the individual guns of the device may be sufficiently spaced from one another so that the bubbles they produce are independent and non-interacting, and it also indicates that the device must be "tuned" to reduce acoustic noise due to the oscillations of the bubbles, by the choice of the ratios of the volumes of the air guns

  according to a specified equation.

  Another design of the device using both single air guns and groups of air guns of equal volume, each gun of a group being sufficiently close to the other guns of the group for the bubbles they produce to enter.

  in coalescence, has been proposed to produce an impulse

  noise ratio including the ratio of the primary component

  re to the bubble component exceeds that of the acoustic signal that would be produced by the replacement of each group by a single volume air gun

  equal to the sum of the volumes of the guns of the group.

  This combination of single guns and groups of guns is indicated as being necessary to improve the overall response of the device. For

  a description of such a device, we can refer to

  to BF Giles et al., "System Approach to Air-Gun Array Design", Geophysical Prospecting, 21, pages 77-101 (1973), and WR Cotton, "The Application of External Sleeve Guns to Marine Source Arrays", presented in the 46th congress of the European Association

  exploration geophysicists, London, Great Britain

  Brittany, 19-22 June 1984. However, the prior art neither describes nor suggests, and it is not known until now, that if sources designed to produce bubbles having substantially equal maximum radii are spaced apart by the indicated critical distance

  in this memo, the resulting acoustic pulse

  aunt will have a surprisingly high ratio of the primary component to the bubble component (rising to 49: 1 with a recording bandwidth of 5-880 Hz under the conditions described in detail below) and will be particularly useful for seismic exploration during a deployment at shallow depths, such as

  only depths of less than 3 meters.

  Another device design for reducing secondary bubble oscillations is described in the United States Patent

  N 2 771 961. This device uses two charges explo-

  sive having a specified relative potential energy spaced a vertical distance large enough that the bubbles produced by the explosions can not coalesce. It is stated in the last patent cited that the preferred vertical distance, D, of the charges must satisfy the equation (A1 + A2) <D <3/2 (A1 + A2), where A1 is equal to the maximum radius of the bubble from the explosion of the first charge and A2 is equal to the maximum radius of

  the bubble from the explosion of the second charge.

  This patent indicates that this device configuration causes energy dissipation in secondary bubble oscillations due to the interaction between the two explosive bubbles. An explosive charge device having the vertical gap indicated in the aforementioned patent would not be suitable for use in shallow water exploration operations and would not produce an acoustic signal suitable for exploration operations requiring a band signal. broad with significant energy in the near frequency band

250 Hz.

  The marine source device according to the invention comprises at least three marine seismic sources designed to be placed in a body of water in order to produce, when they are triggered, gas bubbles having substantially the same maximum radius R. Each source is spaced from each of the other sources closest to a critical distance D chosen so that the ratio of the primary component to the component of the bubbles (associated with the signal produced by the device) is maximized. In all embodiments, D is not less than 1, 2 R and is not greater than the amount 2 R. The sources of the device of the invention are said to be "interdependent" when they are spaced apart. of the critical distance. In a preferred embodiment, the device comprises four sources placed approximately at the corners of a square whose sides have an approximate length of 2 R. In another preferred embodiment, the device comprises three sources placed approximately at the top. of a horizontal triangle having two sides of length approximately equal to i R. The method according to the invention for simultaneously triggering the sources of the device described is particularly advantageous when the bubbles are produced at lower source depths.

at about 3 meters.

  The invention will be described in more detail

  with reference to the accompanying drawings as examples

  FIG. 1 is a perspective view of a device of marine seismic sources according to the invention, comprising four air guns maintained at a desired critical distance from each other, and a frame to which the guns are related; FIG. 2 is a graph showing the time domain characteristic of an acoustic pulse produced using a preferred embodiment of the invention using a device of four air guns each having a volume of 655 cc. and working at 14 MPa, to produce bubbles at a depth of 1.5 meters in a body of water; FIG. 3 is a graph showing

  the frequency domain characteristic of an impulse

  acoustic noise resulting from the operation of the same device used to produce the pulse whose characteristic is given in Figure 2; FIG. 4 is a simplified view in

  perspective of another form of implementation of the

  tif according to the invention which comprises four groups of four sources, each group of four sources being of the type shown in Figure 1; and FIG. 5 is a schematic view showing four bubbles each having a maximum radius

  equal to R, each bubble having been produced by the triggering

  one of the marine seismic sources of an embodiment of the device according to the invention comprising four sources arranged at the vertices of a side square D equal to D R. The apparatus according to the invention comprises three seismic sources or more, each designed to produce a bubble of maximum radius R when

  the source is deployed in the device according to the invention.

  triggered in a body of water. Cannons to

  commercially available air, of

  Equally equal and loaded at substantially equal pressure, may be suitable for use in a preferred embodiment of the invention. The embodiment of the device according to the invention, shown in FIG. 1, comprises four of these air guns with a substantially equal volume (identified as air guns 1, 2, 3 and 4). Each of the air guns has one or more exhaust ports

  allowing the emission of gases in

  of the longitudinal axis of the barrel. Legs 12 protrude from each barrel and are to be used to connect the barrel to other barrels or to a support structure. Chains 8 or similar elements are mounted between pairs of legs 12 to connect the guns 1-4. Channels 6 or elements

  analogues are mounted between the legs 12 and 14,

  5 of the structure 5, to connect the guns 1 to 4 to the structure 5. The latter can be connected to a marine support structure, such as a ship, a barge, a buoy, a float, a dock, a jetty or a honeycomb so that the structure 5 is supported in a desired position relative to the surface of the water. The structure 5, the chains 6 and 8 and the legs 12 and 14 will sometimes be collectively referred to hereinafter as "positioning unit". It appears that many other positioning units can alternatively be used to hold the rear guns of the device in a desired position relative to each other and to the

surface of the water.

  The positioning unit must be suitable

  dimensioned and oriented in relation to the surface of the water so that the sources of the device

  each produce a bubble with an approximate maximum radius

  R, in the water, when this source is triggered, and so that each source is spaced from the critical distance D of each of the nearest other sources of the device. The embodiment of FIG. 1 advantageously comprises four sources arranged at the four corners of a square, each source

  spaced from the nearest sources of distance

  this criticism D, D being approximately equal to the quantity R. In one embodiment, the square is oriented in the water so as to produce bubbles initially centered at a common depth L in the water. It is not essential that each bubble produced by a source of the device according to the invention is initially centered in the same horizontal plane, (that is to say at a single depth). It is rather within the scope of the invention to be able to arrange the sources in the water, when they are triggered, so that the difference in depth between the initial positions of the centers of the individual bubbles is small enough for the maximum radius of each bubble produced is substantially the same. Thus, for example, it is within the scope of the invention to be able to use a device similar to that of FIG. 1, in which the longitudinal axes of the guns 1 and 2 (that is to say the axes on which the orifices 10 are placed concentrically) are arranged at a depth different from that of the longitudinal axes of the guns 3 and 4, provided that the maximum radius of each bubble produced is substantially the same. In another example, it is within the scope of the invention to use a device similar to that of FIG. 1, in which the orifices 10 of the barrels 1, 2, 3 and 4 are all arranged substantially at the same depth when the guns are fired, provided that the maximum radius of each bubble produced

be substantially the same.

  It has been found that the acoustic pulse produced by the source device according to the invention, in the embodiment shown in FIG. 1, is practically unaffected by a rotation of one or more guns of the device, provided that after the rotation, the guns are placed in a new position such that the distances between the centers of the bubbles initially produced by the

  cannons are substantially the same as before the rota-

  tion. It should be noted that the amplitude of the maximum radius R depends on various parameters including the depth L at which the bubble is initially centered, and the characteristics of the associated marine seismic source. It is intended to use in the apparatus of the invention seismic sources other than air guns. In a preferred embodiment in which air guns are used, the amplitude of the radius R depends, in a manner well known in the art, on the volume of the gas chamber and the gas pressure in the chamber immediately. before firing or firing the barrel. For an explanation of how the maximum radius R of bubbles can be estimated from known characteristics of air guns, reference can be made, for example, to the book "Seismic Energy Sources 1968 Handbook", prepared by United Geophysical Corporation Staff Presented at the 38th Annual Society of Exploration Conference

  Geophysicists, Denver, Colorado, October 1968.

  It has been found that in a device comprising four outer sleeve-type air guns and having a compressed gas chamber with a volume equal to 655 cm 3 and loaded to 14 MPa, the optimum configuration of the device for implementation at a depth of 1.5 m (that is, to produce bubbles initially centered at a depth of about 1.5 m) in water is to arrange the sources at the vertices of a square of about 57 side cm. Table

  The following shows the estimated average ratios of the

  primary component to the bubble component (associated with a recording bandwidth of 5-880 Hz) of such a device, implemented at depths

  of 6.4.5, 3 and 1.5 m, respectively.

BOARD

Depth Cannons trigger

  (m) 1,2,3 and 4 1,2 and 3 1 and 2 1 and 3 1

6 12 10 9 5 3

4.5 13 12 10 7 4

3 25 17 13 10 6

1.5 45 28 20 13 10

  The device used to obtain the data of the preceding table is of the type shown in FIG. 1, a replacement outer sleeve barrel

  each of the air cannons with four holes shown.

  The far left column of the table contains data resulting from the simultaneous triggering of the four represented guns of the device. The second column from the left contains data resulting from simultaneous triggering only guns 1, 2 and 3 (as identified in Figure 1). Similarly, the other columns contain data resulting from the simultaneous triggering of only guns 1 and 2, 1 and 3 and 1. The device has been placed in the water under an orientation such that the longitudinal axis of each gun is vertical and that the orifices

  gas emission cannons are arranged horizontally

  compared to each other at depth

designated.

  FIG. 2 shows the time domain characteristics of an acoustic pulse produced by the simultaneous triggering of the guns of the device used to collect data from the preceding table when the gas emission orifices of the guns

  are arranged at a depth of 1.5 m. The horizontal axis

  it shows the elapsed time from the trigger or shot (in milliseconds), a shot or trigger

  being performed at t = 100 ms. The curve shows the pressure

  the acoustic signal as a function of time, received at a detector placed 183 meters below the source device, this pressure being normalized to an equivalent pressure as measured at a distance of one meter, in a manner commonly used in industry. FIG. 3 shows the characteristics of the frequency domain of an acoustic pulse produced in the same way, using the same device, as was the pulse whose characteristics

  time domain are shown in Figure 2.

  It is particularly interesting to note the large and regular bandwidth indicated by the amplitude spectrum shown. It seemed better to deploy

  and trigger the air guns of the device of the invention

  at depths of 3 m or less to achieve a much higher temporal resolution than can be achieved with conventional airgun devices. At such low depths, the ghost line appears at a frequency of not less than 250 Hz, and a one millisecond record is the minimum acceptable standard.

  for optimal use of the source device.

  FIG. 5 schematically shows four bubbles superimposed on each other, each having been produced by the triggering of one of the sources

  of an embodiment of the device according to the invention.

  tion. Each bubble is represented at the moment when it has expanded to its maximum radius R. The sources of the device of FIG. 5 are arranged at the four vertices of a square whose side length is equal to 2 R. Since the areas occupied by the bubbles overlap, it appears that if the four sources of the device are triggered almost simultaneously,

  the four resulting bubbles interact significantly.

  Devices comprising more or less four marine seismic sources are also within the scope of the invention. For example, in one of the preferred embodiments, three sources are used, each of which can produce a bubble of maximum radius R in water. In this other advantageous embodiment, the three sources are held by a positioning unit so that each generates, when it is triggered, a bubble of maximum radius R, and the sources are arranged approximately at the vertices of a triangle having two sides of length Ai R. It is also within the scope of the invention to have three sources so that the bubbles they produce,

  when they are triggered, have start-up centers

  positioned approximately at the vertices of a triangle of which at least two of the sides are of a length

  D in the range 1.2R <D <2R.

  An advantageous form of the three-source device can be achieved by removing a barrel

  the device of Figure 1 and simultaneously triggering

  three other guns. It is also within the scope of the invention that only three of the four guns of the device of FIG. 1 can be triggered simultaneously, the fourth gun not being fired. Better suppression of the secondary pulses is achieved by maintaining the advantageous range between guns indicated above. For example, the data in the second column of the table shows that a pulse having a ratio of the primary component to the bubble component of 28: 1 can be obtained with the three-dimensional embodiment.

canons of the invention.

  It is also part of the inven-

  to use devices with more than

  four sources, the sources of the device being posi-

  to produce bubbles of equal volumes (with a maximum radius R) and each source of the device being separated from sources closest to the critical distance indicated here. In one embodiment, six sources are placed in two parallel rows of three sources each, the distance between each source and the nearest sources being approximately <R. The optimal critical distance in each embodiment depends on characteristics

  particular sources and the type of unity of posi-

  used to position the sources as desired. In the case of a given device, it is advantageous if the optimum critical intervals or spacings between sources are determined experimentally by the execution of several measurements.

  sequences with different time intervals between

  rents, selected in the range between 1.2R and 2R, inclusive. A maximum ratio of the primary component to the bubble component for a width

  given recording tape is obtained at the space-

optimal criticism.

  It is also part of the inven-

  to use devices comprising two "groups"

  from sources or more, each group forming, by

  same, a device according to the invention. For example, the embodiment shown in Fig. 4 comprises four source groups, each of the four-source type shown in Fig. 1. The first group

  sources includes sources 111, 112, 113, and 114.

  The other three source groups include, respectively,

  - Sources 115-118, 119-122 and 123-126.

  The sources are placed in a desired position with respect to a frame 110 by means of chains 130 or the like, and the sources of each group are placed as desired with respect to each other.

  others using chains 131 or similar elements.

  In a variation of the embodiment of FIG. 4, the device comprises air guns of approximately equal volume, charged to approximately equal pressure, and is oriented in the water so that the sources produce bubbles. initially centered at depths within a range sufficiently narrow that the maximum radius of the bubbles produced by the sources

  individual devices is substantially the same.

  In another variant, each group of sources of the device comprises air guns of a volume

  approximately equal, charged to approximately

  Equally equal (the volume and pressure of the air guns may differ from one group to another, or may be the same for all groups of the device), and the groups are held in position by a positioning unit sized differently. of the unit shown in Figure 4 so that the bubbles produced by the sources of a first group are produced at an average depth different from that of the bubbles produced by the sources of a second group.

  group. In this last variant, the bubbles produced

  by the sources of the first group may have a maximum radius different from that of the bubbles produced by the sources of the second group, or may have the same maximum radius as the bubbles produced by the sources of the second group. It is also within the scope of the invention to allow each

  group to understand the sources of various different types

  (such as, for example, air guns of different volumes), provided that the sources of each group all produce, when they are triggered bubbles of a substantially identical maximum radius (this maximum radius may differ from one group to another) and that the sources of each group are separated from the critical distance associated with this maximum bubble radius. Frame members 160, connecting the individual groups of sources, preferably have a length short enough for the device to have the characteristics of a point source. For some applications, the lengths of frame members 160 may be chosen so that

  source groups are mainly independent.

  The maximum bubble radius associated with the source groups can be identical or, in

  alternatively, it may differ from one group to another.

  In one embodiment, groups of air guns are used, each group comprising air guns of a common volume, the ratios of the volumes of the groups being determined according to any known technique for tuning guns.

air guns.

  In the operation of any of the embodiments of the devices described herein, an acoustic pulse, whose bubble oscillations are suppressed with respect to the magnitude of the primary component of this pulse, is produced in the water by the triggering the interdependent sources of the device, substantially at the same time, in order to maximize the amplitude of the downward energy in the primary pulse. for example

  in one embodiment of the device of the invention

  having three groups of sources each arranged at a different depth, but such that all the groups produce, when triggered, bubbles centered at depths within a range of 3 meters, the sources are advantageously triggered at chosen times in a period of about two milliseconds. In this

  example, where the three groups are arranged to produce

  respectively centered at a depth L, (L - 1.5 m) and (L + 1 5 m), the groups are advantageously triggered sequentially at one millisecond intervals.

  It goes without saying that many changes

  can be made to the process and the device

  described and represented without departing from the scope

tion.

Claims (16)

  A method of producing a seismic pulse in a body of water, comprising the steps of: (a) disposing in water a group of at least three marine seismic sources (1, 2, 3, 4 ) each designed to produce in the water a gas bubble having a maximum radius substantially equal to the quantity   R, the sources being disposed to deep   such that each of them produces, when triggered, a bubble of maximum radius R, and the sources being arranged so that each source is separated from each of the sources which are closest thereto, in the group, by a critical distance D such that 1,2R <D 42R, D being chosen so as to   substantially increase the maximum   successive oscillations of the acoustic signal resulting from the   sources in relation to their impulse   primary education; and (b) triggering the sources substantially at the same time to produce a pulse seismic in the water.   2. The process according to claim 1, wherein   characterized in that the sources are air guns having substantially equal chamber volumes, which   are operated substantially at the same pressure.   3. Method according to one of the claims   1 and 2, characterized in that the source group comprises four sources arranged so that the four bubbles they produce have centers initially located approximately at the four vertices of a square.   4. The process according to claim 3, wherein   characterized in that the distance D is approximately equal to the quantity f / R. 5. Method according to claim 1, characterized in that the source group comprises three sources arranged so that the three bubbles they   produce centers initially located   proximally to the three vertices of a triangle of which at least two sides have a length equal to 2 R. A method of producing a seismic pulse in a body of water, comprising the steps of: (a) disposing in the water a first group   at least three marine sources (111-   114) each designed to produce in water a gas bubble having a maximum radius substantially equal to the quantity R, the sources of the first group being disposed at depths such that each of them, when triggered, produces a a gas bubble with a maximum radius R, and the sources being arranged so that each source of the first group is separated from each of the sources which are closest thereto, in the first group, by a critical distance D such that 1, 2R, D 42R, D being chosen so as to raise substantially as much as possible the suppression of the successive oscillations of the acoustic pulse resulting from the triggering of the first   group of sources in relation to their impulse   primary education; (b) disposing in the water a second group of at least three marine seismic sources (115-118) each designed to produce in the water a gas bubble having a maximum radius substantially equal to the amount S, the sources of the second group being arranged at such depths that   each produces when it is triggered   a gas bubble with a maximum radius S, the sources being arranged so that each source of the second group is separated from each of the sources which are closest thereto, in the second group, by a critical distance E such that 1,2S t4 E 1,2S, E being chosen so as to   substantially increase the maximum   successive oscillations of the   acoustic drive, resulting from the   the second group of sources with respect to their primary impulse; and (c) triggering the sources of both groups at substantially the same time to   produce a seismic pulse in the water.   7. The method of claim 6, wherein   characterized in that each source of the first group is an air gun having substantially the same chamber volume as the other sources of the first group, and each source of the second group is an air gun having substantially the same chamber volume as the other sources of the second group.   8. The method of claim 6, wherein   characterized in that the maximum radius R is substantially equal to the maximum radius S.   9. Method according to one of the claims   1 and 6, characterized in that the marine seismic sources are disposed in the water at a depth not exceeding about 3 meters below the surface of the water.   10. Apparatus of marine seismic sources, characterized in that it comprises a first group of at least three marine seismic sources (1, 2, 3, 4) each capable of producing in water a gas bubble having a maximum radius substantially equal to R, and a first positioning unit (5, 6, 8, 12, 14) adapted to position the first group of sources in the water so that each source of the first group produces, when it is triggered, a bubble of maximum radius R at a depth L in the water, and that each source of the first group is spaced from each of the sources which are closest to it in the first group, by a critical distance D such that 1, 2R zD É 2R, D being chosen so as to raise substantially as much as possible the suppression of the successive oscillations of the acoustic pulse resulting from the substantially simultaneous triggering   sources of the first group in relation to their impulse primary education.   Device according to the claim, characterized in that the first group comprises   four marine seismic sources and that the distance   this D is substantially equal to VT R. Device according to claim 1, characterized in that the first group comprises three marine seismic sources and in that the sources are arranged approximately at the apex of a triangle of which at least two sides have a length equal to % / 2 R.   13. Device according to the claim, characterized in that it also comprises a second group of at least three marine seismic sources (115-118) each capable of producing in water a gas bubble having a maximum radius substantially equal to S, and a second positioning unit (110, 131) adapted to position the sources of the second group so that each source of the second group produces, when triggered, a bubble of maximum radius S at a depth M in the water, and that each source of the second group is spaced from each of the nearest sources, in the second group, by a critical distance such that 1.2S <E <2S, E being chosen so as to raise substantially at most the suppression of the successive oscillations of the acoustic pulse resulting from the substantially simultaneous triggering of the sources in   the second group with respect to their primary impulse.   14. Device according to claim 13, characterized in that the bubbles produced by the sources of the first group are mainly independent of the bubbles produced by the sources of the second group.   15. Device according to the claim   13, characterized in that the maximum radius R is sensi-   the maximum radius S. 16. Device according to the claim   13, characterized in that the maximum radius R is sensi-   different from the maximum radius S.