INTERNAL SECONDARY BUBBLE PULSE SUPPRESSION

Abstract: The novel method substantially reduces the pressure pulse generated by an implosion of a cavity within a body of water by aborting the implosion. The abortion of the implosion is produced by generating within the cavity an explosion at the instant that this cavity attains its maximum volume. The method can also be used to generate within a body of water an impulsive acoustic signal by generating a first explosion within the body of water to produce therein a powerful pressure pulse and a cavity of very low pressure, and then generating a second explosion within the cavity so as to establish hydrostatic pressure inside the cavity while the volume of the cavity remains within a few percent of its maximum volume. The apparatus comprises first (10) and second (10) explosive generators which consecutively create two explo­ sions within the body of water; a first explosion which produces within the body of water a powerful acoustic pulse (P₀) and an expanding cavity (1) of very low pressure, and a second explosion which establishes hydrostatic pressure within the cavity (1) while the volume of the cavity (1) remains within a few percent of its maximum volume, thereby substantially reducing the ensuing secondary press­ ure pulses.

Description: This invention generally relates to a method and apparatus for periodically generating underwater sharp, impulsive acoustic signals, and, in particular, to such acoustic sources which abruptly create a gas bubble at a sufficient depth below the water surface to allow the bubble to expand and contract. Certain seismic sources such as explosives, airguns, gas exploders, etc., are fired deep under water to create a gas bubble or cavity which causes the water to acquire oscillatory energy that generates acoustic pressure wavelets. Each wavelet consists of a desired "primary" acoustic pressure pulse P o , which is especially useful for most seismic exploration work. Pulse P o is followed by an oscillating succession of undesired "secondary" (sometimes called "bubble") acoustic pulses of decreasing amplitude. In this specification, the words "bubble" and "cavity" will be used interchangeably. For example, a widely used seismic source known as an "airgun" explosively releases a high pressure gas bubble into the water which creates the desired primary pulse having a maximum amplitude P o . After the released high-pressure gas bubble impulsively contacts the surrounding water, it continues to expand as the water first accelerates outwardly and later decelerates until the cavity attains a maximum diameter and comes to rest, at which time the pressure within the cavity is much less than the surrounding hydrostatic pressure. When the expanding bubble reaches its maximum diameter, there is practically a vacuum inside the bubble, the kinetic energy of the surrounding water is zero, but it possesses maximum oscillatory potential energy which, if not suppressed, will change into kinetic energy, back into potential energy, etc., for a duration of several cycles, each having an oscillatory time period T. At 1/2 T, the water has maximum potential energy and is ready to rush inwardly to implode the gas in the bubble. After one complete cycle, i.e., at time T, the bubble is recompressed into a relatively small-diameter, high-pressure bubble. The surrounding water then comes to an abrupt stop resulting in a first positive acoustic secondary pulse P₂ which is mainly dependent upon the maximum kinetic energy acquired by the inwardly-moving water. The less kinetic energy is acquired by the water, the smaller P₂ will be. Thus, the secondary pulse problem starts when the surrounding water for the first time violently recompresses the bubble to a minimum diameter or volume. Its kinetic energy is then again zero. Its potential energy is mainly contained within the recompressed gas inside the bubble, and is ready to cause the bubble to again explode in its oscillatory scheme. In this manner, the oscillatory energy stored in the water produces several successive secondary pulses of decreasing amplitude until a portion of the energy of oscillation becomes dissipated by natural processes, such as turbulence, and the remaining portion is consumed to produce the undesired secondary pulses. The number of such bubble explosions (expansions) and implosions (contractions) may vary, but typically 4 to 6 significant secondary pulses can be expected after each primary pulse P o which is generated by the seismic source. Hence, a substantial portion of the acoustic energy released by the seismic source goes to waste. This is so because only a portion of the energy contained in the released gas is used to produce the desired primary seismic pulse P o , while the remaining and substantial portion of the energy becomes converted into harmful secondary seismic pulses which must be suppressed. In seismic exploration, both the primary and secondary acoustic pulses act as distinct acoustic disturbances, which travel in the water in all directions, penetrate the earth, strike one or more rock formations or reflectors, and then return into the body of water. The primary and secondary pulses produce reflected seismic wavelets. But, since the secondary pulses and their reflected waves occur at times when the reflected primary waves also return from the subterranean reflectors, it will be apparent that the secondary pulses and their reflected wavelets interfere with the reflected primary waves. Because the reflected secondary waves and the reflected primary waves are similar in shape, no practical way has yet been found for distinguishing between them. The known prior art found neither a practical nor an economical solution for dealing with the "bubble" or secondary pulse problem when using a single explosive-type seismic source. Such secondary pulses are now being attenuated by using large arrays of differently sized airguns. All the airguns in the array are fired simultaneously so that the P o pulses are in phase for addition. Most of the secondary pulses are out of phase and therefore when added they cancel each other out. The secondary-to-primary ratio P₂/P o is the yardstick by which all marine seismic sources are measured as to bubble suppression. An "ideal" source is said to be that source which has a ratio P₂/P o =0 for a frequency range from 0-125 Hz. The extent to which a particular seismic source approaches the ideal seismic source can be readily measured by measuring its P₂/P o ratio. An ideal seismic source produces a single, short, sharp acoustic impulse having sufficient energy and no secondary pulses. Sharp impulses are needed to improve the definition of seismic reflections, because resolution is inversely proportional to the time-width of the impulse: the larger the time-width of the impulse, the less desirable it is. Fired near the water surface, a dynamite charge closely approximates the ideal seismic source, because the bubbles resulting from each explosion are vented immediately into the atmosphere, hence there are no bubble implosions. If not fired near the water surface, explosive seismic sources will produce undesired secondary pulses, unless some form of implosion suppression is utilized. In the past twenty-five years or more many attempts have been made to attenuate the oscillatory secondary pulses and/or to reduce their ill effects. One early mechanical technique attempts to prevent the secondary pulses from traveling vertically downward towards the water bottom by substantially surrounding the gas bubble source with a container or cage having perforations, so that the expanding gas bubble would have to do work in order to force water through the perforations. The work done by the expanding gas bubble dissipates its internal energy. Its ensuing secondary pulses will have reduced amplitudes. This technique was used in a seismic source trademarked FLEXOTIR. A serious limitation inherent in this technique is that the desired primary pulses also become reduced in strength because they can travel freely only through the available perforations. Also, the perforated cage becomes subjected to rapid deterioration, due to the great stresses to which it becomes subjected when large differential pressures become exerted across its wall. Various software programs have been also developed which require the use of expensive computer time and manpower, see US-A-3,592,286. Techniques which are based on injecting air into the expanding bubble for shaping the secondary pulses have produced disappointing results and most of them were abandoned. For example, in US-A-3,371,740, the injection of air during the expansion of the cavity might increase the size of the cavity without reducing the amount of kinetic energy stored in the water. The injected atmospheric pressure is too low and results in a P₂/P o ratio approaching 40%, which is far in excess of the acceptable 10% norm. In US-A-3,454,127, the air injection is started too early during the expansion of the bubble and at a flow rate whcih is subsonic, and therefore it is insufficient to establish hydrostatic pressure inside the cavity within the required time interval. In US-A-3,653,460, Column 5, Lines 1-30, it is shown that the volume of air required to bring the P₂/P o ratio down to 14% is 6.4 times the volume needed to generate the main pulse. US-A-3,804,194 relates to a method for increasing the energy output of a seismic pneumatic energy generator. Instead of using a single compressed air chamber, the arrangement disclosed in this Specification utilises two compressed air chambers. The compressed air chambers are both filled with compressed air, and the second chamber is depressurised in response to depressurisation of the first chamber. Thus the energy from all the compressed air chambers is released substantially simultaneously. The Specification indicates that this arrangement is intended to emit a large amount of acoustical energy in the form of a clear repeatable pulse, and the Specification does not address the problems that arise from secondary pulses. US-A-3,744,018 discloses a method and apparatus of producing a seismic source in water by a combustive explosion. The oscillation of the explosive exhaust gas bubble is reduced, to reduce extraneous seismic impulses. This is achieved by initially injecting an air/fuel mixture of less than the stoichiometric quantity into an ignition chamber, and igniting the mixture to provide a first combustive explosion. After this explosion there is still oxygen present in the exhaust gas. Additional fuel may be injected to provide an additional combustion to increase the pressure in the exhaust gas to dampen the oscillation of the pressure in the gases. The additional fuel is sprayed at a controlled rate over an appropriate period of time, increasing the pressure within the bubble during its expansion to build up pressure therein to prevent contraction of the bubble. The rate and amount of fuel injected is controlled by the surrounding water pressure and the injector characteristics. In any event the fuel injection is such that the bubble pressure is maintained at or above the pressure of the water surrounding the bubble. US-A-3,601,216 discloses a further arrangement in which a high pressure source is utilised, controlled by means of a solenoid valve. The solenoid valve is initially opened to create an air bubble and, after a delay, the air source is opened again, to inject further air into the bubble, in order to minimise secondary pulses. The second pulse of air that is introduced into the bubble is initiated when the bubble has its maximum size, and the duration of the air injection is said to be half the duration of a single cycle of oscillation of the bubble. Only a very small quantity of additional air is injected into the bubble. Due to the inefficiency or impracticability of known bubble suppression techniques, the seismic industry was obliged to employ a "tuned" array of seismic sources. Typically, these sources are airguns of markedly different sizes. When all the airguns in a tuned array, using airguns of different sizes, are fired simultaneously, theoretically the amplitude of the resulting primary pulse of the array will be equal to the sum of the amplitudes of the individual primary pulses generated by the individual acoustic sources in the array, whereas the amplitudes of the secondary pulses will be reduced because (1) they are not in phase, (2) they occur at different times, and (3) they have random frequencies. Nevertheless, the present state of the art in suppressing bubble pulses has been achieved by these "tuned" airgun arrays, which are now widely used. The average characteristics of such a tuned airgun array are: Output: 1 bar-meter per 1 lit of air at 138 bar (peak to peak from 0-125 Hz), and a ratio of P₂/P o of about 1/10 or 10%. Therefore, any source which generates bubble pulses must achieve about the same P₂/P o ratio of 10% or less, peak-to-peak from 0-125 Hz, in order to be competitive in the seismic industry. Even though the array technique is now the standard in this art, it still has serious drawbacks because it is only a composite of individual sources, each source lacking a narrow, sharp acoustic pulse as required. Also, it is very expensive to build such an array because it requires a large number of differently-sized airguns, as well as heavy and expensive air compressors to provide an air volume at about 138 bar, which is the air consumed by the large number of airguns. In general, the known bubble suppression devices have the following drawbacks, taken singly or in combination: they are cumbersome; they require excessive support machinery and space therefore they require very large compressors and excessive energy to run them; they are expensive to maintain and they require an excessive inventory of expensive replacement parts to maintain in operational condition the different sizes of seismic sources now utilised to form arrays. There is thus a need for a practical, economical and effective method for total bubble suppression, and for a seismic source which is bubble-free, which has a substantially flat power spectrum over a relatively broad frequency range, which produces a single sharp acoustic impulse followed by negligible secondary pulses, which has effective bubble suppression means, which is energy-efficient, which lends itself to become easily incorporated into existing marine seismic energy sources and methods of using them without affecting the utility of such known sources and methods, which is simple in design, compact in use, and relatively inexpensive to manufacture and maintain, and in which the volume of air required for aborting the implosion is reduced to a small fraction of what was generally believed to be necessary to achieve adequate bubble suppression, that is, to achieve P₂/P o = 10% or less. According to one aspect of the invention there is provided a method for generating within a body of water an impulsive acoustic signal comprising the steps of generating a first explosion within said body of water to produce therein a powerful pressure pulse and a region of very low pressure, and generating a second explosion in the region wherein the second explosion is such that hydrostatic pressure is established inside said region, while the volume of said region remains at its maximum value within a few percent. Preferably the explosions are generated by high pressure gas charges. Conveniently said first explosion is generated by explosively releasing a first charge of a highly compressed gas to thereby generate within said body of water a powerful acoustic pulse and a first bubble, which comprises said region and said second explosion is subsequently generated by releasing a second charge of a highly compressed gas into said bubble. In one embodiment the region that is established starts to undergo oscillations and said second explosion is initiated at the instant that said region is at its first maximum volume. In another embodiment the region that is established starts to undergo oscillations and the second explosion is initiated at the instant that the region reaches its second maximum volume. Preferably the release of said second charge is started and hydrostatic pressure is established inside said first bubble, while the volume of said first bubble remains within a few percent of its maximum value. Conveniently a subsequent explosion is generated within said region to thereby produce an oscillation of said body of water around said region, and said oscillation generates in said water a low-frequency monochromatic acoustic signal. Advantageously said subsequent explosion is sequentially repeated within said region to thereby gen erate a train of low-frequency monochromatic acoustic signals. The invention also relates to a method of marine seismic exploration comprising the steps of generating, in a body of water, an acoustic signal by a method as described above, and towing a seismic streamer cable through said body of water for detecting the seismic signals produced by said charges. The invention further relates to an acoustic generator comprising a single acoustic source to explosively generate an impulsive acoustic signal in a body of water, said acoustic source including: first means for generating within said body of water a first explosion so as to produce within said body of water a powerful pulse and an expanding region of very low pressure; second means for generating within said region a second explosion and control means wherein the control means are adapted for starting said second explosion so that the second explosion will establish hydrostatic pressure within said region while said region remains at its maximum volume within a few percent. Preferably said signals have a power spectrum substantially flat within 10dB over a frequency range of four octaves. Conveniently the acoustic generator may comprise a signal chamber to contain a first charge of a highly compressed gas; a normally-closed first fast-acting valve means to normally maintain said signal chamber closed and having discharge port means into the water, an injection chamber to contain a second charge of a highly compressed gas; a normally-closed second fast-acting valve means to normally maintain said in jection chamber closed and having outlet means said control means being adapted to explosively open said first valve means so as to release said first charge as said first explosion and thereby generate within said body of water an impulsive acoustic signal comprising said pulse and a first bubble, and subsequently to open said second valve means to thereby release said second charge from the outlet means of said injection chamber and into said bubble as said second explosion while said bubble remains at its maximum volume within a few percent. Preferably said outlet means communicates directly with said water. Alternatively said outlet means establish fluid communication between said injection chamber and said signal chamber, and said first valve means are adapted to release said second charge from said signal chamber through said discharge means and into said first bubble. Preferably the control means is adapted to open the second valve means at the instant that said bubble is substantially at is maximum volume. Conveniently the control means are such that the release of said second charge through said second valve means starts and hydrostatic pressure is established inside said bubble while the volume of said bubble remains at its maximum value within a few percent. Preferably the first valve means is a first airgun, and the second valve means is a second airgun. Advantageously in operation, said outlet means of said second valve is maintained closed when it is unencompassed by said bubble. Preferably a deflector is provided which, in operation, deflects said released second charge from said outlet means toward and into said bubble Conveniently the control means include pneumatic delay means. The acoustic generator may be in the form of a marine acoustic source. The novel preferred method of the invention substantially reduces the pressure pulse generated by an implosion of a cavity within a body of water by aborting the implosion. The abortion of the implosion is produced by generating within the cavity an explosion at the instant that this cavity attains its maximum volume. The explosion is to have sufficiently energy to establish hydrostatic pressure within the cavity. The explosion starts and hydrostatic is established while the volume of the cavity remains within a few percent of its maximum volume. The invention is further described, by way of example, in connection with the accompanying drawings, wherein FIGURE 1 illustrates an oscillating bubble in a body of water, FIGURE 2 is a pressure vs. time signature for Figure 1, FIGURE 3 illustrates the behaviour of bubble 1 when acted upon by bubble 2 in accordance with the invention, FIGURE 4 is a pressure vs. time signature corresponding to Figure 3, FIGURE 5 is an enlarged sectional view of the fast-acting valve in the suppressor shown in Figure 6, FIGURES 6 and 7-11 are sectional views of one embodiment of the invention, showing six operating phases thereof, FIGURES 7A-11A are pressure signatures obtained from the embodiment shown in Figures 7-11, respectively, from the embodiment shown in FIGURES 7-11, respectively; FIGURES 12, 14-19 are sectional views of a second embodiment of the invention, showing six operating phases thereof; FIGURE 13 is a sectional view taken on line 13-13 of FIGURE 12; FIGURES 14A, 15A, 17A-19A are pressure signatures obtained with the embodiment shown in FIGURES 14, 15, and 17-19 respectively; FIGURES 20 and 23-27 are sectional views of a third embodiment of the invention, showing six operating phases thereof; FIGURES 21 and 22 are sectional views taken on lines 21-21 and 22-22 respectively; FIGURES 23A-27A are pressure signatures obtained with the embodiment shown in FIGURES 23-27 respectively; FIGURE 28 is a view partly in section of one deflector embodiment used with the seismic energy source shown in FIGURE 20; FIGURE 29 is a view taken on line 29-29 in FIGURE 28, and FIGURE 28 is a sectional view taken on line 28-28 in FIGURE 29; FIGURE 30 is a view partly in section of a second deflector embodiment used with the seismic energy source shown in FIGURE 20; FIGURE 31 is a view taken on line 31-31 in FIGURE 30, and FIGURE 30 is a sectional view taken on line 30-30 in FIGURE 31; FIGURE 32 is a view partly in section of a third deflector embodiment used with the seismic energy source shown in FIGURE 20; FIGURE 33 is a view taken on line 33-33 in FIGURE 32, and FIGURE 32 is a sectional view taken on line 32-32 in FIGURE 33; FIGURES 34 and 35 are pressure vs. time and power spectrum vs. frequency plots, respectively, of signal generator 10 operating alone; FIGURES 36 and 37 are pressure vs. time and power spectrum vs. frequency plots, respectively, of signal generator 10 operating together with generator 10`a; FIGURE 38 illustrates consecutive explosions spaced 10 seconds apart and generated with generator 10 only; FIGURE 39 shows consecutive pressure signatures obtained from consecutive explosions shown in FIGURE 38; FIGURE 40 illustrates consecutive double explosions also spaced 10 seconds apart but generated with generators 10 and 10'a following the method of this invention; and FIGURE 41 shows consecutive pressure signatures obtained from the consecutive double explosions shown in FIGURE 40. Conventional methods for operating explosive marine seismic sources involve generating a first explosion which releases at a depth D in a body of water, at a time T o (FIGURE 1), a first bubble 1 having a gas G1, a pressure Pa, and a volume Va. The pressure inside bubble 1 varies relative to the hydrostatic pressure, as bubble 1 undergoes cyclic implosions (contractions) and explosions (expansions) at an oscillatory period T. The pressure signature (FIGURE 2) exhibits a desired pressure peak P o as well as undesired positive (P2, P4,....) secondary pressure pulses of decreasing amplitude. It has been suggested that prior to the time that bubble 1 reaches its maximum diameter at T/2, if the pressure inside bubble 1 were raised by an injection of gas, then the amplitudes of the secondary pressure pulses, resulting from the subsequent implosions and explosions of bubble 1, would be substantially reduced, as compared to the amplitudes of the secondary pulses which would be produced without such gas injection. I found this general belief to be incomplete and inaccurate. I have unexpectedly discovered why prior art workers in this art have failed to fill the long-existing need for single explosive marine seismic source, which is characterized by a secondary-to-primary P₂/P o ratio of about 10% or less (FIGURE 4). I have unexpectedly discovered theoretically and experimentally that the secondary pressure pulse generated by the implosion increases very rapidly at the early stage of the collapse of the cavity. For instance, if one wants to maintain the secondary to primary ratio P₂/P o within 10%, one must not allow the radius of the cavity to shrink by more than 1%, or the volume of the cavity to shrink by more than about 3%. I have unexpectedly discovered that what is actually required is: no air injection while the bubble is expanding, and an explosive injection of air only when the bubble is within a few percent of its maximum size. Using my method the injection volume of air required to bring the P₂/P o ratio down to less than 10% is only 2.1 times (as compared to 6.4, as previously mentioned) the volume of air needed to generate the main pulse P o, which constitutes a reduction by a factor of 3.05=6.4/2.1 or more in the volume of compressed air needed for adequate bubble suppression. Surprisingly, I have discovered that the timing and the duration of the required air injection into the bubble must be extremely short. I have found that the gas injection into the bubble must be explosive and that bubble 1 must be stabilized very near to its maximum size, which means that one must abort or suppress the first implosion by making an explosive air injection before the first implosion shrinks the maximum diameter of bubble 1 by more than 1%. The bubble's maximum diameter is that diameter which bubble 1 would attain without such air injection (FIGURE 1). Thus, I generate two consecutive explosions within milliseconds of each other to obtain an ideal seismic impulse similar to that obtained from firing an explosive near the water surface and allowing the bubbles to vent directly to the atmosphere before they can implode. I generate a timely and sufficient suppression explosion (FIGURE 3) which impulsively injects a bubble 2 into bubble 1 during a short time interval Ti. Bubble 2 is formed by a gas G2 having a pressure Pb and a volume Vb. For convenience Pb is made equal to Pa, where Pa is the pressure of gas G1 which was previously used in the first explosion to generate bubble 1. I further found that the volume Vb of gas G2 (FIGURE 3) required for explosively injecting into bubble 1 is only a fraction of the gas volume that was generally believed to be necessary to achieve adequate bubble suppression. Operating Conditions It is now possible with my second explosion, which produces bubble 2, to completely abort the undesired oscillatory secondary pressure pulses (FIGURE 4) resulting from the first explosion, if certain very critical conditions are satisfied: 1. the explosive injection time interval Ti (FIGURE 3) must be equal to or less than .2T, where T is the period of oscillation of bubble 1, as given by the detected acoustic signal, which is known in the art as the "pressure signature"; 2. Ti must start after .4T and end before .6T; and 3. the second explosion must have sufficient energy to establish hydrostatic pressure within bubble 1. Ti should optimally be centered about T/2 (i.e., when bubble 1 reaches its maximum volume). Under optimum conditions P₂/P o =0, and the second explosion will stop and completely abort the first implosion (FIGURE 3). The optimum value for Vb in order to achieve P₂/P o less than 10%, is Vb=kva, where k is a number in the range between 1.8 and 2.6. My acoustic marine source will be illustrated by embodiments A, B, and C which have common elements that will be designated with the same reference characters, to simplify the description. Similar elements will be designated with a prime (') whenever possible. Each one of my embodiments A-C has an explosive signal generator 10 and an explosive suppressor generator 10', which can be the same as or different from generator 10. I can employ, for example, a conventional explosive signal generator 10 (FIGURE 6) for generating at a predetermined and sufficient depth within a body of water a signal explosion, which produces at time T o bubble 1 and a desired primary pulse P o (FIGURES 3,4,7A). In the absence of my suppression explosion, bubble 1 would undergo a series of implosions and explosions (FIGURE 1) at an oscillating period T, which would result in the undesirable secondary peaks (P2, P4...) of decreasing amplitude (FIGURE 2). To abort the first implosion, I employ an explosive suppressor generator 10' which generates a counter explosion inside bubble 1 within a time interval less than 0.2T and which satisfies the above critical conditions. Explosive signal generator 10 can be a commercially available airgun, such as the one manufactured under the trademark PAR, which is fully described in U.S. Patent 3,379,373. Such an airgun has a signal chamber 14 whose volume Va is charged up with pressurized air G1. Chamber 14 communicates directly with an explosive shuttle valve 12 that can be actuated to explosively discharge the pressurized air G1 from signal chamber 14 into the surrounding body of water through discharge ports 42. Valve 12 includes a main piston 70 engageable with a seal 31 for retaining a charge of pressurized gas G1 within signal chamber 14, and a control piston 72 engageable with a seal 71 for controlling the movements of piston 70. Pistons 70 and 72 are held together, in spaced parallel relation, by a hollow shaft 70' having an axial bore 33 therethrough. A compressor on the deck of the seismic vessel (not shown) supplies air pressure to input line 22 at 138 bar, which is fed to a control or return chamber 32 from which it passes through a metering orifice 44 and axial bore 33 into signal chamber 14. The actuation of valve 12 is controlled by a controller such as a solenoid-operated valve 20, which is energized periodically by a short electric pulse produced by a conventional electronic actuating network (not shown) located on the deck of the seismic vessel. The firing of explosive generator 10 is periodically repeated as dictated by the firing cycle of the seismic survey. When solenoid valve 20 is fired, pressurized gas flows from a line 22 through a trigger passage 39 leading to the opposite surface of control piston 72 from that facing control chamber 32. Thus, the holding force of the air pressure in control chamber 32 becomes instantaneously offset, allowing the pressurized gas G1 in the signal chamber 14 to suddenly accelerate main piston 70 away from its seal 31, thereby suddenly opening the discharge ports 42 and allowing them to communicate directly with signal chamber 14. Then, the pressurized gas G1 from signal chamber 14 is explosively released through discharge ports 42 into the surrounding water wherein it generates a long acoustic seismic wavelet or pressure signature (FIGURE 2) having the desired acoustic primary pulse Po, which is followed by the undesired positive (P2,P4...) pressure pulses. After the discharge of gas G1 from signal chamber 14, the pressure in control chamber 32 returns shuttle 12 to its closed position, and generator 10 is ready for a new cycle. I. Source A. With general reference to FIGURES 5-11 of the drawings, my novel single point acoustic marine source A comprises, in addition to the explosive signal generator 10, which can be the PAR airgun as described above, an explosive suppressor generator 10'a having a suppression chamber 14'a which contains a volume Vb of gas G2. For the sake of economy Pb=Pa. Generator 10'a is fixedly secured to and is axially aligned with generator 10. Generator 10'a also has an explosive shuttle valve 12'a which, upon actuation, explosively discharges the gas G2 from suppression chamber 14'a into chamber 14 and thence into bubble 1 surrounding discharge ports 42 of signal generator 10. The high-pressure air from signal chamber 14 (FIGURES 5-6) is transmitted, through axial bore 18 in valve 12'a and through a metering orifice 19, to a delay chamber 36 which keeps explosive valve 12'a closed. When explosive generator 10 is ready to become fired, signal chamber 14 has 138 bar, and valve 12'a is being kept closed by the 138 bar pressure in delay chamber 36. Pneumatic means delay the release of compressed gas G2, after the discharge of gas G1, by a time delay interval Td depending upon the period T of the oscillatory characteristics of bubble 1. In operation, explosive generator 10 (FIGURES 5,7) is fired at T o (FIGURE 4), when solenoid 20 becomes energized by an electric pulse signal arriving on line 11, which causes valve 12 to explosively discharge gas G1 from signal chamber 14 through ports 42 into the surrounding water. After chamber 14 is discharged, the 138 bar air in delay chamber 36 gradually flows across orifice 19 into chamber 14 until the pressure in chamber 36 drops to about 13.8 bar, at which time valve 12'a explosively opens and releases gas G2 from suppression chamber 14'a into channel 34, and thence into signal chamber 14 and outwardly through the discharge ports 42 of explosive valve 12. After about 60 ms, both signal chamber 14 and suppression chamber 14'a start to repressurize to become ready for another cycle. At about 500 ms, explosive valve 12'a closes. Signal chamber 14 always pressurizes at the same rate, whereas chamber 14'a charges through throttling orifice 17, which has to be large enough to allow chamber 14'a to become pressurized within the given firing cycle for signal generator 10, but orifice 17 also has to be sufficiently small to prevent releasing pressurized air from suppression chamber 14'a through chamber 14 into bubble 1 between the time interval 0-35 ms. Detailed Description Of One Operating Cycle The pressure in a particular volume or space "S" will be designated by "Ps" to simplify the description. Thus, for example, P14 means the pressure in the space defined by chamber 14. PHASE 1. Generator 10 Ready To Fire. Solenoid valve 20 (FIGURE 6) is closed. Shuttle 12 is forced downward against ring 31 and seals signal chamber 14. Source A is pressurized: P32 = P33 = P14 = P34 = P35 = P36 = P14'a= P37=138 bar. P38 = P39 = Ph (hydrostatic pressure)=1.2 bar. Valve 12'a is in its up position and seals suppression chamber 14'a with an upward force. F = 138 bar x (S2 - S3) Volumes Signal chamber 14 has a volume Va=V14 + V34 + V35 + V33 =.74 lit. Suppression chamber 14'a has a volume Vb=V14'a =1.56 lit. Vb/Va=2.1 These volumes can be changed with differently-sized inserts such as 40 and 41. PHASE 2. Generator 10 Exploded And P o Generated At t = 0 (FIGURES 7,7A). Firing of explosive generator 10 is initiated by energizing solenoid valve 20 with a short electric pulse on line 11. Shuttle 12 moves up explosively to allow the compressed air G1 (138 bar) from signal chamber 14 to discharge explosively through ports 42 into the surrounding water and to form therein bubble 1. The explosive release of air G1 from chamber 14 generates the desired primary acoustic pulse P o , as shown on the pressure signature (FIGURE 7A). Bubble 1 continues to expand (FIGURE 3). PHASE 3. Bubble 1 Is Close To Its Maximum Size At t = 30 ms (FIGURES 8,8A). Bubble 1 is close to its maximum size. Pressure inside bubble 1 is much less than the hydrostatic pressure; actually bubble 1, at this instant, can be considered a vacuum cavity. A negative pulse P1 appears on the pressure signature at T/2. The compressed air from delay chamber 36 (V = 24 cm³) is throttled across orifice 19, whose diameter is 4 mm. Because the pressure in delay chamber 36 is still high enough to maintain explosive shuttle valve 12'a in its closed position, explosive suppressor generator 10'a is still in its stand-by mode. PHASE 4. Generator l0'a Generates An Explosion Inside Bubble 1 At t = 35 ms (FIGURES 9,9A). When pressure in delay chamber 36 drops to about 13.8 bar, the force against explosive valve 12'a reverses. Valve 12'a starts moving downwardly. The downward force on valve 12'a instantly becomes: F = P14'a (S4 - S2) + P37 (S3) - P36 (S4), or approximately F = P14'a (S1 - S2) = 138 bar x (S1 - S2). Because of its very light weight, explosive shuttle valve 12'a is fully open within about 2 ms, allowing gas G2 from suppression chamber 14'a to explosively discharge and in effect form a second bubble 2 inside bubble 1 (FIGURE 3). Gas G2 flows through signal chamber 14 and through discharge ports 42 into bubble 1. Bubble 2 has a volume of air Vb which is selected to establish hydrostatic pressure inside bubble (1+2) when pressure equilibrium is reached therein. The acoustic pressure signal shows an upward vertical step on the pressure signature. PHASE 5. Hydrostatic Pressure Established Inside Bubble At t = 40 ms (FIGURES 10-10A). The amplitude of the acoustic signal is near zero. Valve 12'a remains open because metering orifice 43 (FIGURE 5) prevents the pressure in chamber 37 from dropping too quickly. The downward force on valve 12'a of explosive generator 10'a is: F = 97 bar x (S3), approximately. Because of the 138 bar pressure in return chamber 32, Shuttle valve 12 of explosive generator 10 will move downwardly and seal against ring 31. PHASE 6. Bubble 1 Is Stabilized And Oscillates Gently Around Its Equilibrium Position The implosion of bubble 1 has been aborted. Bubble (1+2) is stabilized and its boundary (FIGURES 11-11A) undergoes low-amplitude, low-frequency oscillations which generate a very low-amplitude, low-frequency acoustic wave. Signal chamber 14 is again pressurized through orifice 44. When the pressure in delay chamber 36 (FIGURE 5) of valve 12'a rises to about 69 bar, valve 12'a will move up to again seal off suppression chamber 14'a. Gas G2 will again charge up chamber 14'a through throttling orifice 17 having a diameter of 2.5 mm. The upward force on valve 12'a becomes F = P36 (S4) - P35 (S4 - S2) - P37 (S3). Chamber 37 is pressurized through orifice 43 whose diameter is 1.5 mm. Source A is now ready for a new cycle. Detailed Description Of Explosive Shuttle Valve 12'a Explosive valve 12'a (FIGURES 5-6) is entirely pneumatically operated and has a body 23 which defines three air chambers 14'a, 36, and 37, and one water chamber 38. The body 23 also defines bores 25, 26 and sealing seat 62. Pistons 29, 30, 61 and 63 form shuttle valve 12'a. The bottom end cover 24 defines a bore 27 in which piston 63 slides. Pistons 29, 30 and 63 are slidably and sealingly mounted inside bores 25, 26 and 27, respectively. Suppression chamber 14'a continuously communicates with signal chamber 14 through bores 34, 35, 16, and metering orifice 17. However, when shuttle 12'a is in its open or lower position, chamber 14'a also communicates through channel 64 with annular chamber 35. When shuttle 12'a is at its lower position, its piston 61 disengages from seat 62, which allows channel 64 to freely communicate with suppression chamber 14'a and with signal chamber 14. Delay chamber 36 continuously communicates with chamber 35 through radial bore 28, axial bore 18, and orifice 19. Air chamber 37 continuously communicates with suppression chamber 14'a through orifice 43 and a line 65. When shuttle 12'a is in its closed or upper position, its piston 61 sealingly engages seat 62, thereby sealing off suppression chamber 14'a from chamber 35 and from bore 34, except for the small orifice 17 in line 16. Operation Of Shuttle Valve 12'a. At the start, i.e., prior to t=0 (FIGURE 4), volumes 34, 35, 14'a, 36 and 37 are pressurized to 138 bar. Water chamber 38 is filled with water at hydrostatic pressure. Forces exerted on shuttle 12'a push piston 61 upward against its seal 62. Channel 64 is closed. After explosive signal generator 10 is fired, air pressure in volumes 34 and 35 decreases very quickly and the upward force increases on piston 61 against its seal 62, while pressure in delay chamber 36 decreases slowly because its air is throttled across orifice 19 into lines 18 and 28. At the same time, some air escapes from suppression chamber 14'a into chambers 35 and 34 through orifice 17. Because chamber 14'a has a relatively large volume Vb, the pressure in chamber 14'a does not decrease substantially. When the pressure in delay chamber 36 drops to about 13.8 bar, the force on shuttle 12'a is downward. Then the airtightness between piston 61 and its seal 62 breaks. Pressure decreases in chamber 14'a and increases in chamber 35, resulting in a strong downward force on shuttle 12'a, which opens almost instantaneously because of its light weight. Shuttle 12'a is preferably made from a suitable light plastic material. The air from chamber 14'a escapes through volumes 64, 35, 34, 14, and discharge ports 42. Shuttle 12'a is maintained in its open position by the pressure trapped inside chamber 37. In fact, air escapes very slowly from chamber 37 through line 65 and is throttled by orifice 43. But, the pressure in chamber 37 never drops below 83 bar. When shuttle valve 12 in signal generator 10 seals off signal chamber 14, the pressurization of source A starts. Shuttle valve 12'a is at this time still open, so that air from chamber 14 flows out into channel 64 through line 16, and is throttled across orifice 17. In the meantime, pressure increases in delay chamber 36 through passages 28, 18, and orifice 19. Air from chamber 37 continues to throttle across orifice 43 into chamber 14, as long as the pressure inside chamber 37 is greater than the pressure in chamber 14'a. When the pressure in delay chamber 36 increases to about 69 bar, shuttle 12'a moves up and its piston 61 is forced against its mating seal 62, which seals off chamber 14'a. At this time, suppression chamber 14'a is charged only through bore 16 and metering orifice 17. When the gas G2 in chamber 14'a is at about 90 bar, air is throttled across orifice 43 and flows out through line 65 into chamber 37. After about 5 seconds (for a 1.56 lit chamber 14'a), all the air volumes are again pressurized to 138 bar. Source A becomes ready for a new cycle. II. Source B Source B (FIGURES 12-19) includes a similar explosive signal generator 10 as in source A, but a different explosive suppressor generator 10'b. A deflector 15 removably couples them together. The cylindrical housing 73 of deflector 15 defines signal chamber 14 as well as four outlet ports 66 (FIGURES 12, 16), which preferably make a 60° angle with the vertical or longitudinal axis and preferably are angularly aligned with discharge ports 42. Signal chamber 14 of generator 10 is charged up from inlet line 22. The explosive generator 10'b is more fully described in French patent 2,158,730. Housing 74 of generator 10'b defines a suppression chamber 14'b which is filled from inlet 22'. Valve 12'b slides on a piston 52 and is pneumatically operated through inlet lines 57 and 58 which are at different pressures. Inlet 57 supplies the triggering chamber 54 with 69 bar, and inlet 58 supplies the return chamber 56 with 5 bar. At the start, a pulse signal is sent on line 11 to solenoid 20 which allows valve 12 to open explosively. A delayed electric signal is then sent to solenoid valve 20' on line 11' inside conduit 58. The delay is 35 ms. When activated, valve 12'b opens to allow air G2 from suppression chamber 14'b to become explosively released into bubble 1 through ports 66 of deflector 15. Chamber 14 is repressurized after 60 ms, and chamber 14'b is repressurized after 1 second. The optimum ratio Vb/Va is equal to about 2.1 and it can have a range between 1.8 and 2.6, where Vb is the volume of chamber 14'b, and Va is the volume of chamber 14. Detailed Description Of One Operating Cycle. PHASE 1. Generator 10 Ready To Fire Generator 10 (FIGURES 12, 13) and generator 10'b are armed. Solenoid valves 20 and 20' are closed. Shuttle valves 12 and 12'b seal off respectively signal chamber 14 and suppression chamber 14'b. Source B is pressurized: P14 = P33 = P32 = P14'b = 138 bar. P39 = P55 = Ph (hydrostatic pressure)=1.2 bar. P56 = 5 bar. P54 = 69 bar. Volumes: Va=14 + 33 =.74 lit. Vb= 14'b =1.56 lit. PHASE 2. Generator 10 Exploded And P o Generated At time t = 0 (FIGURES 14, 14A). Firing is initiated by energizing solenoid 20 of generator 10 with a short electric pulse on line 11. Shuttle 12 moves up explosively to allow the compressed gas G1 from signal chamber 14 to discharge through ports 42 into the surrounding water. This explosive air release generates the primary acoustic pulse P o on the pressure signature. Bubble 1 is expanding (FIGURE 3). PHASE 3. Bubble 1 Is Close To Its Maximum Size At t = 30 ms (FIGURES 15, 15A). Bubble 1 is close to its maximum size. Pressure inside bubble 1 is much less than hydrostatic pressure; actually the bubble, at this instant, can be considered as a vacuum cavity. A negative pulse P1 appears on the pressure signature. Generator 10'b is still in stand-by. PHASE 4. Generator 10'b Explodes-Gas G2 Injected Into Bubble 1 At t = 35 ms (FIGURES 16, 17, 17A). After a delay of 35 ms, depending upon the volume Va of chamber 14, water depth, and/or firing air pressure in chamber 14'b. Solenoid 20' is energized via line 11'. The gas G2 in triggering chamber 54 is sent to chamber 55 through solenoid valve 20'. The force on shuttle 12'b reverses, causing it to explosively move downwardly. The gas G2 from suppression chamber 14'b is explosively projected into bubble 1 through deflector ports 66 which are encompassed thereby. The acoustic pressure signal shows an upward vertical step on the pressure signature. PHASE 5. Hydrostatic Pressure Established Inside Bubble At t = 40 ms (FIGURES 18, 18A). Vb is selected so that it can establish hydrostatic pressure inside bubble 1 at this time. The amplitude of the acoustic signal is near zero. Shuttle 12 of generator 10 moves downwardly and seals against ring 31, because of the 138 bar pressure in return chamber 32. PHASE 6. Bubble 1 Is Stabilized And Oscillates Gently Around Its Equilibrium Position The implosion of bubble 1 has been aborted. Bubble 1 is stabilized and its boundary undergoes low amplitude oscillations (FIGURES 19, 19A) which generate a negligible low-amplitude acoustic signal. Shuttle valve 12'b recocks because of the pressure in return chamber 56. Pressure in triggering chamber 54 is vented outside through a small orifice (not shown). Source B is repressurized and made ready for another cycle. III. Source C Source C comprises an explosive signal generator 10 and an explosive suppressor generator 10'c which can be two PAR airguns such as described in U.S. patent No. 3,379,373. A gas deflector 13 is also provided. If source C is operated at a low hydrostatic pressure, then bubble 1 will have a very large volume. On the other hand, if source C is submerged at greater depths, the volume of bubble 1 will be much smaller. In the shallow depths, the volume of bubble 1 can be so large that it will encompass the discharge ports 42' of explosive generator 10'c. One the other hand, in deeper waters, bubble 1 may not encompass ports 42' of explosive generator 10'c. When discharge ports 42' of explosive generator 10'c are not encompassed by bubble 1, bubble suppression may not be achieved. Because bubble 1 sometimes may and sometimes may not encompass ports 42', I channel the discharged compressed air into bubble 1 through a deflector 13. This deflector 13 makes source C independent of depth variations. Thus, deflector 13 has a double function: to act as a gas deflector, and to detachably couple suppressor generator 10'c to signal generator 10. Bubble 1 produced by signal generator 10 is in fact not a sphere: it has four well-known lobes (not shown). I have discovered that impulsively injecting a large quantity of gas G2 uniformly and simultaneously into the four lobes of bubble 1 is a critical operation. This can be accomplished with deflector 13 which injects the gas G2 into the individual lobes by directing each air jet into a corresponding lobe of bubble 1. During the first cycle, a pulse signal on line 11 energizes solenoid 20 of explosive generator 10. Explosive shuttle valve 12 moves up and allows the pressure from signal chamber 14 to discharge explosively through ports 42 into the outside water to produce a primary pulse P o . A delayed second pulse is sent to solenoid 20' on line 11' which actuates explosive valve 12'c, causing air pressure from suppression chamber 14'c to discharge through the deflector's four outlet ports into their corresponding lobes of bubble 1. After 60 ms, valve 12 closes, and signal chamber 14 starts to recharge from line 22. Explosive valve 12'c closes after about 95 ms and then suppression chamber 14'c can again receive compressed air from line 22'. Detailed Description Of One Operating Cycle PHASE 1. Generator 10 Is Ready To Fire Generator 10 (FIGURES 20, 22) and generator 10'c are armed. Solenoid valves 20 and 20'c are closed. Shuttles 12 and 12'c seal respectively signal chamber 14 and suppression chamber 14'c. Source C is pressurized: P32 = P14 = P14'c = P32' = 138 bar. Volumes: Va = V14 =.74 lit. Vb = V14'c =1.56 lit. Vb/Va = 2.1. PHASE 2. Generator 10 Exploded And P o Generated At t = 0 (FIGURES 23, 23A). Firing of explosive generator 10 is initiated by energizing solenoid valve 20 with a short electric pulse on line 11. Shuttle 12 moves up and allows the air from signal chamber 14 to explosively discharge through ports 42 into the surrounding water. Bubble 1 and pulse P o are generated. The geometry of bubble 1 is centered about ports 42 of explosive generator 10. Bubble 1 has four lobes: one lobe for each discharge port 42. Bubble 1 is expanding (FIGURE 3). PHASE 3. Bubble 1 Is Close To Its Maximum Size At t = 30 ms (FIGURES 24, 24A). Bubble 1 is close to its maximum size. Pressure inside bubble 1 is much less than hydrostatic pressure; actually bubble 1, at this instant, can be considered as a vacuum cavity. A negative pressure pulse P1 appears on the pressure signature. Generator 10'c is still in stand-by. PHASE 4. Generator 10'c Injects Gas G2 Into Bubble 1 At t = 35 ms (FIGURES 25, 25A). Solenoid 20' of generator 10'c is energized by an electric pulse on line 11'. The gas G2 contained in volume Vb of chamber 14'c is explosively injected into the vacuum cavity of bubble 1. The outlet ports of deflector 13 channel the discharged air from chamber 14'c toward corresponding lobes of bubble 1. The acoustic pressure signal shows an upward vertical step. PHASE 5. Hydrostatic Pressure Inside Bubble 1 At t = 40 ms (FIGURES 3, 26, 26A). Vb is selected so that it will establish hydrostatic pressure inside bubble 1. The amplitude of the acoustic signal is near zero (FIGURE 26A). Shuttle valve 12 has moved to seal off its chamber 14. PHASE 6. Bubble 1 Is Stabilized And Oscillates Gently Around Its Equilibrium Position The implosion of bubble 1 has been aborted. Bubble 1 is stabilized (FIGURES 3, 27, 27A) and its boundary undergoes low-amplitude oscillations which generate a negligible low-amplitude, low-frequency acoustic wavelet. Shuttle valve 12'c has moved to seal off its chamber 14'c. Generator 10 and generator 10'c are again pressurized up to 138 bar and made ready for another cycle. Deflector Shapes Deflector 13 can be made in different shapes (FIGURES 28-33). For example, it can be shaped to have four circular outlet ports 53 (FIGURES 28-29), or semi-circular ports 59 (FIGURES 30-31), or a single conical port 68 (FIGURES 32-33). An annular chamber 51 collects the air discharged by the four ports 42' of generator 10'c. Deflector 13 is preferably designed in two symmetrical parts 13' and 13'' and is also provided with a clamp 66 to replace two pairs of half flanges, which conventionally serve to clamp upper and lower sub-assemblies of generator 10'c. The outlet ports 53, 59, or 68 are pointed in a direction so as to project the discharged gas G2 toward respective discharge ports 42 of signal generator 10. Each outlet port has a small orifice 67 (10 cm diameter) to prevent air from becoming trapped inside the port and to ensure that water damping will exist at the end of each stroke of shuttle 12'c. Advantages 1. Extended Power Spectrum - Single Point Source In addition to having advantages in the pressure-time domain, my novel acoustic source also has very important advantages in the power spectrum-frequency domain. By measuring the pressure signature with a detector positioned close to the source, for example one meter below the source, it is possible to construct a pressure-time signature for known generator 10 operating alone (FIGURE 34) and for my source (A, B, or C) operating in conjunction with a suppressor generator 10'a, 10'b, or 10'c (FIGURE 36), as well as their corresponding power spectra for generator 10 (FIGURE 35) operating alone and for generators 10 and 10' (FIGURE 37) operating consecutively in accordance with this invention. It will be observed from a detailed comparison of the pressure plots (FIGURES 34,36) and power spectra plots (FIGURES 35,37) that: the uneven power spectrum (FIGURE 35) for the known generator 10 is such that at some frequencies the amplitude of the spectrum is maximum, while for other frequencies, in the useful range, the amplitude of the spectrum is near zero; the power spectrum (FIGURE 37) for my acoustic source extends from 6 Hz to 125 Hz and is relatively smooth over the entire useful frequency range, resulting in better resolution and signal-to-noise ratio; my acoustic source can be considered a point source because it produces a single, sharp, narrow-width pressure impulse P o and a relatively flat power spectrum curve which extends from 6 Hz to 125 Hz; the high frequency content of my emitted single pressure impulse P o is very useful to the geophysicists for improving the discrimination of closely-spaced events and for providing better resolution at all depths; the low-frequency content of my emitted impulse is extremely important to the geophysicists because the absorption of the earth varies with the exponential of the frequency, i.e., the lower the frequency the less the earth will absorb the seismic signals and the deeper the penetration will be; and the peak of the power spectrum of my source is positioned at a much lower frequency, as compared to the position of the peak of the power spectrum for the known generator 10 operating without my suppressor generator 10'. This shift in the position of the spectrum peak toward a lower frequency is also of great benefit to the geophysicists. 2. Arrays Because they are not point sources, conventional acoustic sources that generate secondary pulses have to be deployed in tuned arrays which utilize sources of different sizes in order to reduce the amplitudes of the individual bubble trains in the vertical and orthogonal planes. In other directions, and particularly the horizontal planes, such arrays detune, while in certain other directions the bubble energy will become additive. In contrast, my single-point, high-energy source can be used alone or several identical sources can be used to construct a very efficient tapered array. As distinguished from known sources which produce wavelets, no tuning and no bubble cancellations are required when an array is built using a number of my identically-sized acoustic sources. The signature produced by my array will have the same shape as the pressure signature of the individual sources except for amplitude. There will be a linear summation of the individual impulse amplitudes (bar-meters), and the composite energy of the array will become proportional to n² where n is the number of my sources used. Because the shapes of the pressure signature and of the power spectrum of my array produced by a plurality of my identical sources are the same as those of the individual sources forming the array, except for amplitude, if one source becomes inoperative in my array, no change will occur either in the shape of the pressure signature or in the shape of the power spectrum of the array, except for a change in amplitude. Since the sharp impulses emitted by my sources are of minimum phase, processing of the gathered seismic data is greatly simplified. Being able to use identical seismic sources of my design in order to construct a tapered array will greatly simplify their maintenance and will alleviate the inventory problem associated with carrying a wide range of spare parts on the seismic vessel. Because the breakdown of a single conventional airgun in a conventional array will detune the array and cause a substantial change in the shape of the pressure signatures produced by the array, there is a need when using a conventional array to stop the seismic vessel, pull out the defective array from the water, replace the defective airgun or airguns, replace the array in the water, and then continue with the seismic survey, all of which is very costly to the user of the seismic vessel. The loss of one or more of my acoustic sources in my array will not detune the array and the seismic survey can continue without stopping. 3. Practical Considerations Thus, my acoustic impulse source can make use of conventional seismic generators such as airguns, and can be operated in conjunction with already existing seismic vessels, without requiring any major modifications thereto. The cost of constructing and operating my novel energy sources is only a fraction of the corresponding costs involved in constructing and operating arrays using conventional inefficient seismic energy sources, or such systems as are described in the illustrative patents listed in the background section of this specification. The energy needed to operate my source 10 is relatively small compared to the energy consumed by the prior art arrays constructed from known sources. Even when my source has to operate under conditions which are not optimum, bubble 1 will oscillate about a mean value and will generate secondary pulses but only of relatively low frequency and amplitude and, therefore, these are insignificant to the seismic record.

Claims: A method for generating within a body of water an impulsive acoustic signal comprising the steps of generating a first explosion within said body of water to produce therein a powerful pressure pulse and a region of very low pressure, and generating a second explosion in the region characterised in that the second explosion is such that hydrostatic pressure is established inside said region, while the volume of said region remains at its maximum value within a few percent. A method according to Claim 1 wherein the explosions are generated by high pressure gas charges. A method according to Claim 2 wherein said first explosion is generated by explosively releasing a first charge of a highly compressed gas to thereby generate within said body of water a powerful acoustic pulse and a first bubble, which comprises said region and said second explosion is subsequently generated by releasing a second charge of a highly compressed gas into said bubble. A method according to Claim 1 or 2 or 3, wherein the region that is established starts to undergo oscillations and said second explosion is initiated at the instant that said region is at its first maximum volume. A method according to Claim 1 or 2 or 3, wherein the region that is established starts to undergo oscillations and the second explosion is initiated at the instant that the region reaches its second maximum volume. A method according to Claim 3, wherein the release of said second charge is started and hydrostatic pressure is established inside said first bubble, while the volume of said first bubble remains within a few percent of its maximum value. A method according to any one of the preceding Claims wherein a subsequent explosion is generated within said region to thereby produce an oscillation of said body of water around said region, and said oscillation generates in said water a low-frequency monochromatic acoustic signal. A method according to Claim 7 wherein said subsequent explosion is sequentially repeated within said region to thereby generate a train of low-frequency monochromatic acoustic signals. A method of marine seismic exploration comprising the steps of generating, in a body of water, an acoustic signal by a method according to any one of Claims 1 to 7, and towing a seismic streamer cable through said body of water for detecting the seismic signals produced by said charges. An acoustic generator comprising a single acoustic source (A, B, or C) to explosively generate an impulsive acoustic signal in a body of water, said acoustic source including: first means(10) for generating within said body of water a first explosion so as to produce within said body of water a powerful pulse (P o ) and an expanding region of very low pressure(1); second means(10') for generating within said region(1) a second explosion and control means (19, 36 or 20') characterised in that the control means are adapted for starting said second explosion so that the second explosion will establish hydrostatic pressure within said region(1) while said region(1) remains at its maximum volume within a few percent. An acoustic generator according to Claim 10, wherein said signals have a power spectrum substantially flat within 10dB over a frequency range of four octaves. An acoustic generator according to Claim 10 or 11 comprising a signal chamber(14) to contain a first charge (Va) of a highly compressed gas (G1); a normally-closed first fast-acting valve means(12) to normally maintain said signal chamber closed and having discharge port means(42) into the water, an injection chamber(14') to contain a second charge (Vb) of a highly compressed gas (G2); a normally-closed second fast-acting valve means(12') to normally maintain said injection chamber(14') closed and having outlet means(34), said control means(20, or 19, 36 or 20') being adapted to explosively open said first valve means(12) so as to release said first charge (Va) as said first explosion and thereby generate within said body of water an impulsive acoustic signal comprising said pulse (P o ) and a first bubble(1), and subsequently to open said second valve means(12') to thereby release said second charge (Vb) from the outlet means(34, 66 or 42 ) of said injection chamber(14') and into said bubble(1) as said second explosion while said bubble remains at its maximum volume within a few percent. An acoustic generator according to Claim 12 wherein said outlet means(66 or 42') communicates directly with said water. An acoustic generator according to Claim 12 wherein said outlet means(34) establish fluid communication between said injection chamber(14') and said signal chamber(14), and said first valve means(12) are adapted to release said second charge (Vb) from said signal chamber(14) through said discharge means(42) and into said first bubble(1). An acoustic generator according to any one of Claims 12 to 14 wherein the control means is adapted to open the second valve means(12') at the instant that said bubble(1) is substantially at is maximum volume. An acoustic generator according to any one of Claims 13 to 15, wherein the control means are such that the release of said second charge (Vb) through said second valve means(12') starts and hydrostatic pressure is established inside said bubble(1) while the volume of said bubble remains at its maximum value within a few percent. An acoustic generator according to any one of Claims 12 to 16, wherein the first valve means(12) is a first airgun, and the second valve means(12') is a second airgun. An acoustic generator according to any one of Claims 12 to 17, wherein, in operation, said outlet means(66) of said second valve(12') is maintained closed when it is unencompassed by said bubble. An acoustic generator according to any one of Claims 12 to 18, wherein a deflector(13) is provided which, in operation, deflects said released second charge (Vb) from said outlet means(42') toward and into said bubble(1). An acoustic generator according to any one of Claims 10 to 19, wherein the control means include pneumatic delay means(19,36). An acoustic generator according to any one of Claims 10 to 20 in the form of a marine acoustic source.

Inventors: PASCOUET, ADRIEN P.
Applicants: PASCOUET, ADRIEN P.
Application year: 1986
Main classification: G01V1
IPC codes: G01V1