CA1070819A - Implosion acoustic impulse generator - Google Patents

Abstract

ABSTRACT
There is provided a method for generating an acoustic impulse by propelling, at a very-high velocity, along a predetermined trajectory, a main liquid jet into a liquid body. The jet is propelled by a force field which is stopped substantially instantaneously. For very high-power impulses the jet's kinetic energy is such as to create a cavity followed by an implosion in the liquid body. Preferably the main liquid jet is split into at least two branch liquid jets which are deflected. Each branch jet can also have sufficient kinetic energy to create an implosion.
The generator comprises a housing defining a slug chamber which, when the housing is submerged in the liquid body, entraps a liquid slug therein. The slug chamber has an exit port communicating with the liquid body. Means are coupled to the liquid slug for propelling a main liquid jet along a predetermined trajectory. Desirably the generator also includes jet splitting means for splitting the main liquid jet into at least two branch liquid jets, and diverting means for diverting the branch liquid jets in a plane which is inclined relative to the trajectory of the main jet.

Description

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~070819 The present invention relates to a generator for producing acoustic impulses.
~enerators for producing in water acoustic i~lpulses are known - see for example U.S. Patent Speci~ications Nos. 3,3~9,627; 3,642,090; 3,6~2,089;
and 3,711,824.
Another such generator is described in "Ocean Industry", pages 42-43, July, 1973.
According to the present invention there is provided a generator for producing acoustic impulses when submerged in a body of liquid,the generator including: a main housing defining a first bore having a bottom stop wall having a main port, and a top stop wall having a top port; a shuttle having a main piston, a second piston, and a push rod coupling the pistons in a spaced relation-ship, the main piston being slidably mounted in the first bore, which bore defines in use of the generator a) a slug chamber, having the said main port, between the bottom stop wall and the main piston and confining a slug of liqUidtherein and b) a vent chamber, between the main piston and the top stop wall, said housing further defining a second bore in which the second piston is slidably mounted, the second bore providing in use of the generator . r a) a return chamber, between the second piston and a fixed seal ring, which slidably receives the push rod, and b) a trigger chamber between the second piston and the housing; a reservoir chamber in the said housing, the return chamber having a pressure inlet, the trigger chamber having a vent hole and a pressure inlet, the reservoir chamber having a pressure inlet and a pressure outlet, and the vent chamber having a vent hole; a normally-closed, mechanically -operable valvefor coupling ~t'~, - 1 - ~

10708~9 an air pressure source to the inlet of the reservoir chamber, the return chamber being directly coupled to the pressure source in use of the generator; a normally-closed, electrically-operable valve coupling the outlet from the reservoir chamber to the inlet of the trigger chamber; and a plunger movably mounted in a wall of the housing above the trigger chamber, the plunger being displaced in use of the generator by the second piston, when the second piston reaches its uppermost position i~
the second bore, thereby opening the mechanically-operable valve, which admits air pressure to the reservoir chamber, the electrically-operable valve, when energized by an electric signal in use of the generator,admitting air pressure from the reservoir chamber to the trigger chamber, thereby causing the shuttle to execute a forward stroke that propels the li~luid slug,which forms a :liquid jet that exits through said main port into the body of liquid and the said shuttle automatically executing a return stroke in response to the air pressure confined in the return chamber.

The generator could include a suction chamber having a bottom wall for deflecting the said liquid slug in a plane which is inclined relative to the trajectory of the liquid slug, and at least two ports for splitting

2~ the deflected liquid slug into at least two liquid jets, separated from each other by a sufficient angular distance to reduce the recoil on the generator.

~ The invention will now be described by way of example with reference to the accompanying drawings, in which:-Figures 1 - 4 are sectional views in elevation of one embodiment of a generator according to the invention, illustrating various positions of the shut~le therein 107~)819 and of the water jet produced thereby;
Figure 5 illustrates the collapse of a spherical cavity by the surrounding high-pressure water layer whiich gives rise to a desired very-high pressure impulse;
Figure 6 illustrates the rebo.und eIfect of the implosion which tends to produce an undesired bubble impulse;
Figure 7 illustrates the cushioning effect produced by beveled surfaces of the piston;
Figure 8 illustrates the relationship between the cross-sectional area of the main port and the shape of the water jet produced therethrough;
Figures 9, 11, 13 and 15 are sectional views in elevation of another embodiment of a generator according to the invention, having a jet splitter-and-deflector;
Figure 10 is a sectional view on line 10-10 in Figure 9;
Figure 12 is a sectional view on line 12-12 in Figure 11;
Figure 14 illustrates the formation of cavities in the water, and in the suction chamber of the jet splitter, and the implosions of such cavities;
~igure 16a shows a generalized waveform of the pressure signature produced by an imploder-type acoustic impulse generator, Figure 16b is a filtered version of the waveform shown in Figure 16a;
Figure 17a is a waveform produced with the generator of Figure l;
Figure 17b is a waveform produced with the generator of Figure 9;
. Figure 18a is a representation of the direct waveform produced by the generator of Figure 9;

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Figure 18b is a rel~resentation of the direct waveform, after being reflected from the water surface;
Figure 18c is a representation of the combined waveform;
Figure l9a illustrates the pressure signature produced by the generator without an expansion chamber;
Figure l9b shows ~he pressure signature of the generator with an expansion chamber;
Figure 20 illustrates the method of employing the generator of Figure 9 for conducting seismic exploration in a body of water; and Figure 21 shows the use of a suction chamber with a generator employing vapour bubbles.
A first embodiment of an acoustic generator according to the invention and generally designated as 10, is shown by ~igures 1 - 4. It includes a main housing 13 having a cylindrical bore 16 between stop walls 17 and ~ , 18, the latter having piston seats 17a and 18a respectively.
Seat 17_ defines a main port 17b and seat 18a defines a port 18b. When submerged, main port 17b communicates with the liquid body, typically water 12.
A main piston 20 is slidably mounted in bore 16 on a seal ring 21. Seats 17a and 18a have beveled surfaces, and piston 20 is shaped, at the top and bottom, to have matching tapered surfaces 20a and 20b respectively.
The principle of operation of the generator 10 is based on projecting a water slug 22, entrapped inside a slug chamber 23 defined by the bore 16 between ring 21 and stop wall 17. Bore 16 also defines a chamber 24 between ring 21 and wall 18 as the piston 20 moves downwards.

,~q .. . .

The water slug 22 is projected into a water jet 22a by the application of a force field having a resultant force in an axial direction. The means for producing the force field include an actuating member, namely a push rod 26 having one end connected to the piston 20 and another end connected to an auxiliary piston 27 slidably mounted on a ring 28 in a cylindrical bore 29. The bore 29 defines a return chamber 30 under-neath the piston ~7 and a trigger chamber 31 above the piston 27. The chamber 30 is made fluid tight by a ring seal 32 sealingly and slidably receiving the push rod 26. The chamber 31 continuously vents into the water through a vent hole 33.
For any particular size of the generator 10, there will correspond an optimum length ior the chamber 30 which will yield an acoustic impulse of maximum energy.
If the optimum length for the chamber 30 makes the generator 10 undesirably long, the chamber 30 can be "folded", as shown in the embodiment, so that a portion thereof is inside, and another portion 30a thereof lies between the bore 29 and a cylindrical wall 34. The volume between the wall 34 and the housing forms a reservoir chamber 35.
The chamber 24, even when reduced to its smallest dimensions, continuously vents through a very small vent hole 36. Though the vent hole 36 could vent into the water 12, it is preferable,as in the embodiment, that it vent into an expansion chamber 37 of considerable volume, and that chamber 37 vent into the water through a small top vent hole 38.
The generator 10 is powered by an air compressor 45 producing, at the output of a control valve 45a, a regulated, controllable stream of air pressure. The valve .. . .

45a is coupled to an inlet 40 of ~he cham~er 3~a by a channel 41. Chamber 35 has an inlet 42 coupled to the channel 41 through a normally-closed valve 43, which is operated by a plunger 44 sealingly and slidably mounted in the upper wall of the chamber 31. When the piston 27 reaches its uppermost position (Figure 1), the plunger 44 is li~ted from its seat 44a, thereby mechanically opening the valve 43 and admitting high-pressure air into the inlet 42 of the chamber 35. The amount of bac~
pressure on the piston 27 is controlled by the vent hole 33, and the amount of back pressure on the piston 20 is controlled by the vent holes 36 and 38. Thus, the air pressure in the chambers 24 and 31 serves as a fluid shock absorber.
The assembly consisting of pistons 20, 27 and push rod 26 forms a shuttle 46. To propel the shuttle 46 downwardly (Figure 2), trigger pressure is applied to the top of the piston 27. This trigger pressure is obtained from the outlet 47 of the chamber 35. The outlet 47 is coupled to the inlet 48 in chamber 31 through a channel 49 having a valve 50, which is preferably solenoid-operated.
Valve 50 is normally closed and opens only in a response to an applied electrical trigger pulse 64 arriving via a line 51.
An improved embodiment of the generator 10 (see Figure 9) includes a jet splitter-and-deflector and bubbie suppressor, generally designated as 15.
The deflection of the water jet 22a is accomplished by a deflector, in the example a plate 15a which is flat (or it may be conically shaped), that is secured to the main housing 13. Plate 15a is at a suitable distance from the main port 17b.

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~C~708~9 The jet-splitting ;~nd bubble-suppression function is accomplished by a secondary housing 15b de~ining a suction chamber 70 having at least one out;let port 15c communicating with the water body 12. For typical seismic exploration, the optimum number of outlet ports 15c is four, and the same are spaced from each other by an angular distance of approximately 90.
If only a pair of outlet ports 15c is employed, they should be diametrically oppositely spaced.
Though either of the acoustic impulse generators 10 may be applied in various industries, the use of one will be described for conducting seismic exploration.
For such seismic use (Figure ~0), the ~ -' generator 10 is normally towed submerged by a seismic vessel 60 and is cyclically operated $o produce a train of high-power, short duration, sharp acoustic impulses in the water body 12. The generator 10 is supported by an adjustable hanger 61. On the vessel 60 are positioned the air compressor 45 with its associated devices and a signal recording-and-processing unit 62. Compressor 45 is connected to the valve 45a through the channel 41.
A unit 62 receives the detected reflected seismic signals from a towed streamer cable 63, and produces the trigger pulse 64 transmitted via the line 51 for opening the valve 50, thereby "firing" the generator 10. There may be more than one generator 10 towed submerged by the vessel 60.
To make the generator 10 ready for operation~
the means for doing this being as in ~igures 1 to 4, the control valve 45a is opened to pressurize the return chamber 30 through the inlet 40. The admitted pressure causes the piston 27 to automatically return to its .. , . ~. ... ; .. :.. - , .

1~70819 cocked position whereby th( plunger 44 becomes lifted from its seat 44a to open the valve 43. Air pressure, say at 150 bars, can now fill the reservoir chamber 35.
The chambers 24, 31 and 37 are substantially at the ambient hydrostatic pressure. A water slug 22 now fills slug chamber 23. The generator 10 is now pressure-loaded, and the shuttle 46 is in its cocked position.
To allow shuttle 46 to execute its forward stroke (Figure 2), a trigger pulse 64 is transmitted, causing the valve 50 to open, thereby establishing pressure communication between chambers 31 and 35. A downwardly-directed trigger force 52 (Figure 2) becomes exerted against the piston 27. This force is combined with the already existing downwardly-directed force 54 (Figure 1) to produce a resultant force that is exerted on the portion oi piston 21 opposite to the port 18b. The resultant force overcomes the sum o~ all upwardly-directed forces -53 exerted on the shuttle 46. As a result, the shuttle 46 starts moving downwardly to execute its forward stroke.
When the exposed portion 20b of piston 20 'oecomes disengaged from the seat 18a, the high pressure in chamber 35 becomes exerted against the entire piston 20, resulting in the abrupt propulsion of shuttle 46. The high pressure starts venting through the vent hole 36 into the expansion chamber 37 and from there to the water 12 through the vent hole 38.
The beveled surface 17a of the main port 17b serves a very important function (Figure 7). Before the tapered surface 20a approaches the beveled surface 17a, the water slug 22 has no difficulty in exiting through the main port 17b. When the tapered surface 20a approaches the beveled surface 17a, an annular ring of water 22b becomes entrapped therebetween which serves as a liquid shock absorber for the piston 20. Without the cushioning ; . ;

107~
effect produced by the entrapped annular water ring, generator 10 might sustain considerable stresses which would tend to shorten the useful life of the generator .
The shape of the water jet 22a produced from the water slug 22 also depends (Figures 2 and 8) on the cross-sectional area of the main port 17b - the smaller this cross-sectional area is, the longer and thinner water jet 22a will be.
The fast moving water jet 22a, when it separates from the piston 20, first produces in the water a nearly cylindrical cavity CC (Figure 3) and then a nearly spherical cavity SC (Figure 4). Hydrostatic pressure head causes the collapse of the cylindrical cavity which results in a two-dimensional implosion, and the collapse of the spherical cavity produces a three-dimensional implosion, at a safe distance from the generator 10. This distance depends on the velocity of the jet 22a and the cross-sectional area of the main port 17b ~Figures 1 and 8).
Thus, when the force field exerted on the water slug 22 is stopped, as by arresting substantially instantaneously the motion of the piston 20 (Figure 3), the propelled water jet 22a continues its downward motion, away from the stopped piston 20, and in so doing creates a two-dimensional implosion followed by a three-dimensional implosion.
The acoustic energy resulting from the implosion of CC (Figure 3) is at best proportional to the volume of the water slug 22, and the acoustic energy resulting irom the implosion of SC (Figures 4 and 5) is roughly proportional to the ki.netic energy of the water jet 22a.
Thus, the kinetic energy of the jet 22a, which is determined, in part, by the cross-sectional area of the _ g _ '~ ~ ' ' . !

11)708~9 main port 17b and by the Dressure exerted by the shuttle 46 on the water slug 22, plays a predominant role in the quantity of acoustic impulse energy obtained from the generator 10.
Also, from a structural point of view, the further the three-dimensional spherical cavity implosion takes place from the generator 10, the better it is for the generator, since if the entire implosion took place between or too near to the structural members of the generator, there would result a rapid fatigue in the generatorls housing, thereby considerably reducing the life span of the stationary and moving parts in generator 10.
When the shuttle 46 comes to a stop (Figure 3), the chambers 24, 31 and 37 continue to vent until the back pressure is sufficiently reduced, and the shuttle 46 can start on its return stroke. The return velocity oi the shuttle 46 is affected by the rate of venting of the chamber 24. When the volume of chamber 24 diminishes, the pressure therein increases, whereby the upward movement of the shuttle is decelerated. The chamber 31 also continuously vents through the vent hole 33 in to the water 12 so as not to contribute to the deceleration force produced by the back pressure on the returning shuttle 46. By properly sizing the vent holes, the rate oi return for the shuttle can be adjusted to obtain a desired pulse repetition rate.
In practice, the shuttle 46 executes its forward stroke i~ a time interval which is relatively short compared with the time interval for its return stroke, and the generator 10 is operated repetitively to produce in the water body 12 a train of high-powered acoustic impulses which become reflected from the earth formations 12a lying below the water. The reflected seismic signals are .. .. . . ... . ..

1070~

detected by the streamer ca~le 63 whose output signals are recorded and processed by the recording-and-processing unit 62 on the deck of seismic vessel ~0.
The processing of the reflected seismic signals allows a geologist to study the nature of the earth formations be:Low the bed of the water body 12.
When the matching surfaces 20b and 18a re-engage (Figure 1), that is when piston 27 attains its cocked position, the plunger 44 will move up, thereby mecha~ical-ly and automatically re-opening the valve 43 through which high pressure will again be admitted into the reservoir chamber 35. Simultaneously, the chamber 30 is also being re-pressurized, since the return stroke of the piston 27 has caused an increase in the volume of the chamber 30.
Conversely, during the forward stroke of the piston 27, the volume of the chamber 30 decreases (Figure 3), and some of the air pressure from chamber 30 escapes through the inlet 40 to the pressure source 45.
The implosion (Figures 4 and 5) takes place very rapidly even before the shuttle 46 starts on its return stroke. Without expansion chamber 37, chamber 24 would vent directly into the water at a point too close to the site of the implosion, and the vented air would act as an acoustic energy absorber for the high-pressure water, thereby attenuatipg the output acoustic impulse of the generator 10. The difference in the acoustic signals, known as '`pressure signatures", can be seen by comparing Figuresl9a and l9b. By providing the expansion chamber 37, the chamber 24 can first vent into the expansion chamber 37 during the time of implosion. Also, the fact that vent hole 33 is directed upwardly, and at a greater distance from the site of implosion, improves the pressure signature, which is cleaner, narrower, and larger with :

10708~9 chamber 37 (Figure l9b) than without chamber 37 (Figure 19a).
When the generator 10 has no deflector plate 15a (Figures 1 to 4), after each firing of the generator there is produced an upwardly-directed reaction force which causes a recoil on the generator.
It is frequently desired or even necessary to eliminate the recoil of the generator. This is accomplished by the deflector piate 15a which deflects the water jet 22a in a transverse plane so as to produce reaction forces that are perpendicular to the generator's axial direction, thereby suppressing the recoil. Deflector 15a must be fixedly secured to the generator's main housing 13.
When the void of a cavity implodes (Figure 5), it becomes filled with a volume of very high-pressure water which produces a desired primary compression PP
and hence a large acoustic pulse. At the pressures involved, the water as well as the water vapor in the cavity act as a spring which, after the implosion, rebounds to produce a secondary compression (Figure 6), known as a "bubble" pulse BP which constitutes an undesired seismic pulse that also causes reflections from the underlying earth layers 12a. The detected reflected bubble signals greatly complicate the processing of the seismic signals as is well known to those skilled in the seismic art.
The undesired bubble pulses can be eliminated or greatly suppressed by using the suction chamber 70 in the secondary housing 15b (Figures 9-15).

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- ~070819 The chamber 70 forces the main water jet 22a to split. In the case of four ports 15c, it splits into four branch jets 22A-22D exiting through the ports 15c. Each branch jet produces a main cavity (Figures 13 and 14), but the main jet 22a itself produces a secondary or suction cavity 70a inside the suction chamber 70, while jets 22A-22D produce primary cavities 70A-70D, respectively, outside of housing 15b.
The cavity 70a and cavities 70A-70D form one continuous cavity. If more than four ports 15c are employed, the implosions would tend to interfere with each other. With four ports 15c, the primary cavities 70A~70D do n~t appreciably interfere with each other, since they are angularly displaced from each other by about 30.

It was discovered that the secondary cavity 70a produced within suction chamber 70 implodes only after the implosion of the primary cavities 70A-70D, so that the secondary cavity 70a sucks in the rebounding high-pressure water which otherwise would produce the bubble effect resulting in undesired bubble pulses. Thus, the provision of the suction chamber 70 in the secondary housing 15_ has eliminated or substantially suppressed the bubble pulses BP which are highly undesirable for conducting seis~ic surveYs.
The advantages of the suction chamber 70 can also be illustrated, for example, in connection with an implosion produced by a steam generator 45' (Figure 21) discharging superheated steam into the water body 12.

The superheated steam is discharged by an insulated pipe 72, immersed about 3 to 5 metres below the water's surface. At the end of the pipe is a steam valve 73 that `~ periodically ejects into the water a bubble of superheated ~07~;)819 steam having a pressure oi approximately 90 bars at a 100C. The steam ejection from the valve produces the two undesirable effects previously described, that is, the recoil effect and the bubble effect.
S By surrounding the steam valve 73 with a deflector-and jet splitter 15, the ejected steam bubble will be in a plane normal to its regular discharge path thereby eliminating the upward reaction force, whereas the splitting of each steam bubble into four steam bubbles produces a secondary cavity in the suction chamber 70 and four main cavities 74. Again, the implosion of the cavity within the suction chamber 70 takes place after the implosion of the main cavities 74, thereby allowing the cavity in the suction chamber to absorb the rebound effect from the implosions of the main cavities 74.
The essential reguir~ments for the formation of a cavity depend on the conditions surrounding the deceleration or stoppage of the piston 20. On the other hand, in order for a cavity to produce a useful pressure pulse PP, it is necessary that the velocity of the main water jet 22a (Figure 8) prior to the stoppage of the piston 20 be sufflciently great.
It is important to bear in mind the form and the nature of the frequencies of the general pressure 2~ signal produced by an implosive compression. Figure 16a shows the general pressure signal or pressure signature as a function of time, measured at a fixed distance from the centre of implosion. The first portion 1 of this curve shows an increase in the ambient pressure Pha within the water corresponding to the propulsion of the jet 22a. This overpressure reaches a peak ~ Po and thereafter the pressure decreases. The portion Z of this curve shows that ~` ~ when the piston 20 is abruptly stopped, the pressure , ~ . . ,- ~ ;; ., .

~(~708~9 decreases until it beco~es iie~ative relative to the hydrostatic pressure. This negative pres~ure,~hich corresponds to the formation of the cavity ~nd its en:Largement in volume, continues until the depression has reached its maximum value - ~ P. When the voll~e of the cavity is at its maximum~ its potential ener~y transforms into kinetic energy in the water layer surrounding the cavity. Portion 3 of this curve marks the implosion which produces æ pressure reaching a high peak ~P1, which is the maximum pressure in the ambient water at the point of measurement subsequent to the implosion of the cavity (Figure 5). Portion 4 of this curve illustrates the rebound of the mass of the high-pressure water fillin~ the cavity (Figure 6). The rebound results in secondary cavitations followed by secondary implosions which can repea~ themselves successively several times. These cavitations and implosions produce successive peaks ~P2, h P3, etc., which decrease in amplitude and alternate uith valleys corresponding to depressions.
On the scale cf time, T designates the period of the signal measured from the start time to the end of the primary or first implosion. This period T depends on the potential energy of the cavity, and hence on the kinetic energy in the water jet 22a (Figure 3, and also on the distance of the cævity from the water surface, that is on the hydrostatic pressure head above the cavity. The total duration of the pressure signature is T1 which determines the seismic resolution. The resolution is greater when T1 is smaller.
~0 The basic curve shown in Figure 16a is not the one normallY used in geoph~sical exploration. The useful signal is that portion of this basic curve which is left after it is filtered at 8-62 Hz,from the point of view .

10708~9 of penetration, or after i1 is filtered at 0-248 Hz, from the point of view of resolution.
Figure 16b shows what is left of the pressure signature of Figure 16a after being filtered at 8-62 Hz.
One can see that the peak ~Pl corresponding to the first implosion and containing high frequencies does not appreciably look different from the peak ~ P2 correspond-ing to the implosion resulting from tbe first bubble pressure BP. The signal has therefore several peaks, which means that each earth layer 12a will produce many reflected signals that will be detected by the streamer cable 63 and recorded by unit 62 (Figure 20).
The greatest portion of the utilizable energy is situated a~ the maximum of the implosion and is emitted in a relatively high-frequency band which is rapidly absorbed by the ground. The penetration of such a wave is relatively small.
Uslng suction chamber 70, it is possible to suppress the secondary peaks ~P2, ~P3 by suitably shaping 20 the dimensions of the ports 15c (Figures 9 and 10). It is possible in this manner to create different size cavities 70A-70D whose periods of implosion are also different, all of which allows to create at the time of the secondary peaks ~P2, ~ P3 (Figure 16a) opposite peaks of opposite phase which produce the desired pressure cancellations.
- Such shaping of implosion cavities is of particular interest to relatively small-size implosion producing generators wherein the ports 15c have dimensions which do not allow the sufficient absorption of the high-pressure water in the internal suction chamber 70 (Figure 14) during the rebound of the primary cavities 70A-70D subsequent to implosion.

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10708~9 The shortening of the period T1 of the emitted signal is very advantageous - the reflected signal (Figure 18b) from the water's surface becomes superimposed on the direct signal (Figure 18a) to form a combined signal (Figure 18c), but with a phase difference corresponding to the time it takes for the direct signal to complete the round trip from and to the site of implosion. That is, the generator (Figure 9) allows the positive portion of the direct signaI ~Figure 18a?
to be superimposed on the positive portion of the reflected signal (Figure 18b) for relatively shallow implosions.
Combining these signals (Figures 18a and 18b) produces a resultant signal (Figure 18c) which is of particular interest since it contains relatively high energy in the low lS irequency band, especially because of the utilization of the low ~requencies contained in the positive portion of the reflected signal (Figure 18b). Accordingly, for the same amount of input energy to the generator, it is possible to considerably increase the penetration power of the combined signal (Figure 18c).
The actual waveform of the signals monitored with a hydrophone near the site of implosion with a generator according to Figure 1 is shown in Figure 17a, and with a generator according to Figure 9 it is shown in Figure 17b.

The experimental results therefore confirm the theoretical waveforms.
Thus, the generator of Figure 9 produces a waveform containing only a predominant single peak (Figure 18c), which contains a large quantity of energy at relatively low frequencies and is of relatively short duration so that it is capable of having a large penetration into the earth formation as well as good resolution.

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Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A generator for producing acoustic impulses when submerged in a body of liquid, the generator including:
a main housing defining a first bore having a bottom stop wall having a main port, and a top stop wall having a top port; a shuttle having a main piston, a second piston, and a push rod coupling the pistons in a spaced relationship, the main piston being slidably mounted in the first bore, which bore defines in use of the generator a) a slug chamber, having the said main port, between the bottom stop wall and the main piston and confining a slug of liquid therein and b) a vent chamber, between the main piston and the top stop wall, said housing further defining a second bore in which the second piston is slidably mounted, the second bore providing in use of the generator a) a return chamber, between the second piston and a fixed seal ring, which slidably receives the push rod, and b) a trigger chamber between the second piston and the housing;
a reservoir chamber in the said housing, the return chamber having a pressure inlet, the trigger chamber having a vent hole and a pressure inlet, the reservoir chamber having a pressure inlet and a pressure outlet, and the vent chamber having a vent hole; a normally-closed, mechanically-operable valve for coupling an air pressure source to the inlet of the reservoir chamber, the return chamber being directly coupled to the pressure source in use of the generator; a normally-closed, electrically-operable valve coupling the outlet from the reservoir chamber to the inlet of the trigger chamber; and a plunger movably mounted in a wall of the housing above the trigger chamber, the plunger being displaced in use of the generator by the second piston, when the second piston reaches its uppermost position in the second bore, thereby opening the mechanically-operable valve, which admits air pressure to the reservoir chamber, the electrically-operable valve, when energized by an electric signal in use of the generator,admitting air pressure from the reservoir chamber to the trigger chamber, thereby causing the shuttle to execute a forward stroke that propels the liquid slug,which forms a liquid jet that exits through said main port into the body of liquid and the said shuttle automatically executing a return stroke in response to the air pressure confined in the return chamber.

2. A generator according to claim 1, wherein the said main port has a beveled surface tapering inwardly which forms a seat that matches the beveled surface of a portion of the main piston.

3. A generator according to claim 1 and further including a suction chamber having a bottom wall for deflecting the said liquid slug in a plane which is inclined relative to the trajectory of the liquidslug, and at least two ports for splitting the deflected liquid slug into at least two liquidjets, separated from each other by a sufficient angular distance to reduce the recoil on the generator.

4. A generator according to claims 1, 2 or 3 and further including an expansion chamber having a vent hole for venting into theliquid body, the expansion chamber fluidly communicating with the vent hole of the vent chamber.