CA1077166A - Marine gas exploder - Google Patents

Abstract

Abstract Apparatus for generating underwater seismic signals includes an enclosed outer cylinder adapted for support in a submerged state, one or a pair of pistons being movable therein and exposed to an expansible combustion chamber.
Piston rods extend from the respective pistons through the cylinder ends, free-standing annular plates being mounted at the external ends of the piston rods in axial alignment therewith. Means are provided for introducing an explo-sive gas mixture within the combustion chamber and means are provided within such chamber for electrically igniting the mixture to drive the piston or pistons so as to propel the plate or plates through the water away from the cylinder.
Further means are provided within the cylinder between each of the pistons in the respective cylinder end for damping the plate movement, the free-standing plates being substan-tially surrounded by water in spaced-apart relation to the cylinder ends, the rate of travel of the piston or pistons being sufficient to induce substantial cavitation adjacent the trailing surface of each plate.

Description

1077~6f~

Background of the Invention .
1. Field of the Invention. This invention relates generally to a method and apparatus for producing underwater acoustic seismic signals. More particularly, it is concerned with a marine gas exploder of the type which achieves its results by generating cavitation bubbles whose collapse pro-duces such signals.

2. Description of the Prior Art. Acoustic signal sources such as underwater seismic gas exploders are typically operated by electrical ignition of an explosive gas mixture confined under pressure within an expansible combustion cham-ber separating two relatively movable telescoping cylindrical bodies. Movement of at least one of these bodies is adapted to propel a mass, such as a plate, through the water at a rate sufficient to generate an acoustic signal, either by positive compressive force or through collapse of a cavitation bubble.
Prior art devices of the character described typi-cally involve the rapid separation of a pair of opposed, flat plates exposed to a body of water in a direction normal to their plane surfaces. Photographic studies reveal that when the plates are moved in opposite directions cavitation occurs adjacent both facing surfaces. The size of the two cavita-tion bubbles invariably differs and consequently there is a finite time separating the signals resulting from their collapse. If one of the plates is held fast, cavitation appears to occur almost exclusively at the surface of the moving plate. The Applicants have therefore surmised that existing devices of this character could be rendered more efficient by completely eliminating one of the two opposed plates and simply moving a single plate surrounded by water on all sides. Experiment verifies that this is correct.

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1 This construction also makes it possible to vary the configu-ration and dimensions of the plate without essentially modi-fying the driving mechanism. These plate characteristics significantly affect the energy content of the collapsing cavitation bubble and the acoustic signal strength which can be obtained. By contrast, in typical prior art piston and cylinder arrangements for rapidly separating opposed plates, y modification of the dimensions and contour of these plates would involve substantial redesign of the entire device.
The Applicants also learned that a plate accelerated by a marine gas exploder is subjected to severe bending forces in the direction of motion, due either to sudden deceleration at the end of its forward travel or because of the non-uniform pressure distribution on its trailing surface produced by the ;
collapsing cavitation bubble. These forces are severe enough to cause cracks or complete failure if low strength, non-resilient materials are used in plate construction.
It is thus a general object of this invention to provide a method and apparatus of the character described for generating a marine seismic signal which provide for maxi-mum energy utilization in the formation of a cavitation bubble and in which the energy content of the bubble may be readily varied.
Summary of the Invention In this invention, a marine gas exploder, adapted to be supported so that it is completely submerged in water, con-sists generally of an outer enclosed cylinder, a pair of movable pistons within the cylinder separated by a central expansible combustion chamber, a pair of piston rods extending respectively from said pistons through the opposite ends of the cylinder to interconnect with a pair of external annular plates 1 substantially surrounded by water and facing such opposite ends in spaced relation unopposed by any other adjacent or contiguous plates of similar contour and extent, means for introducing and detonating a pressurized explosive gas mixture within the combustion chamber and means for expelling the resultant combustion products. When the device fires, the pistons are driven apart so as to accelerate the annular plates through the water in opposite directions. In conse-quence, cavitation bubbles are generated adjacent their trailing surfaces. The collapse of these bubbles produces a single energetic acoustic pulse. The annular plates are preferably dish-shaped, of diameter significantly greater than that of the cylinder, and oriented so that their respec-tive convex surfaces face the opposite ends of the submerged cylinder. The bubbles are essentially ring-shaped and coin-cide with the periphery of the plates.
In an alternate embodiment of this invention, a stationary enclosed cylinder includes a combustion chamber above a single movable piston. The piston is driven by the expansion of an explosive gas mixture within the chamber to drive a single annular external plate rigidly interconnected therewith through the water. As in the previously described embodiment, the single plate is substantially surrounded by water, spaced apart from the end of the cylinder and unopposed by any other plate of substantially similar extent.
Brief Description of the Drawings Figure 1 is a view in longitudinal section of an improved marine gas exploder in accordance with this inven-tion.
Figure la is a diagrammatic plan view of the com-bined fuel injection and ignition installation for the marine . -. . , . . ~
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1 gas exploder of Figure 1.
Figure 2 is a view in longitudinal section of an improved marine gas exploder in accordance with an alternate embodiment of this invention.
Figures 3a, b and c present detailed sectional views of alternate forms of the external plates shown in Figures 1 and 2.
Figure 4 is a detailed cross-sectional view of a sump and exhaust tube combination in accordance with this invention taken along the line 4-4 in Figure 1.
Figure 5 is an enlarged detail of the piston and ~ , exhaust tube shown in Figure 2.
Figure 6 is an enlarged detail of an alternate form of the sump and exhaust tube combination shown in Figure 4.
Figure 7 is an enlarged detail, partly in longitu-dinal section, of a gas injection tube and its associated holder in accordance with this invention.
Figure 8 is a sectional view taken along the line 8-8 in Figure 1, showing the interior face of the fuel injec-tion installation of Figure la.

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Figure 9 is a detail plan view of the upper end of a gas injection tube in accordance with this invention.
Figure 10 is a view in vertical section of a spark plug housing in accordance with this invention.
Description of the Preferred Embodiments Referring now to Figures 1 and la, a marine gas exploder 10 includes an enclosed cylinder 12, submerged in water and supported at opposite ends by suitable cables 13 and 14 from a stationary or movable support (not shown) such as a float which may carry the customary controls, electrical power, and fuel supply equipment for such a device well known in the art. A pair of similar pistons 16 and 18 are adapted to move in opposite directions within the cylinder 12. Pistons 16 and 18 are connected respectively by piston rods 20 and 22 to external annular plates 24 and 26 adjacent the two ends of cylinder 12. The space between the pistons 16 and 18 within the cylinder 12 forms a combustion chamber 28 which may be charged with an explosive gas mixture under suitable pressure to drive the pistons 16 and 18. This explosive gas mixture may consist, for example, of separate streams of oxygen and propane introduced through tubes 29 and 30 respectively.
Ignition of the explosive gas mixture in chamber 28 is accom-plished by means of a conventional spark plug 31 connected to a source of voltage through insulated cables 32 and 33. As best seen in Figure la, a mounting plate 34 positioned over an aperture in the upper wall of cylinder 12 supports a pair of gas tube holders 35 and 36 through which tubes 29 and 30 respectively communicate with the interior of chamber 28.
Spark plug 31 is in like manner retained in a housing 37 secured to mounting plate 34. The details of the combined fuel injection and spark plug installation for gas exploder 10 form part of this invention and will be described below.

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,~ 77~6 1 An exhaust tube 38, whi.ch may be either of the closed or open type, extends through mounting plate 34 into the chamber 28 to enable t.he expulsion or purging of gaseous combustion products. Since the exploder 10 is subjected to constant cooling by the surrounding body of water, part of the water formed in the combustion process will condense. Unless period-ically removed, this condensat.e accumulates.and decreases the effective volume of chamber 28 and in turn the energy of the explosion produced therein. In order to remove this conden-sate, a sump 40 is formed wi.thin the wall 39 of the cylinder 12 at the bottom of the combustion chamber 28. Condensed water vapor will collect by gravitv flow within the sump 40 and wi.ll be entrained in any spent combustion gases flowing from the chamber 28 into the lower end of the exhaust tube 38 which extends within the sump 40.
If the exhaust tube 38 is of the open type, the interior of the chamber 28 will be constantly exposed to the external atmosphere. In that event, water vapor present in residual air in the chamber 28 not otherwise removed in the filling and exhaust operations constitutes a source of con-densation in addition to that arising as a by-product of the explosion itself. The means for removing this condensation also forms part of this invention and will be detailed below.
. In operation, ignition of the gas mixture within ~ chamber 28 drives the pistons 16 and 18 oppositely against :
the counter pressure of air springs 41 and 42 to accelerate the plates 24 and 26 through the water at a rate sufficient to induce cavitation behind such plates. Collapse of such cavitation bubbles with approximate simultaneity produces an acoustic signal o~ desjl:ed magnitude which may be used for seismic or other purposes. The balanced reaction forces of .'. ~

10771~6 1 the pistons 16 and 18 eliminate the need for any heavy sup-porting float in order to hold the exploder 10 substantially stationary.
The plates 24 and 26, which may be constructed, for example, of steel or aluminum, are suitably fixed to the extrem-ities of piston rods 20 and 22 and spaced from respective cylinder end caps 43 and 44 by means of brass bushings 45 and 46 secured therein. It is significant that prior to ignition of the exploder 10 plates 24 and 26, in their initially retracted positions, are substantially surrounded by water on all sides except for that portion of their trailing surfaces 47 and 48 abutting the bushings 45 and 46 respectively. When the plates 24 and 26 are accelerated in opposite directions to their moved positions shown in dotted outline, underwater photographs reveal that cavitation occurs at the trailing sur-faces 47 and 48 in the form of a ring or torus 49, having an outer diameter at least as large as the periphery of each plate. The collapse of these toroidal cavitation bubbles 49 produces the acoustic or seismic signal of interest. No detectable cavitation appears to occur adjacent the surfaces of end caps 43 and 44 because cylinder 12 remains essentially stationary in the water.
Since the operation of this invention does not lnvolve the separation of opposed flat plates or other sur-faces, there is no necessary relationship between the diam-eter of the plates 24 and 26 and that of the cylinder 12.
For example, an efficient design for exploder 10 may consist of a cylinder 12 of a diameter of six inches while the plates 24 and 26 may be circular and given a diameter of twele to sixteen inches in order to increase the size of the cavitation bubbles produced thereby and the consequent peak strength of , -8-. :

1 the resultant acoustic signal. As the plate diameter increases, the fill time of the explosive mixture introduced within the combustion chamber 28 will have to be correspondingly increased to provide explosi.ve energy to develop the necessary accelera-tion. An advantage, however, of increasing the diameter of t.he plates 24 and 26 is to resist t.he spring rebound effect of air springs 41 and 42 to a greater extent than would plates of smaller diamet.er, thus damping undesirable osci.llation.
Further, the contour of the plates 24 and 26 need not conform to that of the end caps 43 and 44. Thus, any of a variety of plates of differing dimensions and contour can readily be affixed to the ends of the rods 20 and 22 without any altera-tion in other features of the exploder 10.
Turning now to the alternate embodiment of Figure 2, there is shown a marine gas exploder 50 in accoLdance with this invention consisting generally of an outer vertically oriented cylinder Sl supported from its upper end plate 52 by means of cables 53 and 54 suitably suspended similarly to exploder 10. The piston 56 is interconnected by means of a rod 57 with an external plate 58 adjacent and spaced from :
the lower end cap 59 of the cylinder 51. The space within cylinder 51 above the piston 56 forms a combustion chamber 60. Separate streams of propane and oxygen may be introduced within combustion chamber 60 through flexible fill lines 62 and 63. The electrodes of a spark plug device 64 supplied with power through lead 67 are conveniently situated so that they are exposed to the interior of the chamber 60. In order to vent, purge or otherwise expel spent combustion gas from the chamber 60, an exhaust tube 65 is introduced downwardly through the end 52 into the chamber 60.
The operation of the exploder 50 is similar to that of the exploder 10 with t.he exception that only one external _ g_ .~ ..
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1077~6 1 plate 58 is employed. Upon firing, the plate 58 is accelerated downwardly at a rapid rate against the counter-pressure of an air spring 69, reaching the position shown in dotted outline.
In this embodiment, it is convenient to illustrate the forma-tion of a cavitation bubble 70 of generally toroidal shape adjacent the upper or trailing surface 71 of the plate 58.
It is hypothesized that because of its toroidal shape the collapse of the bubble 70 is accompanied by a relatively greater force on the periphery of the plate 58 than upon portions thereof closer to its central axis~ Experimental firing of the exploder 50 with plates 58 of varying dimensions and strengths suggests that severe bending forces are exerted in the direction of acceleration. This hypothesis has led in part to the adoption of alternate forms for the acoustic plates of this invention as will now be discussed.
In Figure 3a, there is illustrated a plate 80 of laminar construction supported at the end of a piston rod 82 extending through the lower end cap 84 of the cylinder of a gas exploder similar to the exploder 10 or 50. It will be understood in what follows that the description and operation of this embodiment of the invention is equally applicable to single or dual plate versions and in either vertical or hori-zontal orientation.
The plate 80 may consist conveniently of a plurality of curved circular segments such as segments 86, 87 and 88 of thin, high strength spring steel. In order to secure the plate 80 to the end of the rod 82, an adapter 90 is square-threaded to its lower end and provided with a shank portion 92 of reduced diameter over which the segmented plate 80 fits and is thereafter clamped in place by means of threaded nut 94. The lower end of the rod 82 is also tapped to receive a threaded stud 96 extending through the adapter 90 so as to ~077~;6 1 accommodate a washer 97 and jam nut 98 to complete the plate assembly.
The purpose of the laminar or segmented construction of the plate 80 is to provide a degree of resilience or spring action which will allow the periphery of the plate 80 to flex in the direction of travel without breaking. In this way, the entire plate 80 may be made thinner than would otherwise be permissible in order to withstand the considerable stresses generated in the firing of high energy underwater devices of this character.
The Applicants have determined surprisingly that by curving at least the periphery of the plate 80 in the direction of its forward motion that the peak amplitude of an acoustic signal produced thereby is substantially enhanced. Since this increases the hydrodynamic drag, it might be supposed that acceleration would be lessened and the size of the resultant cavitation bubble would be decreased. It develops, however, that the shape of plate 80 contributes to the formation of a larger cavitation bubble. The degree of curvature of the plate 80 appears to be primarily significant along the convex surface 99 of the upper or rear most segment 86. For example, bowing the periphery of the plate 80 forward a distance of approximately one-tenth its diameter gives good results. As compared to a flat plate of similar weight and dimensions, the plate 80 so configured very nearly doubles the peak acoustic signal strength obtainable. While an upper limit to the acceptable degree of this curvature has not been determined, it seems reasonable that one exists.
Another alternate embodiment of the acoustic plate employed in this invention is shown in Figure 3b. The laminar flat plate 100 is threaded onto the lower end of the shaft 57 ~, . .

1077~6 1 of a veetically oriented exploder of the type shown in Figure 2. The plate 100, for example, of lightweight aluminum, may consist of a pair of thicker leading and trailing laminations 103 and 104 and a thinner pair of intermediate laminations 105 and 106. In operation, this laminar construction will lend resilience to the composite plate 100 in order to resist peripheeal forces in the direction of motion. If desired, any of plates 24, 26, 58 or 100 may be constructed of beryllium copper of suitable thickness to add to their resilience.
Figure 3c illustrates another alternate form of moving plate 200 provided with a hollow, cone-shaped cap 201 tapering in the direction of motion. Such a plate 200 may be square-threaded onto the lower end of a rod 202 similar in construction and operation to the previously described explo-der piston rods. Prior to positioning of cover or cap 201, the plate 200 may be secured by tightening the nut 204 on threaded stud 205 extending within the lower end of the piston rod 202. This entire assembly may be of lightweight aluminum, and the cap 201 may be filled with polyurethane foam 205 to further reduce its weight. These factors, together with the hydrodynamic streamlining of cone 201, enable a high accel-eration of the plate 200 in relation to the chemical energy of explosion supplied. Therefore, such a plate produces an acceptable degree of cavitation even though it lacks the par-ticular advantages ascribed to the configuration of Figure 3a.
Condensation Removal Features __ __ _ Attention is now directed to the detail of Figure 4. The sump 40 is preferably machined so that it has a gener-ally circular configuration of any desired dimensions having - a depth approximating that of the thickness of the wall 39, i~ -12-...

-10771~6 1 although the precise shape is not critical. A nut 122 may be welded to the external surface of the cylinder 12 adjacent the sump 40 and threaded to receive a tapered pipe plug 123 to form a seal against water leakage into the chamber 28.
The mounting plate 34 secured to the upper portion of the wall 39 receives and supports the exhaust tube 38 to enable it to extend downwardly through the chamber 28 and into the sump 40. The upper end of the plug 123 may be machined to form a cone 124 projecting into the open lower end of the exhaust tube 38 to increase the flow velocity of liquid and gaseous combustion products. In order to enhance the venturi effect, the bottom of the exhaust tube 38 may be provided with a flared lip 126 extending adjacent the bottom of the sump 40. In an operating prototype, a vertical clearance of approximately one-eighth of an inch is established between the lip 126 and the sump 40.
In operation, liquid 128 will collect by gravity flow within the sump 40. Owing to the relative positions of the lip 126 and the sump 40, any gaseous combustion products flowing into the exhaust tube 38 must pass through the sump 40. If the level of the liquid 128 lies above the lip 126, the flow of gas will tend initially to sweep out the liquid ahead of such flow into the exhaust tube 34 in relatively large drops. If this liquid level is below the lip 126, there is a greater likelihood that the liquid 128 will be atomized and entrained in the gas prior to entering the exhaust tube 38.
In either event the exhaust tube 38 carries off a mixture of gaseous and liquid combustion products from the chamber 28.
A conventional trap 130 formed in the exhaust tube 38 prvents re-entry of liquids removed in this manner.
In accordance with this invention, this gas and liquid mixture may be ejected by the force of the explosion . ~

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1 in chamber 28 or partially squeezed out by the return of the pistons 16 and 18 to their original positions under the counter-pressure of air springs 41 and 42. Filling of the combustion chamber 28 with fresh gases also pushes out such mixture. If desired, separate purging means (not shown) well-known in the art may be introduced within the chamber 28 for the same purpose. Finally, the exhaust tube 38 may be provided with external vacuum means (not shown) of conventional construction to suck out the contents of the chamber 28. Thus, the removal of collected condensate automatically accompanies the outflow of gaseous combustion products, regardless of the phase of operation of gas exploder 10 at which such outflow occurs. --~
In the alternate embodiment of Figure 2, in order to vent, purge or otherwise expel spent combusted gas from the chamber 60, the exhaust tube 65 provided with a liquid trap 66 is introduced downwardly through the end plate 52 and through the chamber 60.
The detail of Figure 5 shows the construction of the piston 56 in greater detail. The outwardly flared lower lip 132 of the exhaust tube 65 is brought into close proximity with the base of the sump 133. An upwardly projecting cone 134 formed in the top surface of piston 56 is positioned centrally within the sump 133 so that it~projects within the end of the exhaust tube 65.
The operation of the condensation removal apparatus of the alternate embodiment of exploder 50 is substantially similar to that previously described. Gravity flow will cause condensate 135 formed within the chamber 60 to collect within the sump 133 where it may be flushed out by the flow of spent combustion gas into the exhaust tube 65 by any of the various mechanisms described above. As before, the flared shape of .
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1~77166 1 the lower lip 132 together with the upwardly projecting cone 134 facilitate the flow of the mixture of gaseous and liquid combustion peoducts~
Within the scope of this invention, it will be possi-ble to provide a further alternate embodiment thereof as shown in Figure 6. The exhaust tube 38 in the exploder 10 of Figure 1 is replaced by a flexible exhaust tube 137 which is adapted to extend downwardly from a supporting float tnot shown) so that it reaches the under surface of the exploder 10. The lower end of the tube 137 terminates in a swivel nozzle 138 which threads onto the tapered fitting 139 which is bored to provide direct communication with the base of the sump 40.
In operation, condensate collecting in the sump 40 drains directly into the exhaust tube 137. Entrainment of condensate in the flow of gaseous combustion products occurs by any of the various mechanisms previously described to enable the com-bined ejectment of gas and liquid products from the combustion chamber 28.
It should be understood that the greatest need for condensate removal in gas exploders is in a marine environment because of the cooling effect of the water. However, the structure and mode of operation of the apparatus described is compatible with a land-operated device if that should be deemed necessary or desirable.
Misfire Prevention Features The side wall 39 of the cylinder 12 of Figure 1 is provided with a rectangular aperture 146 communicating with the combustion chamber 28. With additional reference to Figure la, a rectangular adapter frame 147 is bolted or otherwise secured t.o the out.er surface of the wall 39 in ali.gnment. with the aperture 146. The mounting plate 34 is in turn secured r~ --15 10771~6 1 to the upper surface of the adapter frame 147 so as to support and position the structures to be described.
The pair of similar gas tube holders 35 and 36 are connected to t.he supply tubes 29 and 30 and threaded into spaced apart apertures provided for that purpose adjacent one end of the mounting plate 34. The holders 35 and 36 support the gas injection tubes to be described and also couple with supply tubes 29 and 30, whi.ch furnish propane and oxygen respectively. The watertight spark plug housing 37 for coupling the electrical cable 32 to the spark plug 31 is affixed adjacent the opposite end of the mounting plate 34.
Finally, an aperture extends through the mounting plate 34 intermediat.e the spark plug housing 37 and the spaced apart holders 35 and 36 in order to receive the exhaust tube 38.
The holders 35 and 36 preferably consist of commer-cially available fuel gas and oxygen hose couplings modified in accordance with this invention. For purposes of brevity, only holder 35 will be described in detail. With one exception to be not.ed, its construction is identical to that of holder 36. As best seen in Figure 7, a gas injection tube 150 for the injection of propane is supported within a hollow, cylin-drical member such as modified male pipe fitting 152 which in turn threadably engages a hose adapter 153. The injection tube 150 is provided with an enlarged head 155 at its upper end which is dimensioned to rest agai.nst a shoulder 156 machined into the bore of the pipe fitting 152. This upper end of the gas injection tube 150 communicates with the bore 158 of the adapter 153 while its lower end, which projects below the mounting plate 34 into communication with the com-bustion chamber 28, is sealed. An orifice 160 of predetermined size is drilled through the thin side wall of the tube 150 .
~ -16-; 10771~6 1 adjacent its sealed end so as to control the injection of pro-pane into the chamber 28.
In order to install the holder 35 and injection tube 150, the male fitting 152 is threadably secured to the mount-ing plate 34, the injection tube 150 is inserted therein until it seats against the shoulder 156, and the adapter 153 is there-after joined to the fitting 152 with left hand threads signi-fying the presence of fuel gas, so that respective tapered edges 162 and 164 of these two elements form a positive seat.
To insure that there is no leakage into the chamber 28 along the bore of the fitting 57, an O-ring 165 may be provided encircling the periphery of the head 155. The upper end of the adapter 153 is now threadably coupled with tube 29 and is provided with a one-way check valve 167 of conventional con-struction to function as a flame arrester or to eliminate back-flow from chamber 28. The holder 36 (Figure la) is constructed similarly to holder 35 except that the components thereof (not shown) corresponding to fitting 152 and adapter 153 are mated with right hand instead of left hand threads to indicate the presence of oxygen. The holder 36 contains an injection tube 151 (Figure 8) in all respects equivalent to tube 150 except that its orifice 161 is larger to accommodate the flow of oxygen.
As seen in Figure 8, which is a view looking upward at the interioc surface of the mounting plate 34, the lower sealed ends of injection tubes 150 and 151 are positioned in spaced apart relation adjacent one end of the mounting plate 34. Their respective orifices 160 and 161 are positioned so that the propane and oxygen streams injected into the com-bustion chamber 28 therethrough are directed against the elec-trodes of the spark plug 31. The size of the orifice 161 is ~0771~6 1 approximately five to six times that of orifice 160 so that the two gases fill the chamber 28 in ploper stoichiometric propor-tion. In a prototype version of this invention, the orifices 160 and 161 have diameters of .0785" and .209" respectively.
In order to insure proper angular indexing of the orifices 160 and 161 after the injection tubes 150 and 151 are in place, an arrangement of fiducial marks may be employed. For example, as shown in Figure 9, the head 155 of the injection tube 150 may be rotated by means of a screwdriver slot 170 in order to align arrow 171 with a corresponding arrow 173 (Figure la) on the upper surface of the mounting plate 34. It is under-stood that an equivalent adjustment feature is provided for the injection tube 151. Since the tubes 150 and 151 are tightly secured within their respective holders 35 and 36, the force of an explosion of the gas mixture within the cham-ber 28, which is exerted equally in all directions, will not normally disturb the orientation of orifices 160 and 161.
Thus, these orifices do not require adjustment in connection with repetitive operation of the exploder 10.
The spark plug 31 is threaded into the mounting plate 34 as shown in Figure 10 and surrounded with the water-tight housing 37. A male pipe fitting 176, adapted to receive the spark plug 31 with suitable clearance, is welded to the surface of the mounting plate 34. A cover 177, of plastic or other suitable insulative material, is screwed onto the tapered threads of the fitting 176 and a cap 178 is tightened down on the cover 177 to squeeze O-ring 179 against the wall of the spark plug insulated lead 32. Different techniques for mount-ing the spark plug 31 so as to prevent water leakage into

3() the coml)ustion chambel 28 will be equally compatible with this invention.

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In operation, during the fill period of the gas exploder 10, separate streams of propane and oxygen are injected into the combustion chamber 28 through the respective orifices 160 and 161 so that they are directed toward the electrodes of the spark plug 31. In this manner, any residual moisture on such electrodes will be blown off or vaporized prior to spark initiation. At the same time, there is a high probabil-ity that the gaseous environment immediately surrounding these electrodes will contain propane and oxygen in the proper ratio for ignition at the conclusion of the fill time. Thus, the installation described eliminates two major sources of plug misfire in devices of this character.
An alternate embodiment of this feature of the inven-tion is shown in the gas exploder 50 in Figure 2 which is sup-ported beneath the water with its longitudinal axis extending vertically.
Propane and oxygen may be introduced through the upper end plate 52 into the combustion chamber 60 in accord-ance with this invention by means of the hoses 62 and 63. The end plate 52 may be appropriately machined or counter sunk to receive a pair of gas injection tube holders 180 and 181 in all respects similar to the holders 35 and 36, so as to sup-port and project a pair of orificed gas injection tubes 183 and 184 into the chamber 60. In like manner, the electrodes of the spark plug 64 may be positioned within the chamber 60 intermediate the injection tubes 183 and 184, protected by watertight housing 187, and provided with electrical power through the insulated cable 67. Propane and oxygen will be injected through a pair of appropriately dimensioned orifices through the tubes 183 and 184 so that oppositely directed streams of propane and oxygen are blown across the electrodes .
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1~)77~;6 1 of plug 64. The beneficial results of this operation are in all respects identical to that previously described in detail in connection with the exploder 10. Clearly within the scope of this invention, it will be possible to make appropriate gas and electrical connections into the combustion chamber 60 through the side wall of the cylinder 51 instead of the end plate 52. Furthermore, if desired, a form of gas exploder may be employed in which the constituent gases are premixed in a conventional mixing chamber or carburetor prior to injection 10 within chamber 28. In that event, a single gas stream con-taining both propane and oxygen may be directed toward the electrodes of a spark plug 31 for drying action.
It should be carefully emphasized that the foregoing detailed description is to be construed as illustrative of this invention and not in a limiting sense. In consequence, those skilled in this art should have no difficulty in devising other alternate embodiments incorporating modifications in structure and arrangement of parts, all falling within the scope of this invention as more particularly set forth in the appended claims.

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Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of generating underwater acoustic signals for use in seismic explorations characterized by propelling a wholly submerged free-standing annular plate rapidly through the water in a direction normal to its trailing surface at a rate sufficient to induce substantial cavitation adjacent its trail-ing surface and controlling the forward motion of the plate so that it travels a predetermined distance, such trailing free-standing surface being initially unopposed by any other adjacent or contiguous surface of similar contour or area.

2. The method according to Claim 1 characterized in that the plate is propelled by means of an internal combustion device such as a gas exploder.

3. Apparatus for generating underwater seismic signals comprising an enclosed cylinder adapted for support in a sub-merged state, one or a pair of pistons movable therein and exposed to an expansible combustion chamber, piston rods ex-tending from the respective pistons through the cylinder ends, free-standing annular plates mounted at the external ends of the piston rods in axial alignment therewith, means for intro-ducing an explosive gas mixture within the combustion chamber, means within the combustion chamber for electrically igniting the mixture to drive the piston or pistons so as to propel the plate or plates through the water away from the cylinder and means within the cyliner between each of the pistons and the respective cylinder end for damping the plate movement, the free-standing plates being substantially surrounded by water in spaced apart relation to the cylinder ends, the rate of travel of the piston or pistons being sufficient to induce substantial cavitation adjacent the trailing surface of each plate.

4. Apparatus according to Claim 3 characterized in that the plates have a concave-convex shape oriented so that their leading surfaces are concave, the plates being adapted to flex elastically in the direction of travel responsive to the pressure distribution on their trailing surface resulting from the collapse of the cavitation bubble or bubbles.

5. Apparatus according to Claims 3 or 4 characterized in that the plates have a diameter of at least twice that of the cylinder.

6. The apparatus according to Claim 3 characterized in that a single annular plate is arranged in use vertically below the lower end of the cylinder.