PATENT ABSTRACT
A marine acoustic seismic generator carried on the deck of a boat is coupled to a body of water in which the boat floats by a column of water in a pipe extending between the generator and the body of water. Post explosion oscillations are damped or terminated by venting the pipe of gas, either through an orifice or a valve, at a selected time, to a selected pressure, to the atmosphere or below water.

PATENT DESCRIPTION
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of applicant&#39;s prior copending application Ser. No. 521,966 filed Nov. 8, 1974, entitled Seismic Exploration, whose priority is claimed, the latter being a continuation of the inventor&#39;s prior application Ser. No. 338,306 filed Mar. 5, 1973 for Seismic Exploration, whose priority is claimed. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention pertains to seismic exploration. In particular the invention relates to seismic exploration employing a marine acoustic seismic generator of the type shown in my prior U.S. Pat. No. 3,588,801 issued June 28, 1971. 
     In using marine acoustic seismic generators there is difficulty in handling connecting lines and hoses extending from the operator&#39;s boat to the submerged generator. The present invention is directed toward elimination of such lines and hoses. A further difficulty encountered in the use of such apparatus is the oscillatory action following the initial explosion. Usually it is desirable to have only a single pulse of brief duration produced in the water, for otherwise the reflections and refractions from the initial pulse may be masked by reflections and refractions received from later energy pulses and it may be difficult or impossible to sort out the several reflections and refractions appearing at the seismic receivers. The invention overcomes that problem too. 
     SUMMARY OF THE INVENTION 
     According to the invention a column of water enclosed in a pipe is used to couple an on-board marine acoustic seismic generator to the body of water in which the operator&#39;s boat is floating. The water column can be elevated into the pipe from the body of water by a vacuum pump or the pipe can be filled with water from the top, e.g. by means of a pump. A pressure can be maintained in the column of water in excess of its static head. Vent means may be provided in the pipe to exhaust therefrom the gases released to the pipe from the seismic generator, thereby to reduce or terminate or control possible oscillations in the water following the initial energy pulse produced by the generator. The vent means may exhaust to atmosphere or to the body of water. Preferably the exhaust is under water, but to a pressure less than that at the bottom of the pipe. The exhaust may be through a selected orifice or through a valve, the latter being opened at a selected pressure or time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a detailed description of preferred embodiments of the invention, reference will now be made to the accompanying drawings wherein 
     FIG. 1 is an elevational view of a seismic exploration boat carrying apparatus embodying the invention. 
     FIG. 2 is a detail of a modified form of the apparatus; 
     FIG. 3 is an elevation showing a further modification; and 
     FIGS. 4-6 are elevations showing other modifications. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1 there is shown a boat 11 floating in a body of water 13, the interface between the water and the earth&#39;s atmosphere being indicated at 15. A marine seismic generator 17, e.g. as in the aforementioned U.S. Pat. No. 3,588,801, is suitably supported outboard of the boat from its deck, e.g. by boom frame 21 connected to a flange 23 extending from the generator housing. Generator 17 provides means to release suddenly a quantity of high pressure gas or other fluid into the medium at the generator&#39;s outlet. 
     To the lower end of the generator 17, e.g. by companion flanges 22, 24, one on the generator and one on the pipe, is connected a pipe 25. The flanges are suitably fastened together, e.g. by bolts not shown, providing a fluid tight connection between the pipe and the generator, placing the pipe in communication with the generator outlet. The pipe may be made of steel or other sturdy material. Usually pipe 25 will be rigid, or if flexible it will not be expansible to any great degree. To prevent recoil upon actuation of the generator, the bottom of the pipe is preferably closed by plate 27, and side outlets 29 are provided adjacent the lower end of the pipe, as shown in FIG. 4 of U.S. Pat. No. 3,588,801. 
     Alternatively, as shown in FIG. 2, the bottom of the pipe may be substantially open, as shown at 31, reliance being placed on the weight of the pipe and generator to minimize recoil. The construction will then be substantially the same as shown in FIG. 5 of the aforementioned U.S Pat. No. 3,588,801 to the extent of including a long pipe attached to the lower end of the generator, although the function of the pipe in the present invention is different. In U.S. Pat. No. 3,588,801 the function of the pipe is to contain the exhaust gases from the submerged generator whereas in the present invention the purpose of the pipe is to conduct the seismic pulse from the generator on the boat to beneath the water through the intervening atmosphere. Nevertheless the constructional details are alike so that reference may be made therefor to U.S. Pat. No. 3,588,801. 
     Returning to FIG. 1, in order to fill pipe 25 with water to conduct a seismic pulse from the generator 17 to the water 13, a vacuum pump 33 is connected to the top end of pipe 25 by a fluid conduit 35. By this means the pipe 25 is evacuated of air, and atmospheric pressure on water 13 forces it up into the pipe 25 into contact with closure 36 of the generator 17, sealing around the outlet 37 from its pressure chamber 39. Such outlet would correspond to outlet 54 of FIG. 1 of the generator shown in U.S. Pat. No. 3,588,801. 
     Vacuum pump 33 will usually be operated to maintain a subatmospheric pressure to line 35 more than sufficient to elevate the water in the pipe to outlet 37. For example pump 33 may be located at an elevation above outlet 37 and water may be drawn up a distance in stand-pipe 41 connected between pipe 35 and pump 33. The column of water in pipe 25 is then at a pressure above atmospheric at the upper end of the column and at greater pressures therebelow. 
     Referring now to FIG. 3, the apparatus is the same as in FIG. 1 except that for the vacuum pump 33 has been substituted a water pump 43. For clarity, boom frame 21 has been omitted in this and subsequent figures. Pump 43 draws in water via inlet tube 45 which extends down into the body of water 13. Water from pump 43 is discharged via conduit 35 to the top of pipe 25. To retain water in pipe 25, a spring loaded, downwardly opening check valve 47 is positioned in the pipe near its lower end but above ports 29. Valve 47 retains water in pipe 25 until a pressure pulse is created by generator 17 but thereupon opens to communicate the pulse through the ports 29 to the surrounding water 13. In any of the embodiments the lower end of the pipe may be open, closed or, as shown in FIG. 3, partially closed. 
     Valve 47 is spring loaded sufficiently to stay closed when pump 43 has filled pipe 25 and a pressure head 41 has been placed thereon. However the valve opens immediately in response to release of pressure fluid from generator 17. The water in pipe 25, as in the FIG. 1 and other embodiments hereof, is in contact with the generator 17 around the outlet opening thereof so that the pressure pulse therefrom acts immediately on the water in the pipe and is conducted by the water down the pipe and out ports 29 into the surrounding body of water 13. 
     Referring now to FIG. 4, there is shown a further modification of the invention wherein the apparatus is essentially the same as that shown in FIG. 1 except that exhaust gas vent means is provided comprising a vent tube 51 extending from near the upper end of pipe 25 above interface 15 to a level in water 13 below the surface. An orifice 53 of a size suitably selected to prevent too rapid venting of pipe 25, is provided adjacent the lower end of tube 51. By this means after the generator 17 has been operated to create a high pressure pulse against the water in pipe 25, the spent gases from the generator can be vented slowly underwater through tube 51. Such venting allows the spent gases to be dissipated in an unobjectionable manner. Venting is desirable to reduce secondary seismic pulses. 
     A similar venting means can be provided for the FIG. 3 embodiment of the invention, same being shown in FIG. 5. In place of orifice 53, the lower end of the vent tube 51 is provided with a spring loaded downwardly opening check valve 55. Valve 55 remains closed during filling of pipe 25 by pump 43 but opens when the pressure from generator 17 occurs. The size of vent tube 51 is small compared to that of pipe 25, e.g. in the range of 1/10 to 1/100 of the cross section of pipe 25, so that most of the pressure energy is transmitted down pipe 25 to outlet ports 29. When the pressure in pipe 25 drops, water rushes back in to at least the level of interface 15, and the low pressure spent gases are expelled through valve 55. 
     In the embodiments of FIGS. 4 and 5, the spent gases from the generator are vented underwater to a pressure greater than atmosphere. If there is no environmental problem, the spent gases can be vented to the atmospher as shown in FIG. 6. The apparatus of FIG. 6 is substantially the same as that of FIG. 4 except for the fact the vent pipe 65 does not extend down below the surface of the water, and a spring loaded, outwardly opening check valve 63 is provided in the vent pipe. When the vacuum pump is operated, valve 63 closes, so that a vacuum is created in pipe 25 and water rises therein. But upon actuation of generator 17 valve 63 opens and remains open, slowly to vent the spent gases from pipe 25. FIG. 6 further illustrates a construction wherein the bottom of the pipe 25 is substantially open, as in FIG. 2, the ports 19 being omitted. 
     The FIG. 5 construction is also adapted for use with atmospheric venting. It is only necessary to dispose the end of the vent pipe 51 above water. 
     As compared with the prior art according to which the whole generator 17 is operated submerged below the surface of the water, being supplied with power and controlled via a hose bundle, the present invention eliminates the need for the underwater hose bundle, and keep the generator above water where it is easier to operate and maintain. 
     While preferred embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit of the invention.