US3679021A

US3679021A - Acoustic pulse generating system

Info

Publication number: US3679021A
Application number: US22425A
Authority: US
Inventors: Seymour Goldberg
Original assignee: EG&G Inc
Current assignee: PerkinElmer Inc
Priority date: 1970-03-25
Filing date: 1970-03-25
Publication date: 1972-07-25
Anticipated expiration: 1989-07-25
Also published as: FR2085116A5; DE2114590A1; AU2631371A; DE7111463U

Abstract

An acoustic pulse generator and a method of producing acoustic pulses in a fluid medium is provided. In a preferred embodiment of the invention, the acoustic pulse generator includes a piston slidably disposed within a cylindrical housing having one end open and one end closed to the fluid medium. When the generator is submerged, the piston is accelerated toward the closed end of the cylindrical housing by the ambient pressure of the fluid medium and rebounds from the closed end of the cylindrical housing to produce an acoustic pulse. Means are provided for capturing the piston after its rebound from the closed end of the cylindrical housing to prevent subsequent inward movement of the piston by the ambient pressure.

Description

United States Patent [151 3,679,021 Goldberg [451 July 25, 1972 [541 ACOUSTIC PULSE GENERATING 3,506,085 4/1970 Lofer ..l8l/.5 H

SYSTEM [72] Inventor: Seymour Goldberg, Lexington, Mass.

[73] Assignee: EG&G, lnc., Bedford, Mass.

[22] Filed: March 25, 1970 [21] App]. No.: 22,425

Related U.S. Application Data [63] Continuation-impart of Ser. No. 813,625, April 4,

1969, Pat. No. 3,610,366.

[52] U.S. Cl. ..l8l/.5 H, 181/.5 AG, 340/3 A, 340/12 R [51] Int. Cl. ..G0lv 1/14 [58] Field ofSearch ..l8l/.5 H, .5 AG; 340/3 A, 12R

[56] References Cited UNITED STATES PATENTS 3,277,437 10/1966 Bouyoucos. ..18l/.5 H 3,522,862 8/1970 Lister ..l8l/.5 BM

Primary Examiner-Benjamin A. Borchelt Assistant Examiner-N. Moskowitz Atlorney-Ralph L. Cadwallader and Finnegan, Henderson & Farabow [5 7] ABSTRACT An acoustic pulse generator and a method of producing acoustic pulses in a fluid medium is provided. in a preferred embodiment of the invention, the acoustic pulse generator includes a piston slidably disposed within a cylindrical housing having one end open and one end closed to the fluid medium.

20 Claims, 8 Drawing Figures Patented July 25, 1972 3,679,021

5 Sheets-Sheet 1 1 w 1 I I E I g g All, "yIh. ai x mm \2 k I \g fll I l- \a g 8 N Ki k N INVENTOR B SEYMOUR GOLDBERG z adwa/zza a and 3 Pr 3172067620,] /ZdliSO/Z Qfiaw ATTURNI'IYS Patented July 25, 1972 3,679,021

5 Sheets-Sheet 2 mv EN'IOR SEYMOUR GOLDBERG Patented July 25, 1972 3,679,021

5 Sheets-Sheet 5 INVENTOR SEYMOUR GOLDBERG ATTORN EYS FIG. 4

Patented July 25, 1972 3,679,021

5 Sheets-Sheet 4 3'0 INVENTOR SEYMOUR GOLDBERG ATTORNEYS ACOUSTIC PULSE GENERATING SYSTEM This application is a continuation-in-part of copending application, Ser. No. 813,625, filed on Apr. 4, 1969 now U.S. Pat. No. 3,610,366, and entitled System for Marine Seismic Exploration.

The present invention relates to an acoustic pulse generator and a method of producing acoustic pulses and, more particularly, to an acoustic pulse generator which produces acoustic pulses when submerged in a fluid medium and can be used in performing marine seismic exploration.

Seismic exploration techniques have been used in the prior art to determine the geologic structure of underwater land areas. These techniques have been particularly useful in the exploration of offshore areas to determine the locations of sub-surface oil deposits.

The prior art techniques for seismic exploration of a marine area utilize sound sources for generating intense pulses of acoustic energy in the water covering the area and devices for recording the echoes that result from the reflection of the acoustic pulses by the various geologic layers of the underwater land area. The exploration is usually carried from a moving vessel which travels over the surface of the water in a predetermined course. By recording the echoes reflected by the geologic layers from successive acoustic pulses, an echo pattern is obtained which can be used to form a geologic map of the underwater area.

In the seismic exploration of a marine area, it is desirable to use an acoustic pulse generator capable of producing an intense pulse of acoustic energy and which can be operated at a predetermined repetition rate. Acoustic pulse generators heretofore used in seismic exploration have produced primary, intense pulses of acoustic energy followed by a succession of secondary acoustic pulses. In mechanical structures for producing acoustic pulses, the secondary acoustic pulses occur as a result of oscillations of the mechanical parts after the primary acoustic pulses are generated or cavitation in the water adjacent to the mechanical structure.

Since the secondary pulses are also reflected by the geologic layers of the underwater land area, the echoes obtained from the primary acoustic pulses are compounded by the echoes of the secondary acoustic pulses from the geologic layers. In forming a geologic map of the land area it is often difficult to distinguish the echoes produced by primary acoustic pulses from the echoes by secondary acoustic pulses. This problem makes it difficult to obtain an accurate, unambiguous geologic map of the land area.

The present invention contemplates a method of and apparatus for producing acoustic pulses in a fluid medium. The method and apparatus have significant advantages over the prior art. The invention substantially eliminates secondary acoustic pulses from the output signal of an acoustic pulse generator, thereby providing an echo pattern from the geologic layers of an underwater land area in which echoes of the primary pulses are readily identified. The invention also conserves the amount of mechanical energy which is required to operate the acoustic pulse generator.

An important feature of a preferred embodiment of the invention is the provision of a unique fluid-operated valve. The valve can be utilized to operate an acoustic pulse generator. It permits fluid lines having a low pressure rating to be used where large fluid flow rates are required at low pressure and where small fluid flow rates are required at high pressure. Thus, the valve enables more flexible fluid lines to be connected to the acoustic pulse generator, In addition, the valve design provides an initially large operating force which is rapidly decreased after the valve is opened to conserve the amount of fluid needed to operate the valve and to operate the valve more rapidly.

Additional advantages of this invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention.

To achieve its objects and in accordance with its purpose, this invention provides an expandable and collapsible structure for establishing a chamber of variable volume in the fluid medium. It also includes means connected to the structure for expanding the chamber against the ambient pressure of the fluid medium and'for permitting the chamber to be collapsed by the ambient pressure and to rebound against the ambient pressure to produce an acoustic pulse in the fluid medium. Means connected to the structure and operable after the rebound of the chamber are provided for preventing a subsequent collapse of the chamber by the ambient pressure to prevent secondary acoustic pulses from being produced in the fluid medium.

In a preferred embodiment of the invention, the acoustic pulse generator includes a cylindrical housing having one end closed and one end open to the fluid medium and a piston mounted for sliding movement within the cylindrical housing. The piston is in sealing engagement with the cylindrical hous ing to provide a chamber of variable volume between the closed end of the housing and the piston.

In the preferred embodiment of the acoustic pulse generator, means are provided for displacing the piston relative to the cylindrical housing to an extended position relative to the closed end of the housing and for releasing the piston to permit the ambient pressure of the fluid medium to drive the piston inward relative to the cylindrical housing toward its closed end. In addition, means operatively connected to the piston are provided for capturing the piston after rebound of the piston from the closed end of the cylindrical housing to prevent subsequent inward movement of the piston by the ambient pressure. By capturing the piston after its rebound from the closed end of the cylindrical housing, the acoustic pulse generator of the present invention arrests the mechanical motion of the piston to eliminate secondary acoustic pulses from the output signal of the generator and to conserve the amount of mechanical energy required for the operation of the generator.

Further, in a preferred embodiment of the invention, a fluid-operated cylinder, e.g., a hydraulic cylinder, is used as a displacing means for the piston. In addition, fluid control means are provided for operating the hydraulic cylinder. The fluid control means charge the hydraulic cylinder with pressurized fluid to move the piston to an extended position relative to the closed end of the cylindrical housing and then discharge the pressurized fluid from the hydraulic cylinder to permit the ambient pressure of the fluid medium to drive the piston toward the closed end of the housing. The fluid control means also admit pressurized fluid into the hydraulic cylinder during rebound of the piston away from the closed end of the cylindrical housing and then prevent that fluid from discharging from the hydraulic cylinder after the rebound of the piston to capture the piston and prevent secondary pulses.

The method of the present invention includes the steps of establishing a chamber of variable volume in the fluid medium, expanding the chamber against the ambient pressure of the fluid medium, permitting the chamber to be collapsed by the ambient pressure of the fluid medium, causing the chamber to rebound and expand against the ambient pressure to produce an acoustic pulse in the fluid medium and preventing a subsequent collapse of the chamber after the rebound to prevent secondary acoustic pulsesfrom being produced in the fluid medium.

In accordance with a preferred method of the present invention, a cylindrical housing having an open end and a closed end with a piston slidably disposed in the housing is submerged in a fluid medium. The piston is displaced against the ambient pressure of the fluid medium toward the open end of the cylindrical housing. Then the ambient pressure of the fluid medium is permitted to drive the piston toward the closed end of the cylindrical housing and the piston is permitted to rebound away from the closed end of the cylindrical housing. The rebound of the piston produces a primary acoustic pulse in the fluid medium. Finally, the motion of the piston is arrested after its rebound from the closed end of the cylindrical housing to prevent secondary acoustic pulses from being produced.

It should be noted that there are three modes of operation for the acoustic pulse generator, and each mode of operation is intended to be covered by the description and claims. The three modes are as follows:

I. Where the combined mass of the cylindrical housing and the effective water load mass coupled to the cylindrical housing is substantially equivalent to the combined mass of the piston and the effective water load mass coupled to the piston, so that both the cylindrical housing and the piston move with respect to each other when the acoustic pulse generator is fired.

2. Where the combined mass of the cylindrical housing and the effective water load mass coupled to the cylindrical housing is large relative to the combined mass of the piston and the effective water load mass coupled to the piston, so that the cylindrical housing is stationary with respect to the piston and the piston moves when the acoustic pulse generator is fired.

3. Where the combined mass of the piston and the effective water load mass coupled to the piston is large relative to the combined mass of the cylindrical housing and the effective water load mass coupled to the cylindrical housing, so that the piston is stationary with respect to the cylindrical housing and the cylindrical housing moves when the acoustic pulse generator is fired.

The effective water load mass of the piston and the cylindrical housing is approximately determined by the radius of the piston and cylindrical housing according to the following formula: M= 8/3 r R where M the effective water load mass,

r= the density of the water, and R the radius of the cylindrical housing or the piston.

It is presently believed that mode three is best for acoustic pulse generators in which the ratio of the bore of the cylindrical housing to the stroke length of the piston is small; that mode two is best for large bore to stroke ratios; and that mode one is best for intermediate bore to stroke ratios.

As will be apparent to those skilled in the art, where both the cylindrical housing and the piston move in the operation of the acoustic pulse generator, both will rebound causing a large positive acoustic pressure. Where only the cylindrical housing moves, it will rebound causing a large positive acoustic pressure; and where only the piston moves, its rebound will cause a large positive acoustic pressure.

Thus, when reference is made to movement of the piston toward the closed end of the cylindrical housing in the detailed description it is intended that there be relative movement but that either the piston or the cylindrical housing, or both, may move. Similarly, references to displacing the piston relative to the cylindrical housing to an extended position relative to the closed end of the cylindrical housing are intended to refer to situations in which either the piston, or the cylindrical housing, or both, move. Further references to capturing the piston on arresting movement of the piston are intended to refer to situations in which either the pistons, the cylindrical housing or both are captured.

In order that the invention be easily understood, it is assumed in the detailed description of this invention that the piston moves and the cylindrical housing is stationary with respect to the piston.

The present invention provides an acoustic pulse generator and a method of producing an acoustic pulse in a fluid medium which produce a series of primary acoustic pulses without undesirable secondary acoustic pulses. When utilized in seismic exploration of a marine area, the acoustic pulse generator and method of the present invention permit a very accurate geologic map of the land area to be obtained from the echo pattern produced from the primary acoustic pulses. Further, the present invention permits seismic exploration to be conducted with a minimum expenditure of mechanical energy.

The accompanying drawings illustrate a preferred embodiment of the invention, and, together with the description, serve to explain the principles of the invention.

OF THE DRAWINGS FIG. I is a plan view of an acoustic pulse generator constructed in accordance with the principles of the present invention;

FIG. 2 is a horizontal section of the acoustic pulse generator of FIG. 1 which illustrates a hydraulic cylinder and a valve mounted at the end of the hydraulic cylinder for operating the acoustic pulse generator;

FIG. 3 is an enlarged fragmented sectional view of the valve and hydraulic cylinder of FIG. 2;

FIG. 4 is an enlarged fragmented vertical section taken along line 4--4 of FIG. 1, showing the valve in its open position;

FIG. 5 is an end elevation of the acoustic pulse generator taken along line 55 of FIG. 1;

FIG. 6 is a schematic diagram of a fluid control circuit for supplying fluid to the valve to operate the acoustic pulse generator;

FIG. 7 is a schematic diagram of an electronic control circuit for operating the fluid control circuit of FIG. 6; and

FIG. 8 is a schematic diagram of a time delay circuit which can be used in the electronic control circuit of FIG. 7.

FIGS. I and 2 show an acoustic pulse generator which operates in accordance with the principles of the present invention. The acoustic pulse generator is designed to be submerged in a fluid medium to produce acoustic pulses. In performing seismic exploration of a marine area, e.g., an offshore underwater area, the acoustic pulse generator is normally suspended on a cable which is attached to a vessel which moves across the surface of the water.

In accordance with the invention, the acoustic pulse generator includes an expandable and collapsible structure for establishing a chamber of variable volume in the fluid medium. In a preferred embodiment, the acoustic pulse generator includes a housing having one end closed and one end open to the fluid medium, and a piston mounted for sliding movement within the housing. The housing and piston define a chamber of variable volume. In this structure the chamber is collapsible and expandable along a single axis.

As here embodied, a cylindrical housing or cylinder, generally 12, having an open end 14 and a closed end 16 is provided. A piston, generally 18, is slidably disposed within cylindrical housing 12. Closed end 16 of cylindrical housing 12 is curved and piston 18 has a contoured inner surface 19 to match the curve of closed end 16.

The piston is in sealing engagement with the cylindrical housing to provide a chamber 20 (FIG. 2) of variable volume between the closed end of the cylinder and the piston. Referring to FIG. 2, an annular cup seal 22 is mounted on the outer edge of piston 18 to provide sealing engagement with the inner wall of cylindrical housing 12. Seal 22 is mounted by an annular clamp 24 held in place by a plurality of bolts 26 which extend through clamp 24 and cup seal 22 into appropriate internal threads in piston 18.

As shown in FIGS. 2 and 5, piston 18 includes a hollow cylindrical hub 28 and six ribs 29 radiating outwardly from hub 28 to the outer perimeter of piston 18 to add strength to the piston. Piston I8 is secured to a shaft 30 by a clamp 32 which is mounted on the shaft and secured to hub 28 by a plurality of bolts 34. Clamp 32 can be of any conventional type, such as a split ring clamp, which can be secured to shaft 30 at a groove 36 in the shaft. The sections of clamp 32 may be secured together in any conventional manner.

Shaft 30 has a second groove 38 formed therein and located within hub 28 of piston 18. An 0-ring seal 40 is inserted in groove 38 to provide a seal between the inner surface of hub 28 and shaft 30.

One end of shaft 30 is slidably mounted in a hub 42 of a spider, generally 44, mounted on open end 14 of cylindrical housing 12 by a plurality of bolts 46 passing through the outer rim of spider 44 and into suitable internal threads formed at the periphery of cylindrical housing 12. A bushing 48 is mounted between shaft 30 and hub 42.

The other end of shaft 30 extends through a neck 54 formed at the closed end of cylindrical housing 12. Neck 54 has an internal bore in which a bushing 58 is mounted. The bushing permits shaft 30 to slide with respect to neck 54. An -ring seal 60 is mounted in a groove formed in the interior wall of neck 54 to provide a seal between shaft and neck 54.

In accordance with the invention, means connected to the structure are provided for expanding the chamber against the ambient pressure of the fluid medium and for permitting the chamber to be collapsed by the ambient pressure and to rebound against the ambient pressure to produce an acoustic pulse in the fluid medium. In the preferred embodiment, displacing means are provided for displacing the piston relative to the cylindrical housing-to an extended position relative to the closed end of the cylindrical housing and for releasing the piston to permit the ambient pressure of the fluid medium to drive the piston inward relative to the cylindrical housing toward its closed end. As here embodied, the displacing means comprises a fluid-operated cylinder 62 (FIG. 2), preferably a hydraulic cylinder, having a plunger 64 slidably mounted therein. Plunger 64 is mounted on shaft 30 which extends into one end of hydraulic cylinder 62 through a packing seal 66 located in a cylindrical base 68 of the hydraulic cylinder.

Hydraulic cylinder 62 is used to displace piston 18 relative to cylindrical housing 12. When the hydraulic cylinder is charged with pressurized fluid, plunger 64 and shaft 30 are moved outward with respect to the hydraulic cylinder and piston 18 moves, against the ambient pressure of the fluid medium, to an extended position at the open end of cylindrical housing 12. When the pressure of the fluid in the hydraulic cylinder is decreased, the ambient pressure of the fluid medium drives piston 18 toward the closed end of cylindrical housing 12 and plunger 64 is moved inward with respect to the hydraulic cylinder to discharge the fluid from the hydraulic cylinder.

In a preferred embodiment of the acoustic pulse generator, hydraulic cylinder 62 is mounted on cylindrical housing 12 at its closed end. Referring to FIG. 1, the cylindrical housing is provided with a mounting block 76 having a pair of

pins

72 and 74. A pair of

arms

78 and 82 is connected at one end to the mounting block by

pins

72 and 74.

Arms

78 and 82 have shaft-like portions 80 and 86 (FIG. 2) which have threads formed at their ends.

As shown in FIGS. 1 and 2, hydraulic cylinder 62 is provided with a flange 84 which projects outward from the sides of the hydraulic cylinder. The shaft-like portion 80 of arm 78 extends through an opening formed in the flange. A resilient washer 88 and a metal washer 90 are fitted over the shaft-like portion of arm 78, and a nut 92 is screwed on the threaded end of shaft-like portion 80 to secure the hydraulic cylinder to the cylindrical housing.

Similarly, shaft-like portion 86 of arm 82 (FIG. 2) extends through an opening in flange 84. A resilient washer 94 and a metal washer 96 are fitted over the shaft-like portion of arm 82. A nut 98 is screwed on the threaded end of the shaft-like portion of arm 82 to secure the hydraulic cylinder to the cylindrical housing.

Alternatively, any convenient arrangement for securing hydraulic cylinder 62 to the cylindrical housing may be used in place of the particular mounting arrangement described. In an alternative arrangement, the hydraulic cylinder can be attached to the piston and its plunger connected to the cylindrical housing.

In accordance with the invention, means operable after rebound of the chamber are provided for preventing a subsequent collapse of the chamber by the ambient pressure to prevent secondary acoustic pulses from being produced in the fluid medium. The preferred embodiment of the acoustic pulse generator includes means operatively connected to the piston for capturing the piston after rebound of the piston from the closed end of the cylindrical housing to prevent inward movement of the piston by the ambient pressure of the fluid medium. In the preferred embodiment of the invention,

fluid control means connected to the hydraulic cylinder are provided for supplying pressurized fluid to the interior of the hydraulic cylinder to operate the acoustic pulse generator. The fluid control means enables the piston to be captured after its rebound from the closed end of the cylindrical housmg.

The capturing means preferably includes a valve connected to the hydraulic cylinder for admitting pressurized fluid into the hydraulic cylinder during rebound of the piston from the closed end of the cylindrical housing and for preventing the fluid from discharging from the hydraulic cylinder after the piston completes its rebound. As here embodied, a unique fluid-operated valve, generally 100, (FIGS. 1 and 2) is secured by welding or other conventional means at one end of hydraulic cylinder 62. The valve includes a valve housing 102 having a circular slot 104 (FIG. 3) formed at one end thereof for receiving the cylindrical side wall of hydraulic cylinder 62. As shown in FIG. 3, an 0-ring seal 106 is mounted in a groove formed in the exterior side wall of hydraulic cylinder 62 to provide a seal between valve housing 102 and the hydraulic cylinder.

Referring to FIG. 3, a slot 110 is formed in valvehousing 102 and a cylindrical valve stem 112 is slidably mounted within slot 110. One end of valve stem 112 extends into a fluid passage 113 formed in valve housing 102. A circularly-shaped poppet valve 114 is mounted at this end of the valve stem. Poppet valve 114 is normally maintained in contact with a valve seat 116 formed on the valve housing at the periphery of fluid passage 113, so that it is normally closed. In its open position, as shown in FIG. 4, poppet valve 114 is moved away from valve seat 116 and the interior of hydraulic cylinder 62 is opened to fluid flow from ports 120 and 122 (FIG. 3) which are formed in valve housing 102. Hydraulic tubes 121 and 123 (FIGS. 1 and 3) are connected to

ports

120 and 122, respectively.

The other end of valve stem 112 extends through slot 110 into a generally cylindrical fluid chamber 126 formed in valve housing 102. Valve stem 112 has an enlarged cylindrical portion 128 located in fluid chamber 126 which defines a shoulder 130 on the valve stem. The enlarged cylindrical portion of valve stem 112 provides an enlarged end surface 129 (FIG. 3) on the valve stem. I I

As shown in FIG. 3, an annular flange 132 projects from the enlarged cylindrical portion 128 of valve stem 112 into fluid chamber 126. The flange has opposite annular surfaces 133 1 and 134 to which fluid pressure can be applied by fluid admitted into fluid chamber 126.

In addition, flange 132 has an annular slot 135 formed therein for receiving one end of a coil spring 136. The other end of coil spring 136 is received in an annular recess 138 formed in valve housing 102. The coil spring biases valve 100 into a normally closed position by urging poppet valve 114 into contact with valve seat 116.

Valve 100 is also provided with a free piston 140 (FIGS. 3 and 4) slidably disposed within fluid chamber 126. The free piston is provided with a cylindrical opening 142 for receiving the enlarged cylindrical portion 128 of valve stem 112 and for permitting the free piston to slide relative to valve housing 102 and relative to the enlarged cylindrical portion of valve stem 112. An annular end surface 143 of the free piston is located adjacent to end surface 129 of the valve stem. Free piston 140 is normally in contact with surface 134 of flange 132. The flange provides means for transmitting pressure applied to surface 143 of the free piston to the valve stem.

As shown in FIG. 3, valve 100 is provided with a cover plate 146 secured to valve housing 102 by a plurality of bolts 148 which extend through the cover plate and into threaded openings formed in the valve housing. An O-ring seal 150 is mounted in an annular slot formed in valve housing 102 to provide a seal between cover plate 146 and the valve housing.

An annular projection 152 (FIG. 3) is formed on cover plate 146 and extends into the interior of valve housing 102 to provide a stop for free piston 140. The valve stem is dimensioned so that with the poppet valve is in its closed position, as shown in FIG. 3, there is a small clearance between free piston 140 and projection 152. In this position, end surface 143 of free piston 140 and the enlarged end surface 129 of valve stem 112 are spaced from the interior wall of cover plate 146 to provide a small space 154.

In the preferred embodiment, means are provided for supplying pressurized fluid to the end of the valve stem in the fluid chamber and to the end of the free piston to open the poppet valve.

Referring to FIG. 3, cover plate 146 has a port 158 formed therein through which pressurized fluid may be admitted into space 154 to open the poppet valve. As shown in FIGS. 1 and 3, a hydraulic tube 160 is connected to port 158 of cover plate 146 and is used to supply pressurized fluid to space 154.

When pressurized fluid is applied to space 154 (FIG. 3) through port 158, fluid pressure is applied to end surface 129 of valve stem 112 and to end surface 143 of free piston 140. The pressure applied to surface 143 of the free piston is transmitted to the valve stem through flange 132. Thus, the total force acting on the valve stem to open poppet valve 114 is the sum of the forces applied to the end surfaces of valve stem 112 and free piston 140.

As the poppet valve is opened, free piston 140 and valve stem 112 move to the right, as viewed in FIG. 3. Free piston 140 is limited in its rightward movement by a shoulder 162 which is formed on valve housing 102 in fluid chamber 126. Valve stem 112 is also limited in its rightward movement by shoulder 130 formed on the enlarged portion 128 of the valve stem. Thus, free piston 140 moves to the right until it is stopped by shoulder 162. Thereafter, the fluid pressure applied to its end surface 143 is no longer transmitted to valve stem 112, and valve stem 112 moves to the right due only to the pressure applied to end surface 129. Valve stem 112 continues to move until shoulder 130 contacts an annular surface 164 of valve housing 102 surrounding slot 110 in which the valve stem is slidably mounted.

Referring to FIG. 4, valve 100 is provided with a port 166 formed in valve housing 102 which extends into fluid chamber 126. As shown in FIG. 1, a hydraulic tube 168 is connected to port 166. Pressurized fluid can be applied through hydraulic tube 168 and port 166 to fluid chamber 126 (FIG. 4).

In addition, a port 170 is formed in valve housing 102. As shown in FIG. 4, port 170 extends into the interior of hydraulic cylinder 62.. A hydraulic tube 172 (FIG. 1) is connected to port 170. Pressurized fluid can be applied through hydraulic tube 172 and port 170 to the interior of the hydraulic cylinder.

In the preferred embodiment of the invention, resilient means is provided in the chamber established by the closed end of the cylindrical housing and the piston for preventing the piston from contacting the closed end when the piston is driven inward relative to the cylindrical housing by the ambient pressure of the fluid medium and for causing the piston to rebound outwardly with respect to the closed end of the housing.

As here embodied, the resilient means comprises a compressible fluid, such as a gas, which is introduced into the chamber 20 (FIG. 2). The compressible fluid is applied to chamber 20 at a pressure below the ambient pressure of the fluid medium in which the acoustic pulse generator is submerged. 'When chamber 20 is expanded by displacing piston 18 to the open end of cylindrical housing 12, the pressure of the compressible fluid is less than the ambient pressure of the fluid medium. Thus, when the piston is released the ambient pressure is sufficient to drive the piston toward the closed end of the cylinder. In the preferred embodiment of the acoustic pulse generator, air can be used as the compressible fluid.

As shown in FIG. 2, the air can be introduced into the chamber through a port 182 formed in the closed end of cylindrical housing 12. A connector 184 (FIG. 1) which is secured to the end of an air line 186 is threaded into port 182. Air line 186 is connected to a conventional vacuum regulator (not shown) for regulating the pressure of the air in the chamber.

As shown in FIG. 2, a port 176 is fonned in base 68 of hydraulic cylinder 62. A hydraulic tube 178 (FIG. 1) is connected to a coupling 180 which is threaded into port 176. For purposes of illustration, port 176 is shown in FIG. 2 at a position which is out of alignment with hydraulic tube 178 and coupling 180 of FIG. 1. Port 176 is provided to expel fluid from the hydraulic cylinder that leaks past plunger 64 in the operation of the acoustic pulse generator.

In FIG. 6, a schematic diagram of a fluid control circuit for operating the hydraulic cylinder of the acoustic pulse generator is shown. The fluid control circuit includes a solenoidoperated directional control valve, generally 200, which is a 4- way, S-piston valve. The three positions of the control valve are illustrated by valve symbol 202 and are designated as positions X, Y, and Z. The control valve is operated by solenoids S,and S As shown in FIG. 6, the fluid control circuit includes a source 204 of pressurized fluid connected by an elongated hydraulic tube 206 to a first port 208 of the control valve. A fluid reservoir 210 is connected by an elongated hydraulic tube 212 to a one-way valve 214 which is connected to a second port 216 of the control valve by a hydraulic tube 215.

Hydraulic tubes

160 and 172, which are connected to

ports

158 and 170, respectively, of valve (FIG. 4), are con nected to

ports

218 and 220, respectively of control valve 200 (FIG. 6).

The purpose of control valve 200 is to selectively vary the fluid connections between

hydraulic tubes

160 and 172 and

hydraulic tubes

206 and 212. For example, when neither solenoid S nor 8, is operated, the control valve is in its neutral position (position Z). With the control valve in this position, hydraulic tube 160 is connected to hydraulic tube 215 and there is no connection between the

hydraulic tubes

172 and 206.

When solenoid S, is operated, control valve 200 is moved to position X. In this position, hydraulic tube 160 is connected to hydraulic tube 206 through the control valve, and hydraulic tube 172 is connected to hydraulic tube 215. When solenoid S is operated, control valve 200 is moved to position Y. In this position, hydraulic tube 160 is connected to hydraulic tube 215, and hydraulic tube 172 is connected to hydraulic tube 206.

The fluid control circuit also includes a plurality of fluid accumulators A,, A and A;,. Any conventional fluid accumulator may be employed in the fluid control circuit of the present invention. An example of an accumulator which has been used in a preferred embodiment of the present invention is one known as the I-Iale 4A Accumulator, manufactured by the Hale Fire Pump Company of Conshohocken, Pennsylvania.

Accumulator A, is connected by a hydraulic tube 224 to hydraulic tube 215 which is connected to port 216 of the control valve. This accumulator serves as a local reservoir for the acoustic pulse generator. It is submerged in the fluid medium along with the generator, while reservoir 210 remains above the surface of the fluid medium and is connected by the elongated hydraulic tube 212 to the generator. The local reservoir provides for rapid operation of the fluid control circuit and conserves the amount of energy expended in operating the circuit by eliminating the necessity of driving fluid rapidly through along hydraulic tube to a remote reservoir.

Accumulator A is connected by a hydraulic tube 226 to hydraulic tube 206 which connects source 204 of pressurized fluid to port 208 of the control valve. This accumulator provides a local pressure source for the acoustic pulse generator. It is also submerged in the fluid medium along with the generator, while source 204 is above the surface of the fluid medium and is connected to the generator by the elongated hydraulic tube 206. The local pressure source permits more rapid operation of the fluid control circuit and conserves the amount of energy expended in the operation of the circuit by eliminating the necessity of rapidly driving pressurized fluid from a remote source through a long hydraulic tube to the submerged generator.

Accumulator A is connected by a small diameter hydraulic tube 228 to hydraulic tube 215. Hydraulic tube 168 which is connected to port 166 of valve 100 (FIG. 4) is connected to accumulator A,,. In addition,

hydraulic lines

121 and 123 which are connected to

ports

120 and 122, respectively, of valve 100 are also connected to accumulator A Accumulator A, provides a back pressure of plunger 64 of hydraulic cylinder 62 and augments the resilient means in chamber to cause rebound of piston 18 from the closed end of cylinder 12. The degree to which accumulator A augments the resilient means in chamber 20 can be controlled by the adjusting of the initial volume of the accumulator and its initial gas pressure. In an alternative arrangement of the acoustic pulse generator, accumulator A, can be used to provide the primary rebound force for piston 18 and the resilient means in chamber 20 can be eliminated.

Hydraulic line 178, which is connected to port 176 (FIG. 2) of hydraulic cylinder 62, is connected to hydraulic tube 212 extending from reservoir 210. Further, a relief valve 230 (FIG. 6) is connected to hydraulic tube 172.

In the preferred embodiment of the acoustic pulse generator of the present invention, the fluid control circuit (FIG. 6) excluding source 204 of pressurized fluid and reservoir 210, is mounted on a frame (not shown) which is secured to cylindrical housing 12. Thus, when the acoustic pulse generator is submerged in a fluid medium, the entire fluid control circuit, except for source 204 and reservoir 210, is submerged along with it.

Source 204 of pressurized fluid and reservoir 210 are preferably located on a vessel which traverses the surface of the fluid medium in which the acoustic pulse generator is submerged. Source 204 and reservoir 210 are connected by eIon-' gated

hydraulic tubes

206 and 212, respectively, to the remainder of the fluid control circuit and, as explained above, accumulators A, and A serve as local reservoir and source, respectively, for the fluid control circuit.

In FIG. 7, an electronic circuit for operating control valve 200 (FIG. 6) is shown. The circuit includes a transistor T, having its collector electrode connected to a lamp 240 which is connected to one terminal of solenoid S,. The other terminal of solenoid S, is connected to a conductor 242 which, in turn, is connected to a source of potential V (not shown). A resistance 244 connects the emitter electrode of transistor T, to conductor 242, and a diode 246 connects the emitter electrode to a conductor 248. Conductor 248 is connected to ground through a diode 250.

The circuit also includes a transistor T having its collector electrode connected through a lamp 252 to one terminal of solenoid S The other terminal of solenoid S is connected to conductor 242. The emitter electrode of transistor T is connected to conductor 248.

Biasing resistances

254 and 255 are provided which connect the base electrode of transistor T to conductor 242.

As shown in FIG. 7, the circuit includes a normally open switch 256 which is connected to the base electrode of transistor T through resistance 255 and to ground. In a preferred embodiment of the acoustic pulse generator, switch 256 is a normally open microswitch which is mounted on spider 44 (FIGS. 2 and 5). Switch 256 is used to detect inward movement of piston 18 from the open end of cylindrical housing 12.

When piston 18 is moved to the open end of cylindrical housing 12, normally open switch 256 is closed. Thereafter, when the piston is moved inward toward the closed end of cylindrical housing 12, switch 256 is opened to indicate inward movement of the piston.

Referring to FIG. 7, the circuit for operating the control valve also includes a transistor T having its emitter electrode connected to conductor 248. A resistance 260 connects the collector electrode of transistor T to conductor 242. In addition, a resistance 262 connects the collector electrode of transistor T through a diode 264 to the base electrode of transistor T,.

The circuit is also provided with transistor .1, and T, which together comprise a one shot multivibrator. As shown in FIG. 7, the collector electrode of transistor T, is connected through resistance 260 to conductor 242. The emitter electrode of transistor T, is connected by a conductor 266 to a first terminal 268 of a two position switch, generally 270. A second terminal 272 of the two position switch is connected by a conductor 274 through biasing resistance 255 to the base electrode of transistor T,. Switch 270 is provided with a movable contact 276, connected to conductor 248 at terminal 320, which can be moved into contact with either terminal 268 or terminal 272.

The collector electrode of transistor T, is also connected to the base electrode of transistor T through a diode 280 and a resistance 282. The base electrode of transistor T is connected to ground through a resistance 284, The emitter electrode of transistor T is connected to conductor 248, and its collector electrode is connected by

resistances

286 and 288 to conductor 242.

As shown in FIG. 7, a Zener diode 290 connects the common terminal of

resistances

286 and 288 to ground. A capacitance 292 is provided which connects the collector electrode of transistor T to the base electrode of transistor T,. In addition, a resistance 294 is provided which connects the base electrode of transistor T, to conductor 242.

Finally, the circuit for operating the fluid control valve includes a pulse generator 296 for applying input pulses to the base electrode of transistor T,. In a preferred embodiment of the present invention, the frequency of the pulses produced by pulse generator 296 can be adjusted to control the rate at which acoustic pulses are produced by the acoustic pulse generator.

In FIG. 8, there is shown a delay circuit which can be incorporated in a preferred embodiment of the control circuit of FIG. 7. The delay circuit (FIG. 8) includes a Zener diode 300, a capacitance 302, and a variable resistance 304 which are connected at a common terminal 306. When the delay circuit (FIG. 8) is incorporated in the circuit of FIG. 7, terminal 308 of the Zener diode, terminal 310, and terminal 312 of the variable resistance are connected to corresponding

terminals

318, 320, and 322, respectively, of the control circuit.

In considering the operation of the acoustic pulse generator of the present invention, it is assumed that piston 18 (FIG. 2) is completely advanced to the open end of cylindrical housing 12 at the start of the operation. At this time, plunger 64 and shaft 30 are moved to their fully extended positions and the interior of hydraulic cylinder 62 is charged with pressurized fluid. The acoustic pulse generator is submerged in a fluid medium wherein the ambientpressure of the fluid medium acts on the exterior of piston 18.

At the beginning of the operation, both transistors T, and T (FIG. 7) are in non-conducting states. Transistor T, is maintained in a non-conducting state by transistor T,, the normally conducting transistor of the one shot multivibrator. When transistor T, is conducting, its collector electrode which is connected to the base electrode of transistor T,, is at ground potential to maintain transistor T, non-conducting. Transistor T is maintained in a non-conducting state by normally open switch 256, which is held closed when piston 18 is located at the open end of cylinder 12. With switch 256 closed, the base electrode of transistor T is held at ground and the transistor is thereby rendered non-conducting.

Thus, both solenoids S, and S, are deenergized and control valve 200 (FIG. 6) is in its neutral position (i.e. position Z) at the start of the operation.

To fire the acoustic pulse generator, a positive input pulse is applied from pulse generator 296 (FIG. 7) to the base electrode of transistor T, to drive the transistor into a non-conducting state. Transistor T is switched into a conducting state, and transistor T, remains in a non-conducting state for a time which is determined by capacitance 292 and resistance 294 which comprise an R-C timing circuit.

When transistor T, is turned off, an operating potential is applied to the base electrode of transistor T, through

resistances

260 and 262. Transistor T, is thereby rendered conductive to energize solenoid S Solenoid S, remains energized during the time that transistor T is non-conducting. As explained above, this time is determined by the RC timing circuit consisting of capacitance 292 and resistance 294. Thereafter, when the potential applied to the base electrode of transistor T, is sufficient to drive the transistor into conduction, transistor T, is turned off and solenoid S, is deenergized.

Considering the fluid control circuit of FIG. 6, when solenoid S, is energized control valve 200 is moved to position X. With the control valve in this position, hydraulic line 160 is connected to source 204 of pressurized fluid by hydraulic line 206. Referring to FIG. 3, pressurized fluid is thus supplied through port 158 into space 154 to urge free piston 140 and valve stem 112 to the right.

At the same time, with control valve 200 in position X( FIG. 6), hydraulic line 172 is connected through the control valve to hydraulic line 215. Since hydraulic line 215 is connected to accumulator A, and to reservoir 210 by hydraulic line 212, the pressure of the fluid within hydraulic cylinder 62 is immediately decreased. Thus, piston 18 (FIG. 2) can begin to move inwardly toward the closed end of cylindrical housing 12.

The fluid flow capacity of control valve 200 and the hydraulic lines connected to it is limited so that additional means are required for the rapid discharge of fluid from hydraulic cylinder 62. This additional fluid discharge means is provided by valve 100.

Referring again to FIG. 3, the pressure of the fluid applied to space 154 of valve 100 acts on

end surfaces

129 and 143 of valve stem 112, and free piston 140, respectively, and urges both free piston 140 and valve stem I12 rightward. Since the free piston engages flange 132 of valve stem 112, the pressure exerted on end surface 143 of the free piston is transmitted to the valve stem. Thus, the total force applied to valve stem 112 includes both the force exerted on its end surface and the force exerted on the end surface of free piston 140. This force is sufficient to overcome the opposing forces exerted on the valve stem by the fluid in hydraulic cylinder 62, by coil spring 136, and by the fluid applied to chamber 126 through port 166 (FIG. 4). The fluid in chamber 126 exerts pressure on shoulder 130 of the valve stem and produces a pressure differential on

surfaces

133 and 134 of flange 132.

Poppet valve 114 is opened as the free piston and valve stem move to the positions shown in FIG. 4. The movement of free piston I40 terminates when it moves into contact with shoulder 162. Thereafter, the fluid pressure applied to end surface 143 of the free piston does not act on valve stem 112. The valve stem continues to move rightward, due to the fluid pressure applied to its end surface 129 until the valve stem moves into contact with surface 164 of the valve housing.

Free piston 140 (FIG. 3) of valve 100 permits an initially high force to be applied to valve stem 112 to open the poppet valve. The valve design conserves the amount of fluid required to open the valve by permitting valve stem 112 to be driven a greater distance than free piston 140. After the initial opening of the poppet valve, less pressure is required to move valve stem 112 to its completely open position. Thus, the movement of free piston 140 is limited by shoulder 162 (FIG. 3) to conserve the amount of fluid which flows into space 154 through port 158 during the opening ofthe poppet valve.

As piston 18 moves inward toward the closed end of cylindrieal housing 12, switch 256 (FIG. 7) is opened to disconnect biasing

resistances

254 and 255 from ground. An operating potential is thus applied through biasing

resistances

254 and 255 to the base electrode of the transistor. Transistor T is thereby rendered conductive and solenoid S is energized. It should be noted that solenoid S, also remains energized at this time, and due to the non-linear force characteristics of the solenoids, control valve 200 remains in position X until solenoid S, is deenergized.

As piston 18 continues to move inward relative to cylindrical housing 12, the fluid which remains within hydraulic cylinder 62 is discharged primarily through ports I20 and 122 into accumulator A increasing the pressure in the accumulator. There is a large fluid flow rate through ports I20 and 122 at a low pressure.

In addition, the inward movement of piston 18 compresses the air in chamber 20 (FIG. 2) to increase the air pressure therein. At some point during the inward movement of piston 18, the internal air pressure in chamber 20 equals the external ambient pressure of the fluid medium acting on the exterior of piston 18. The piston continues, however, to move toward the closed end of cylindrical housing 12 due to its inertia. The inertia of the piston is overcome by the increase in internal air pressure in chamber 20 and the pressure buildup in accumulator A, during further movement of the piston toward closed end 16 of the cylindrical housing 12. The combined pressure of the compressed air in chamber 20 and the pressure of accumulator A prevents piston 18 from striking the closed end of the cylindrical housing.

When the inward movement of piston 18 is completed, the internal air pressure in chamber 20 causes the piston to rebound against the external ambient pressure of the fluid medium. The rebound of the piston from the closed end of the cylinder causes a large positive acoustic pulse to be produced in the fluid medium.

As explained above, a predetermined time after transistor T, is rendered conductive to energize solenoid S transistor T is driven into conduction to turn off transistor T, and to decnergize solenoid 8,. Since solenoid S has already been energized by the opening of switch 256, control valve 200 (FIG. 6) moves to position Y. The predetermined time is selected so that the control valve moves to this position shortly after piston 18 reaches its inwardrnost position relative to cylindrical housing 12. With control valve 200 in the Y position, hydraulic line 160 is connected through the control valve to hydraulic line 215, and hydraulic line 172 is connected to hydraulic line 206.

Referring to FIG. 4, with the control valve in position Y, port 158 is connected to reservoir 210 and accumulator A, (FIG. 6), and the pressure of the fluid in space 154 is immediately reduced. In addition, port 170 is connected to source 204 of pressurized fluid and to accumulator A, and, thus, pressurized fluid is again supplied to the interior of hydraulic cylinder 62.

As piston 18 rebounds from the closed end of cylindrical housing 12 toward the open end of the housing, plunger 64 (FIG. 2) is moved away from the poppet valve and the pressure in accumulator A, forces the previously discharged fluid through ports and 122 and the poppet valve into the hydraulic cylinder. By forcing the fluid into hydraulic cylinder 62, accumulator A prevents cavitation from occurring in the hydraulic cylinder during the rebound stroke of piston l8. If the pressure from accumulator A., were not provided, a vacuum would occur in hydraulic cylinder 62 during the rebound of piston 18 to cause cavitation in the hydraulic cylinder. At the same time, pressurized fluid from source 204 is supplied to the hydraulic cylinder through port 170 (FIG. 4).

Accumulator A also applies fluid under pressure through port 166 (FIG. 4) into fluid chamber 126 of valve 100. The pressure 0 the fluid supplied to fluid chamber 126 drives free piston leftward to return the free piston to its original position, as shown in FIG. 2. Valve stem 112 does not return to its original position at this time, since the pressure of the fluid flowing into the hydraulic cylinder through

ports

120 and 122 and acting on poppet valve 114 together with the fluid pressure acting on surface 134 of flange 132 is sufficient to balance the forces exerted on the valve stem in the opposite direction by coil spring 136 and the fluid pressure acting on surface 133 of flange 132.

As piston 18 nears the completion of its rebound stroke, the flow of fluid into the hydraulic cylinder through

ports

120 and 122 decreases. An increasing force is applied to valve stem 112 and poppet valve 114 by the fluid in the hydraulic cylinder to urge the valve stem to the left, as viewed in FIG. 4. When the leftward forces acting on valve stem 112, i.e., the force exerted on valve stem 112 by coil spring 136 and the forces applied to shoulder 130 and surface 133 of flange 132 by the pressurized fluid in chamber 126, exceed the forces acting on the valve stem in the opposite direction, i.e., the force exerted on surface 134 of flange 132 by the pressurized fluid in chamber 126and the rightward force exerted on poppet valve 114 by virtue of the fluid

flow form ports

120 and 122 to the interior of the hydraulic cylinder, the valve stem is returned to its original position, as shown in FIG. 3. Thus, the poppet valve is closed to prevent further fluid flow between accumulator A and the hydraulic cylinder. The closure of the poppet valve occurs automatically at the end of the rebound stroke of piston 18 and the piston is thus captured to prevent it from moving toward the closed end of cylindrical housing 12.

The rebound force applied to piston 18 by the compressed fluid in chamber 20 is not sufficient to completely return the piston to the open end of the cylindrical housing. In some cases, for example, the piston only rebounds A: to B of the distance back to the open end of the cylindrical housing. After the poppet valve is closed, however, the pressure of the fluid in hydraulic cylinder 62 continues to build up since pressurized fluid is supplied to the interior of the hydraulic cylinder through port 170 (FIG. 4). Thus, the piston continues to move outward until it reaches the open end of the cylindrical housing. At this time, a large force due to the pressure buildup in hydraulic cylinder 62 acts on poppet valve 114 and valve stem 112 to maintain the poppet valve in a closed position.

When piston 18 reaches the open end of cylindrical housing 12, switch 256 (FIG. 7) is closed and transistor T is returned to a non-conducting state. Solenoid S is thereby deenergized, and control valve 200 (FIG. 6) is moved to its neutral position (position Z). With the control valve in position Z, hydraulic line 172 is cut off from source 204 of pressurized fluid while hydraulic line 160 is maintained in communication with reservoir 210 and accumulator A,.

Since the poppet valve is automatically closed and the interior of hydraulic cylinder 62 is charged with pressurized fluid when piston 18 completes its rebound stroke, the piston is captured after the rebound stroke. Any inward movement of the piston after the rebound stroke is resisted by the pressurized fluid supplied to the interior of hydraulic cylinder 62. The pressure buildup in the hydraulic cylinder exerts a force on plunger 64 which is sufflcient to overcome the inward force exerted by the ambient pressure of the fluid medium on piston 18.

By preventing subsequent inward movement of piston 18 after its rebound stroke, the acoustic pulse generator of the present invention eliminates secondary acoustic pulses from the output signal of the acoustic pulse generator. By capturing the piston when its rebound stroke is completed, oscillation of the piston after rebound is prevented. Thus, the principle cause of secondary acoustic pulses in a mechanical acoustic pulse generator is eliminated.

Furthermore, the piston is captured at the end of its rebound stroke when its velocity is zero and, thus, there are no large or violent accelerating forces applied to the piston to prevent its further motion. As a result, any possibility of external cavitation in the fluid medium near the piston is eliminated and another potential source of secondary acoustic pulses is removed.

In addition, the capturing of the piston after its rebound stroke conserves the amount of energy required to operate the acoustic pulse generator. Since inward movement of the piston after rebound is prevented, it is only necessary to move the piston a minimum distance to return it to the open end of the cylindrical housing to prepare the acoustic pulse generator for a subsequent firing.

In the above description of the electric control circuit of FIG. 7, the operation of the circuit is described with switch 270 in the position shown in FIG. 7, i.e., with movable contact 276 engaging terminal 268. The movable contact 276 of switch 270 can be moved into engagement with terminal 272 if it is desired to continuously energize solenoid S, to maintain piston 18 in a collapsed position at the closed end of cylindrical housing 12.

With movable contact 276 engaging terminal 272, the base electrode of transistor T is continuously connected to ground through switch 270 and diode 250. Thus, transistor T is maintained in a non-conducting state and solenoid S cannot be energized as long as switch 270 is in this position.

At the same time, the emitter electrode of transistor '1, is disconnected from ground, and transistor T is switched from its normally conducting state to a state of non-conduction. Thus, an operating potential is applied to the base electrode of transistor T, through

resistances

260 and 262 which drives transistor T, into conduction to energize solenoid S,. Transistor T, is maintained in a conducting state and solenoid S, is continuously energized as long as the movable contact 276 of switch 270 engages terminal 272.

With solenoid S, continuously energized, control valve 200 (FIG. 6) is continuously maintained in position X. With the control valve in position X, the poppet valve is held in an open position by pressurized fluid supplied to hydraulic line from source 204, and the interior of hydraulic cylinder 62 is connected by hydraulic line 172 to reservoir 210 and accumulator A,. The ambient pressure of the fluid medium maintains piston 18 at the closed end of cylindrical housing 12. Thus, the movable contact 276 of switch 270 can be moved into engagement with terminal 272 when it is desired to terminate the operation of the acoustic pulse generator.

Further, in the operation of the control circuit of FIG. 7, the one shot multivibrator, which includes transistors T, and T provides a fixed delay between the time that transistor T, is turned on to energize solenoid S, and the time that transistor T, is turned off to deenergize solenoid S, to allow solenoid S to operate control valve 200. If it is desirable to provide an adjustable time delay, the time delay circuit of FIG. 8 may be incorporated in the control circuit of FIG. 7. As explained above, the time delay circuit (FIG. 8) is connected to the control circuit (FIG. 7) by connecting

terminals

308, 310, and 312 of the time delay circuit to

terminals

318, 320, and 322, respectively, of the control circuit. The delay provided by the time delay circuit of FIG. 8 can be varied by adjusting the value of resistance 304.

In the operation of the control circuit (FIG. 7) incorporating the time delay circuit of FIG. 8, transistor T, is turned on, in the same manner as described above, by a pulse from pulse generator 296 applied to the base electrode of transistor T After a time delay determined by capacitance 302 and variable resistance 304, a potential is applied to terminal 318, Le, the base electrode of transistor T to switch transistor T into conduction.

When transistor T is rendered conductive, the base electrode of transistor T, is connected to ground through transistor T and transistor T, is switched to a non-conducting state to deenergize solenoid S,. Thereafter, solenoid S is allowed to operate control valve 200 and the operation of the control circuit is identical to the operation described above.

After piston 18 is returned to the open end of cylindrical housing 12, it remains there until a subsequent pulse is applied from pulse generator 296 to transistor T In the normal operation of the acoustic pulse generator, a series of evenly spaced operating pulses from generator 296 is applied to transistor T, to operate the acoustic pulse generator at a desired frequency. In the seismic exploration of underwater land areas, the acoustic pulse generator of the present invention normally operates at a repetition rate of I to 6 seconds between firings.

If, during the time between successive firings of the acoustic pulse generator, piston 18 moves inward relative to cylindrical housing 12 a sufficient distance to open sensing switch 256 (FIG. 7), transistor T is turned on and solenoid S is energized. Control valve 200 (FIG. 6) is moved to position Y, and

pressurized fluid from source 204 is applied through hydraulic line 172 and port 170 (FIG. 4) to the interior of hydraulic cylinder 62 to move plunger 64 (FIG. 2) to the end of the hydraulic cylinder to return piston 18 to the open end of cylindrical housing 12.

Referring to FIG. 6, the small diameter hydraulic line 228 connected between accumulator A and hydraulic line 215 allows accumulator A to exhaust excess fluid into reservoir 210 during the time between successive firings of the acoustic pulse generator. In addition, relief valve 230 which is connected to hydraulic line 172 limits the pressure buildup in hydraulic cylinder 62 to a predetermined value.

As explained above, accumulator A (FIG. 6) serves as a local reservoir for the acoustic pulse generator. When control valve 200 is moved to position X to fire the acoustic pulse generator, the interior of hydraulic cylinder 62 is connected by hydraulic line 172 through the control valve to accumulator A,. As piston 18 moves inward toward the closed end of cylinder 12, any fluid which is expelled from hydraulic cylinder 62 through port 170 is driven into accumulator A,. In the acoustic pulse generator of the present invention, it is desirable that this fluid be driven into a local drain or reservoir, i.e., accumulator A,, which is submerged with the acoustic pulse generator rather than requiring that the fluid be driven into reservoir 210 which is located above the surface of the fluid medium.

Similarly, accumulator A serves as a local source of pressure for the acoustic pulse generator. Accumulator A is preferably a high pressure accumulator for supplying fluid under high pressure to the acoustic pulse generator. It is preferred that a local pressure source be submerged along with the acoustic pulse generator, to provide rapid pressure changes for operating the poppet valve and hydraulic cylinder when control valve 200 is switched to position X or position Y. The poppet valve and hydraulic cylinder are able to respond more rapidly to pressurized fluid from a local pres sure source than to pressurized fluid from source 204 which is normally located above the surface ofthe fluid medium.

The acoustic pulse generator of the present invention can utilize different types of mechanical structures without departing from the principles of the invention. For example, the expandable and collapsible structure for establishing a chamber of variable volume in the fluid medium can include a cylindrical housing open at both ends with a pair of pistons mounted for sliding movement within the cylindrical housing. Stops can be provided at both ends of the cylindrical housing to prevent either piston from moving out of the housing. In this structure, the hydraulic cylinder 62 is secured to one of the pistons and plunger 64 of the hydraulic cylinder is secured to the other piston. Another example of a structure which can be used in the acoustic pulse generator is a pair of plates connected by a set of bellows, or some other flexible element, to define an expandable and collapsible chamber. The exterior surfaces of the plates can be dome-shaped for increased strength, and the interior surfaces are designed to permit the volume of the chamber to decrease to a small value without the plates moving into contact with each other. In this structure, hydraulic cylinder 62 is secured to one of the plates, and plunger 64 is secured to the other plate.

In a further example of a structure for the acoustic pulse generator, two flexible plates are connected together along their edges to provide a chamber of variable volume between the flexible plates. Hydraulic cylinder 62 is connected to one of the flexible plates and plunger 64 is secured to the other plate. In the operation of this structure, the mid-portion of the flexible plates are forced apart when the hydraulic cylinder is operated to expand the chamber defined by the flexible plates against the ambient pressure of the fluid medium.

These examples illustrate structural arrangements which can be used in an acoustic pulse generator constructed in accordance with the present invention. The description of the structures is not intended to limit the present invention to any particular structural arrangement, and it is recognized that additional structures can be designed which operate under the principles of this invention.

The acoustic pulse generator of the present invention produces primary acoustic pulses in a fluid medium while substantially eliminating secondary acoustic pulses. Thus, the acoustic pulse generator permits seismic exploration of underwater land areas to be conducted with extremely good accuracy.

The present invention also provides an acoustic pulse generator which conserves the amount of mechanical energy required to operate the generator. In the acoustic pulse generator, cavitation is prevented in a hydraulic cylinder for operating the generator by using a fluid accumulator to drive fluid into the hydraulic cylinder during the rebound stroke of the generator. Further, since the unique valve of the acoustic pulse generator allows large fluid flows to occur at relatively low pressure, the valve permits smaller, more flexible hydraulic tubes to be used in a fluid control circuit of the generator.

The invention in its broader aspects is not limited to the specific details shown and described, and modifications may be made in the details of the acoustic pulse generator without departing from the principles of the present invention.

What is claimed is:

1. An acoustic pulse generator for producing acoustic pulses when submerged in a fluid medium which comprises:

a cylindrical housing having one end closed and one end open to the fluid medium;

a piston slidably disposed within and in sealing engagement with said cylindrical housing to provide a chamber of variable volume between the closed end of said cylindrical housing and said piston;

a hydraulic cylinder mounted on said cylindrical housing and having a plunger movable therein which is connected to said piston; and

fluid control means connected to said hydraulic cylinder for:

l. charging said hydraulic cylinder with pressurized fluid to move said plunger outward and said piston to an extended position relative to the closed end of said cylindrical housing,

2. decreasing the pressure of the fluid in said hydraulic cylinder to permit the ambient pressure of the fluid medium to drive said piston toward the closed end of said cylindrical housing and said plunger inward to discharge the remaining fluid from said hydraulic cylinder,

3. admitting pressurized fluid into said hydraulic cylinder during rebound of said piston from the closed end of said cylindrical housing, and

4. preventing the admitted fluid from discharging from said hydraulic cylinder after the rebound of said piston from the closed end of said cylindrical housing.

2. The acoustic pulse generator of claim 1, wherein the fluid control means includes:

a normally closed valve mounted at one end of said hydraulic cylinder for controlling the discharge of fluid therefrom; and

means for opening said valve to permit the fluid in said hydraulic cylinder to be discharged when said piston is driven toward the closed end of said cylindrical housing.

3. The acoustic pulse generator of claim 2, wherein:

said valve automatically closes after the rebound of said piston to prevent the discharge of fluid from said hydraulic cylinder.

4. The acoustic pulse generator of claim 2, which includes:

an accumulator connected to said valve for receiving the fluid discharged from said hydraulic cylinder.

5. The acoustic pulse generator of claim 1, wherein the fluid control means includes a fluid control circuit comprising:

a source of pressurized fluid;

a fluid reservoir;

a control valve for selectively connecting said source and said reservoir to said hydraulic cylinder; and

means for operating said control valve to connect said source to said hydraulic cylinder during the charging of said hydraulic cylinder to supply pressurized fluid to said hydraulic cylinder and to connect said hydraulic cylinder to said reservoir during the discharge of said hydraulic cylinder.

6. The acoustic pulse generator of claim 5, which includes:

a first fluid accumulator connected to said control valve to serve as a local source of pressurized fluid for said hydraulic cylinder and to supplement said source of pressurized fluid; and

a second fluid accumulator connected to said control valve to serve as a local reservoir for said hydraulic cylinder and to supplement said fluid reservoir.

7. The acoustic pulse generator of claim 6, which includes:

a third fluid accumulator connected to the normally closed valve for receiving the fluid discharged from said hydraulic cylinder.

8. The acoustic pulse generator of claim 2, wherein the valve includes:

a housing having a slot, a fluid chamber, and a fluid passage formed therein, said slot extending from said fluid passage into said fluid chamber;

a valve stem slidably mounted within said slot and extending from said fluid chamber into said fluid passage;

a poppet valve mounted on said valve stem for closing said fluid passage;

biasing means for maintaining said valve stem and poppet valve in a normally closed position;

a free piston mounted for sliding movement within said fluid chamber relative to said valve stem and said housing;

means for supplying pressurized fluid to the end of said valve stem in said fluid chamber and to the end of said free piston to move said valve stem and poppet valve against said biasing means; and

means for transmitting pressure applied to said free piston by the pressurized fluid to said valve stem.

9. The acoustic pulse generator of claim 8, wherein the means for transmitting pressure to said valve stem comprises:

a flange formed on said valve stem and projecting into said fluid chamber, said flange being normally in contact with said free piston.

10. The acoustic pulse generator of claim 9, wherein the biasing means comprises:

a coil spring mounted within said fluid chamber, one end of said spring engaging the flange of said valve stem and the other end of said spring engaging said housing.

11. The acoustic pulse generator of claim 2, wherein the valve includes:

said housing also having a valve seat formed thereon and located at the periphery of said fluid passage;

a valve stem slidably mounted within said slot and extending from said fluid chamber into said fluid passage toward said valve seat, said valve stem having a flange formed thereon which projects into said fluid chamber;

a poppet valve mounted on one end of said valve stem for engaging said valve seat to close said fluid passage;

means for biasing said poppet valve into engagement with said valve seat to maintain the poppet valve in a normally closed position;

a free piston slidably disposed within said fluid chamber and having an opening formed therein for receiving the other end of said valve stem and for permitting said free piston to move relative to said valve stem, said free piston being normally in contact with said flange; and

means for supplying pressurized fluid to the end of said valve stem in said fluid chamber and one end of said free piston to move said valve stem against said biasing means and to displace said poppet valve from said valve seat.

12. The acoustic pulse generator of claim 11, wherein:

said housing has a shoulder formed thereon which projects into said fluid chamber to limit the movement of said free piston relative to said housing and to said valve stem.

13. The acoustic pulse generator of claim 11, wherein the biasing means comprises:

14. The acoustic pulse generator of claim 11, wherein:

said valve stem has a shoulder formed thereon and located in said chamber for limiting its movement relative to said housing.

15. The acoustic pulse generator of claim l 1, wherein:

said housing has a port formed therein through which pressurized fluid is supplied to said fluid chamber to coun teract the forces exerted on said free piston and said valve stem by the pressurized fluid supplied to the end of said valve stem and the end of said free piston.

16. The acoustic pulse generator of claim 11, wherein:

said housing has a port formed therein which extends into said hydraulic cylinder for supplying pressurized fluid to 1 the interior of said hydraulic cylinder.

17. A method of producing an acoustic pulse in a fluid medium, which comprises:

submerging in the fluid medium a cylindrical housing having an open end and a closed end with a piston slidably disposed therein;

displacing the piston against the ambient pressure of the fluid medium toward the open end of the cylindrical housing;

permitting the ambient pressure of the fluid medium to drive the piston toward the closed end of the cylindrical housing;

permitting the piston to rebound from the closed end of the cylindrical housing against the ambient pressure of the fluid medium to produce a primary acoustic pulse in the fluid medium; and

arresting the motion of the piston after its rebound from the closed end of the cylindrical housing to prevent secondary acoustic pulses from being produced.

18. The method of claim 17, which includes:

applying a predetermined volume of gas at a pressure below the ambient pressure of the fluid medium to the chamber provided by the cylindrical housing and piston to prevent the piston from striking the closed end of the cylindrical housing during its movement toward the closed end of the housing;

said gas producing a rebound force on the piston when it is compressed to cause the piston to rebound from the closed end of the cylindrical housing.

19. A method of producing an acoustic pulse in a fluid medium utilizing an acoustic pulse generator including a cylindrical housing having an open end and a closed end with a piston slidably disposed therein to provide a chamber of varia-' ble volume, and a hydraulic cylinder mounted on said cylindrical housing having a plunger slidably disposed therein and connected to said piston, which comprises:

charging the hydraulic cylinder with pressurized fluid to move the plunger relative to the hydraulic cylinder and to move the piston to an extended position at the open end of the cylindrical housing;

decreasing the pressure of the fluid in the hydraulic cylinder to permit the ambient pressure of the fluid medium to drive the piston toward the closed end of the cylindrical housing and to move the plunger relative to the hydraulic cylinder to discharge the remaining fluid from the hydraulic cylinder;

admitting pressurized fluid into the hydraulic cylinder during rebound of the piston from the closed end of the cylindrical housing; and

preventing the admitted fluid from discharging from the hydraulic cylinder after the rebound of the piston from the closed end of the cylindrical housing.

20. The method of claim 19, which includes:

Claims

1. An acoustic pulse generator for producing acoustic pulses when submerged in a fluid medium which comprises: a cylindrical housing having one end closed and one end open to the fluid medium; a piston slidably disposed within and in sealing engagement with said cylindrical housing to provide a chamber of variable volume between the closed end of said cylindrical housing and said piston; a hydraulic cylinder mounted on said cylindrical housing and having a plunger movable therein which is connected to said piston; and fluid control means connected to said hydraulic cylinder for: 1. charging said hydraulic cylinder with pressurized fluid to move said plunger outward and said piston to an extended position relative to the closed end of said cylindrical housing, 2. decreasing the pressure of the fluid in said hydraulic cylinder to permit the ambient pressure of the fluid medium to drive said piston toward the closed end of said cylindrical housing and said plunger Inward to discharge the remaining fluid from said hydraulic cylinder, 3. admitting pressurized fluid into said hydraulic cylinder during rebound of said piston from the closed end of said cylindrical housing, and 4. preventing the admitted fluid from discharging from said hydraulic cylinder after the rebound of said piston from the closed end of said cylindrical housing.

2. The acoustic pulse generator of claim 1, wherein the fluid control means includes: a normally closed valve mounted at one end of said hydraulic cylinder for controlling the discharge of fluid therefrom; and means for opening said valve to permit the fluid in said hydraulic cylinder to be discharged when said piston is driven toward the closed end of said cylindrical housing.

3. The acoustic pulse generator of claim 2, wherein: said valve automatically closes after the rebound of said piston to prevent the discharge of fluid from said hydraulic cylinder.

4. The acoustic pulse generator of claim 2, which includes: an accumulator connected to said valve for receiving the fluid discharged from said hydraulic cylinder.

5. The acoustic pulse generator of claim 1, wherein the fluid control means includes a fluid control circuit comprising: a source of pressurized fluid; a fluid reservoir; a control valve for selectively connecting said source and said reservoir to said hydraulic cylinder; and means for operating said control valve to connect said source to said hydraulic cylinder during the charging of said hydraulic cylinder to supply pressurized fluid to said hydraulic cylinder and to connect said hydraulic cylinder to said reservoir during the discharge of said hydraulic cylinder.

6. The acoustic pulse generator of claim 5, which includes: a first fluid accumulator connected to said control valve to serve as a local source of pressurized fluid for said hydraulic cylinder and to supplement said source of pressurized fluid; and a second fluid accumulator connected to said control valve to serve as a local reservoir for said hydraulic cylinder and to supplement said fluid reservoir.

7. The acoustic pulse generator of claim 6, which includes: a third fluid accumulator connected to the normally closed valve for receiving the fluid discharged from said hydraulic cylinder.

8. The acoustic pulse generator of claim 2, wherein the valve includes: a housing having a slot, a fluid chamber, and a fluid passage formed therein, said slot extending from said fluid passage into said fluid chamber; a valve stem slidably mounted within said slot and extending from said fluid chamber into said fluid passage; a poppet valve mounted on said valve stem for closing said fluid passage; biasing means for maintaining said valve stem and poppet valve in a normally closed position; a free piston mounted for sliding movement within said fluid chamber relative to said valve stem and said housing; means for supplying pressurized fluid to the end of said valve stem in said fluid chamber and to the end of said free piston to move said valve stem and poppet valve against said biasing means; and means for transmitting pressure applied to said free piston by the pressurized fluid to said valve stem.

9. The acoustic pulse generator of claim 8, wherein the means for transmitting pressure to said valve stem comprises: a flange formed on said valve stem and projecting into said fluid chamber, said flange being normally in contact with said free piston.

10. The acoustic pulse generator of claim 9, wherein the biasing means comprises: a coil spring mounted within said fluid chamber, one end of said spring engaging the flange of said valve stem and the other end of said spring engaging said housing.

11. The acoustic pulse generator of claim 2, wherein the valve includes: a housing having a slot, a fluid chamber, and a fluid passage formed therein, said slot extending from said fluid passage Into said fluid chamber; said housing also having a valve seat formed thereon and located at the periphery of said fluid passage; a valve stem slidably mounted within said slot and extending from said fluid chamber into said fluid passage toward said valve seat, said valve stem having a flange formed thereon which projects into said fluid chamber; a poppet valve mounted on one end of said valve stem for engaging said valve seat to close said fluid passage; means for biasing said poppet valve into engagement with said valve seat to maintain the poppet valve in a normally closed position; a free piston slidably disposed within said fluid chamber and having an opening formed therein for receiving the other end of said valve stem and for permitting said free piston to move relative to said valve stem, said free piston being normally in contact with said flange; and means for supplying pressurized fluid to the end of said valve stem in said fluid chamber and one end of said free piston to move said valve stem against said biasing means and to displace said poppet valve from said valve seat.

12. The acoustic pulse generator of claim 11, wherein: said housing has a shoulder formed thereon which projects into said fluid chamber to limit the movement of said free piston relative to said housing and to said valve stem.

13. The acoustic pulse generator of claim 11, wherein the biasing means comprises: a coil spring mounted within said fluid chamber, one end of said spring engaging the flange of said valve stem and the other end of said spring engaging said housing.

14. The acoustic pulse generator of claim 11, wherein: said valve stem has a shoulder formed thereon and located in said chamber for limiting its movement relative to said housing.

15. The acoustic pulse generator of claim 11, wherein: said housing has a port formed therein through which pressurized fluid is supplied to said fluid chamber to counteract the forces exerted on said free piston and said valve stem by the pressurized fluid supplied to the end of said valve stem and the end of said free piston.

16. The acoustic pulse generator of claim 11, wherein: said housing has a port formed therein which extends into said hydraulic cylinder for supplying pressurized fluid to the interior of said hydraulic cylinder.

17. A method of producing an acoustic pulse in a fluid medium, which comprises: submerging in the fluid medium a cylindrical housing having an open end and a closed end with a piston slidably disposed therein; displacing the piston against the ambient pressure of the fluid medium toward the open end of the cylindrical housing; permitting the ambient pressure of the fluid medium to drive the piston toward the closed end of the cylindrical housing; permitting the piston to rebound from the closed end of the cylindrical housing against the ambient pressure of the fluid medium to produce a primary acoustic pulse in the fluid medium; and arresting the motion of the piston after its rebound from the closed end of the cylindrical housing to prevent secondary acoustic pulses from being produced.

18. The method of claim 17, which includes: applying a predetermined volume of gas at a pressure below the ambient pressure of the fluid medium to the chamber provided by the cylindrical housing and piston to prevent the piston from striking the closed end of the cylindrical housing during its movement toward the closed end of the housing; said gas producing a rebound force on the piston when it is compressed to cause the piston to rebound from the closed end of the cylindrical housing.

19. A method of producing an acoustic pulse in a fluid medium utilizing an acoustic pulse generator including a cylindrical housing having an open end and a closed end with a piston slidably disposed therein to provide a chamber of variable volume, and a hydraulic cylinder mounted on said cylindrical housing having a plunger slidably disPosed therein and connected to said piston, which comprises: charging the hydraulic cylinder with pressurized fluid to move the plunger relative to the hydraulic cylinder and to move the piston to an extended position at the open end of the cylindrical housing; decreasing the pressure of the fluid in the hydraulic cylinder to permit the ambient pressure of the fluid medium to drive the piston toward the closed end of the cylindrical housing and to move the plunger relative to the hydraulic cylinder to discharge the remaining fluid from the hydraulic cylinder; admitting pressurized fluid into the hydraulic cylinder during rebound of the piston from the closed end of the cylindrical housing; and preventing the admitted fluid from discharging from the hydraulic cylinder after the rebound of the piston from the closed end of the cylindrical housing.

20. The method of claim 19, which includes: applying a predetermined volume of a compressible fluid to the chamber established by the closed end of the cylindrical housing and the piston at a pressure below the ambient pressure of the fluid medium to prevent the piston from striking the closed end of the cylindrical housing.