US3612210A

US3612210A - Pneumatic sound source employing an electromagnet for controlling its release valve

Info

Publication number: US3612210A
Application number: US14904A
Authority: US
Inventors: George B Loper
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1970-02-27
Filing date: 1970-02-27
Publication date: 1971-10-12
Anticipated expiration: 1988-10-12

Abstract

The specification disclosed a pneumatic sound source comprising a pressure chamber for receiving gas and having a controllable valve for opening and closing an outlet port. An electromagnet including an electrical coil is energized to hold the valve in its closed position and is deenergized for releasing the valve for the generation of an acoustic pulse. Current is applied to the coil by way of a normally closed switch which is opened to deenergize the coil. Sensing means is provided for sensing the current change in the coil to produce signals representative of the time that current flow is interrupted and representative of the time that the valve begins to move to its open position. Opposing surfaces of the valve and electromagnet are coated with a thin layer of aluminum to provide an nonmagnetic gap and to provide corrosion protection. In addition, a simple seal is provided for sealing the chamber. This seal comprises a metal rim surrounding the port for contacting a disklike resilient member coupled to the valve surface facing the electromagnet.

Description

United States Patent [7 21 Inventor George B. ILoper Duncanville, Tex.

[21] Appl. No. 15,904 a [22] Filed Feb. 27, I970 [45] Patented Oct. 12, 1971 [73] Assignee Mobil Oil Corporation Continuation'in-part oi application Ser. No. 663,800, Aug. 28, 1967, now Pagegt No. 3,506,085, time Apr. 4, 1970.

[54] PNEUMATIC SOUND SOURCE EMPLOYING AN ELECTROMAGNET FOR CONTROLLING ITS RELEASE VALVE 8 Claims, 7 Drawing Figs.

52 U.S. C 181/.5,

[SI] Int. G0lv 1/38 [50] Field ofSearch 181/5;

[56] References Cited UNITED STATES PATENTS 3,379,273 4/1968 Chalminski 181/.5 3,327,264 6/1967 Rodaway 251/129 Primary Examinn-Rodnj D. Bennett, 15

Assistant Examiner-N. Moskowitz Attorneys-William J. Scherback, Frederick E. Dumoulin, Arthur Fv Zobal, Andrew L. Gaboriault and Sidney A. Johnson ABSTRACT: The specification disclosed a pneumatic sound source comprising a pressure chamber for receiving gas and having a controllable valve for opening and closing an outlet port. An electromagnet including an electrical coil is energized to hold the valve in its closed position and is deenergized for releasing the valve for the generation of an acoustic pulse. Current is applied to the coil by way of a normally closed switch which is opened to deenergize the coil. Sensing means is provided for sensing the current change in the coil to produce signals representative of the time that current flow is interrupted and representative of the time that the valve begins to move to its open position. Opposing surfaces of the valve and electromagnet are coated with a thin layer of aluminum to provide an nonmagnetic gap and to provide corrosion protection. In addition, a simple seal is provided for sealing the chamber. This seal comprises a metal rim surrounding the port for contacting a disklike resilient member coupled to the valve surface facing the electromagnet.

PATENTED 061121971 3,612,210

SHEET 1 [IF 4 7A lj FIG I AIR 39 I COMPRESSOR X 44b 4 I Awv i 40 I 105 l DC 32 SOURCE 34 T 1 I :06 SWITCHING I08 GEORGE B. LOPER INVENTOR (WM 3 W ATTORNEY PATENTEnucnelsn 3.612.210

SHEEI k [If 4 FIG. 5

FIG. 7

GEO B. LOPER NVENTOR MG'P ATTORNEY PNEUMATIC SOU D SQ RCE EMPLOYING AN ELECTROMAGNET FOR CONTROLLING ITS RELEASE VALVE This application is a continuation-in-part of application Ser. No. 663,800, filed Aug. 28, 1967, and now U.S. Pat. No. 3,506,085, issued Apr. 4, 1970.

BACKGROUND OF THE INVENTION This invention relates to improvements incorporated in and employed with a pneumatic sound source having a valve controlled by an electromagnet.

In U.S. Pat. No. 3,506,085, there is disclosed a pneumatic acoustic source for marine seismic operations and having an electromagnet which is controlled to hold and release the sources gas pressure release valve. The source comprises a chamber for receiving gas and holding gas under pressure. An outlet ,port is provided through which pressurized gas is released to generate an acoustic pulse. The electromagnet is energized to form a magnetic force for holding the valve in its closed position for confining pressurized gas in the chamber. The flow of current through the electromagnet's coil is interrupted to reduce the magnetic force to allow the pressurized gas in the chamber to move the valve to its open position to release the gas to generate an acoustic pulse.

In one embodiment, control means is employed to normally allow electrical current of a predetermined value to flow from a voltage source through the coil for energizing the coil to produce the magnetic holding force. This control means is actuated to interrupt the flow of electrical current from the voltage source through the coil to decrease the magnetic force to allow the gas pressure to move the valve to its open position. Following the interruption of flow of current through the coil, the valve begins to move prior to the complete decay of the magnetic flux generated by the clectromagnet. The moving valve cuts the magnetic flux and acts as a voltage or current generator.

SUMMARY OF THE INVENTION In accordance with one aspect of the present invention, a sensing means is provided for sensing the current flowing through the coil. In addition, signal-producing means responsive to the sensing means produces a signal representative of the time that current flow through the coil changes to a value different from the predetermined value. Changes in current occur when the current flow to the coil is interrupted and also when the valve begins to move to its open position. Thus, either one or both of these events may be detected by the sensing and signal-producing means. In the embodiment disclosed, the signal-producing means produces a first signal representative of the time that current flow through the coil is nt up and it a so p oduce a second signa epr s ntati e of the time that the valve begins to open.

In the source of he embodimen disclosed. the e ec romagn p ises ferromagne ic st u tu e aving a u ers farin in the direction of the valve. The valve also comprises ffi m gri ti st uctu e hav ng a su face facing in the di ec ion of the electromagnet. A thin layer of nonmagnetic metal is coated r a eas n o he su a es f r ormin a nonma netic gap between the tw. surfaces and in the path of the magtic flux fo med by th coil when t l e is mo ed to ts s d posi ion In th embodimen d sclosed, h norma s netic m ta em ye i aluminum wh h i c at d over bo surfaces.

The chamber port of the source is circular in cross section and extends through the electromagnet. A simply seal is provided comprising a circular-shaped, resilient member coupled to the valve and a rigid, annular ridge coupled to the electromagnet around the port for contacting the resilient member for forming a fluidtight seal for sealing the port when the valve is in its closed position.

BRIEF DESCRIPTION OF THE DRAWINGS FI 1 il u tra a pn umati soun u ce employe in marine seismic surveying operations;

FIG. 2 illustrates in detail the internal and external structure of the source of FIG. 1;

FIG. 3 illustrates circuitry for obtaining signals representa: tive of the time that time current flow through the coil is interrupted and representative of the time that the valve begins to DESCRIPTION OF THE OPERATION OF THE PNEUMATIC SOUND SOURCE Referring now to FIGS. 1 and 2; an acoustic source 20 is shown supported in water from a boat 21 by a cable 22. The acoustic source comprises enclosing wall structure 24 forming a pressure chamber 25 which has an outlet port 26 A quickopening valve 27 of ferromagnetic material is provided for opening and closing the port. A spring 28 is provided for moving the valve to its closed position. An electromagnet comprising an electrical coil 29 and a ferromagnetic core 30 is energiz ed to form a magnetic holding force for holding the valve 27 in its closed position. Electrical current is applied to the coil from DC power supply 32 shown in the enclosed dotted bolt 33 located on the boat 21. Current is applied to the coil from the power supply 32 by way of a normally closed switch included in switching circuitry 34 and

electrical leads

35a and 35b which are coupled to the source.

When the valve is in its closed position, the pressure chamber 25 is pressurized from air supplied by air compressor 37. A seal 38 maintains a pressure seal between the core 30 and the valve 27 to seal the pressurized air in the chamber 25. Air compressor 37 is coupled to the acoustic source by con duit 39, solenoid-controlled valve 40, check valve 41, and flexible conduit 42, the latter of which extends to the chamber 25. An acoustic pulse is generated by interrupting the flow of current through the coil 29. When this occurs, the magnetic holding force decreases and the high-gas pressure in the chamber 25 rapidly moves the valve 27 to its open position whereby the pressurized gas rapidly flows through chamber port 26 and through venting ports 43 into the water to nera a ac u is Pu e- In e p o h ur n through the coil 29 is carri d out by opening the switch provided in switching circuitry 34. A timer 44 is provided to control the solenoid valve 40, by way of conductor and the tching i ui y 4. by ay o conduc o Mb. I e eti ey generate a oust c pul es fo am e a a epe i io t of s acous ic Pul e P9 m DETAILED DESCRIPTION or THE iNVENTlQN Referring to FIG. 3, the circuitry illustrated is located on the towboat except for the coil 29. The switching circuitry may comprise a mechanical switch controlled by timer 44 or an electronic switch. The former is described in the aboveidentified U.S. Pat. No. 3,506,085. A suitable type of electronic switch may be a normally conducting silicon-controlled rectifier which is rendered nonconductive or opened by timer 44. Such a switch is disclosed in U.S. application Ser. No. 15,110 tiled concurrently herewith by Silvan E. McAlpin (Mobil Case No. 7428).

The current applied from the QC source 32 may be of the order of 10 amperes. Coupled in series with coil 29 is a small resistor 50 which may be a 0.1-ohm resistor. Current flow through this resistor and hence through the coil 29, is sensed by an arrangement including transformer 51, having its primary coupled across the resistor 50. The current sensed across h si t 0 is u r e in Q- :l- Th t me tha hriw s 34 is opened and current cut OFF is illustrated at a. Current oug h coil h de ease anis y- There is. w ver. ag in h d c y o h ma ne i P 85399 t 995 i cu r nts h valve begins to move prior to the complete decay of the magnetic flux, and as it moves it cuts the decaying magnetic flux and acts as a current generator. Movement of the valve begins at time b. As can be seen, the current generated in the coil by movement of the valve through the decaying magnetic flux increases rapidly to a peak value above the normal flow of current through the coil and then decreases as the valve moves away from its closed position. The system shown in FIG. 3 produces a first signal representative of the time that current is cut OFF and also a second signal representative of the time that the valve begins to move to its open position. In this respect, the secondary of transformer 51 has one end 51a coupled to a normally nonconducting transistor 52 and its other end 51b coupled to a normally nonconducting transistor 53. Coupled across the secondary of the transformer 51 are two diodes 54 and 55. Coupled to the collectors of

transistors

52 and 53 are two normally

nonconducting transistors

56 and 57, respectively. Outputs from the

transistors

56 and 57 are taken from their collectors. Coupled to the emitters of

transistors

56 and 57 are

capacitors

58 and 59, respectively, which are normally charged to 12 volts.

Operation of the system of FIG. 3 is as follows. Prior to current cutoff, the ends of terminals 51a and 51b of the secondary of transformer 51 are at the same potential, for example, at or near ground potential. The transformer leads are connected in such a manner that when the current decreases following cutoff, the potential at terminal 51a becomes positive with respect to that at terminal 51b. Diode 55 holds the potential of terminal 51b near ground level. The positive potential at terminal 510 causes transistor 52 to conduct. When this occurs, the drop in potential at the collector of transistor 52 causes transistor 56 to conduct. When this occurs, capacitor 58 discharges through transistor 56, resulting in a positive pulse 60 at the collector output from transistor 56. This pulse is representative of the time that current to the coil 29 is cut off.

When the current again increases due to movement of the valve, the potential at terminal 51b will become positive relative to that at terminal 51a. Diode 54 holds the potential at terminal 51a near ground. Transistor 53 and hence transistor 57 become conductive when the current through the coil increases above the IO-ampere level. When transistor 57 conducts, capacitor 59 discharges through this transistor, thereby resulting in a positive pulse 61 at its collector output. This pulse is representative of the time that the valve begins to move to its open position. Pulse 61 may be applied to conductor 62 coupled to recording equipment 63 for recording the time that the valve opens and hence the time that the acoustic pulse is generated. When recorded in this manner, this pulse is used to indicate zero time on the data recorded by seismic detectors employed to detect reflection signals from the subsurface formations.

The

pulses

60 and 61 also may be employed to produce an indication of the time that the source fires following cutoff of the current to its coil. In this respect, pulse 60 is applied to a one-shot multivibrator 64 which produces a positive-going pulse 65 of relatively long duration. In one embodiment, the pulse 65 may have a duration of about 2.5 seconds. Pulse 60 also is employed to trigger a flip-flop 66. Pulse 61 in turn resets the flip-flop 66 for the production of a negative pulse 67 whose time duration is representative of the time required for the source to fire following cutoff of current to the coil. Zener diode 68 is employed to hold the negative pulse 67 at 8 volts. This pulse is applied to the inverting input of an operational amplifier illustrated at 70. Coupled across this amplifier is a capacitor 71 to form an integrator 72. The output of multivibrator 64 is coupled to the gate of a normally conducting field-effect transistor 73. Thus, normally, capacitor 71 is shorted or discharged. Pulse 65, however, when produced by one-shot multivibrator 64, renders the transistor 73 nonconductive. Thus, the integrator 72 will begin to integrate during the duration of pulse 67. In one embodiment, the source fires in about 12 milliseconds following current cut off. Thus, the pulse 67 will have a duration of about 12 milliseconds. During this time, the output of integrator 72 begins to build up as illustrated by waveform 74 in FIG. 3. When pulse 67 terrninates, build up of the output of integrator 72 will also terminate. The integrator will hold this higher level for the duration of the pulse 65 which is much longer than the duration of pulse 67. The output of integrator 72 is applied to a meter 75 which produces a reading indicative of the time required for the source to fire following cutoff of the current to its coil. The meter 75 will hold its reading for the time duration of the pulse 65 which is sufficient for an operator to observe the meter reading during each cycle of the operation of the source. When pulse 65 terminates, transistor 73 will become conductive whereby capacitor 71 is discharged and the reading of the meter 75 drops to zero.

Referring now to FIGS. 5 and 6, there will be described the nonmagnetic gap formed by the aluminum coatings on the opposing

surfaces

27a and 30a of the valve 27 and core 30, respectively. These coatings are illustrated in heavy line. The aluminum coating on the valve surface 27a is illustrated at 27b, while the aluminum coating on the core surface 300 is illustrated at 30b. In one embodiment, these coatings or layers had a thickness of about 0.005 inch and were flame-sprayed onto the valve and core surfaces. In the embodiment disclosed, the coating of aluminum was applied to the total area of the valve, while the coating of aluminum was applied to the core surface 30a as well as to its side surfaces.

The nonmagnetic gap formed by the aluminum coatings between the valve and the core reduces the opening time of the valve and, in addition, increases the reliability or repeatability of the valves opening time. Thus, the nonmagnetic gap formed of aluminum minimizes the valves opening time as well as variations thereof. In one embodiment, the opening time is of the order of 12 milliseconds following current cutoff with :1 millisecond in variation.

The aluminum coatings also have advantages in that they minimize corrosion due to salt water in which the source will operate.

Referring now to FIG. 7, there will be described a simple and effective seal which may be used in the present source. The seal comprises a resilient urethane disk located and held in the circular slot 81 (FIG. 6) formed in the surface of valve 27. In addition, it comprises a metal ridge 82 formed on the surface of a metal ring 83 which is located and held in the slot 84 (FIG. 5) formed in the opposing surface of the core 30. When the valve 27 is moved to its closed position, the ridge 82 bites into or is impressed into the resilient member 80 to form an effective seal for sealing the pressurized gas in the chamber 25.

The metal ring 83 is secured to the core 30 by machine screws 85. An O-ring 86 located in an annular slot formed in the other side of ring 83 forms a seal between the ring 83 and the core 30. The resilient disklike member 80 is held in place by a truncated, cone-shaped washer 88 secured to the valve 27 by a machine screw 89. A wire member 90, coupled to the screw 89 and to a pin 91, prevents the screw 89 from turning.

In one embodiment the ring 83 had an inside diameter of about 5 inches. The thickness of ring 83 was 0.010 to 0.020 inch less than the depth of the slot 84. The ridge 82 extended outward from the ring 83 about 0.045 inch and was spaced about one-sixteenth of an inch from the inside edge of ring 83. The urethane disk 80 had a diameter of about 6 inches and was about oneeighth of an inch thick.

Referring again to FlGS. l and 2, there will be described more of the mechanical details of the source and equipment. Cable 22 is coupled to a harness which in turn is coupled to nose member 101 and vertical tail fin 102. Also coupled to the harness is a strain cable 103 which extends to the boat 21 and is securely affixed to the boat at 104. The cable 22 is supported at the end of the boat by a reel 105 and winch 106. A motor and reel system 107 is employed to reel the cable in and out to raise and lower the source, respectively. The conduit 42,

electrical leads

35a and 35b, and cable 103 are bound together by tape to form a flexible and lightweight conduit 108 which is let out and pulled in by hand.

At the source, the conduit 42 extends through tubing 110 coupled to the harness 100 and then to an inlet 111 which leads to the pressure chamber 25. The electrical leads 35a and 35b extend through tube 112' coupled to the harness and then through

smaller tubes

113 and 114 coupled to the harness and to the source. The electrical leads 35a and 35b then extend to a terminal box 115 where they are connected to the two ends of the electrical coil 29. The electrical coil 29 is secured in a slot 116 formed in the core 30: An encapsulating epoxy may be used to secure the coil in the slot.

Coupled to the backend of the valve 27 by machine screws 118 is a cylinder 119 which guides the valve 27 in its movement and also supports one end of the retract spring 28. Cylinder 119 is supported for movement by bearing 120 located in a cylinder 121. Cylinder 121 is coupled to the core 30 by an arrangement including annular plate 122 and cylinder 123 in which the venting ports 43 are formed. Cylinder 123 is welded to annular plate 122 and is coupled to the core 30 by machine screws 124. The dimensions of the valve 27 and the skirt portion of the cylinder 123 extending from the core 30 are such that there is a lack of fluid seal, allowing fluid flow clearance between the outer periphery of the valve and the interior surface of the skirt portion.

The other end of the spring 28 is supported in an annularshaped member formed by cylinder 126 coupled to cylinder 127 by way of spokes 128. Cylinder 127 in turn is threaded into cylinder 121. Secured to the exterior of cylinder 121 is an annular member 130. A truncated, cone-shaped member 131 is coupled to the annular plate 122 and annular member 130 for additional support for the back end portion of the source. A plate 132 is coupled to the annular member 130 by way of machine screws 133. Coupled to the plate 132 are the

fins

102, 135, and 136.

The valve 27 is slowed and stopped at the end of its opening movement by water located in container formed by plate 122 and the back end of cylinder 123. The outside diameter of valve 27 is slightly smaller than the interior diameter of cylinder 123 as indicated above, whereby water may flow between these members. As the valve moves within this container, water is squeezed between the outside surface of the valve and the inside surface of the container to decelerate the valve. Water within cylinder 119 may flow by way of apertures 137 extending through cylinder 119 and by way of aperture 138 extending through plate 132.

What is claimed is:

1. In a system for carrying out marine seismic operations useful in the investigation of underwater formations comprisan acoustic source comprising a gas pressure chamber formed of rigid wall structure having an outlet port for releasing high-gas pressure rapidly from said chamber to generate an acoustic pulse in water; valve means movable to open and closed positions for opening and closing said ,outlet port, respectively; an electromagnet comprising an electrical coil for producing a magnetic holding force for application to said valve means; and valve retract means for moving said valve to said closed position following the generation of an acoustic pulse,

a source of electrical energy to be coupled to said electrical coil by way of electrical conductors,

control means for controlling the application of electrical energy from said source to said coil,

said control means normally allowing electrical current of a predetermined value to flow from said source through said coil for energizing said coil to produce said magnetic holding force to hold said valve means closed for confining said gas under pressure in said chamber, and

means for controlling said control means to interrupt the flow of electrical current from said source through said coil to decrease said magnetic force to allow said gas pressure'to move said valve means to its open position to releasepressurized gas from said chamber to generate an acoustic pulse,

said valve meansupon movement toward said open position resulting in the generation of a large transient current in said coil prior to the decay of magnetic flux in said electromagnet following the interruption of the flow of electrical current through said coil,

the combination therewith of:

sensing means for sensing the current flowing through said coil, and

signal-producing means responsive to said sensing means for producing a signal representative of the time that current flow through said coil changes to a value different from said predetermined value.

2. The combination of claim 1 wherein:

said signal-producing means produces a signal representative of the time that current flow through said coil is interrupted.

3. The combination of claim 1 wherein:

said signal-producing means produces a signal representative of the time that said valve begins to move to its open position.

4. The combination of claim 1 wherein said signal-producing means comprises:

a first means for producing a first signal representative of the time that current flow through said coil is interrupted, and

second means for producing a second signal representative of the time that said valve begins to move to its open position.

5. The combination. of claim 4 comprising:

means responsive to said first and second signals for producing an indication of the time between said first and second signals.

6. A source for generating acoustic energy for application to the earth for exploratory purposes comprising:

a chamber for receiving gas and holding gas under pressure to be released for the generation of an acoustic pulse,

said chamber being formed of rigid wall structure and having a port for releasing pressurized gas,

valve means supported for movement between a closed position and an open position for closing and opening said outlet port means, respectively,

electromagnet means including a coil for forming a magnetic flux for holding said valve means in its closed position for confining pressurized gas in said chamber and for releasing said valve means for movement to its open position for releasing pressurized gas from said chamber to generate an acoustic pulse,

said electromagnet means comprises ferromagnetic structure having a surface facing in the direction of said valve means,

said valve means comprises ferromagnetic structure having a surface facing in the direction of said electromagnet means,

said valve means being supported to move adjacent said electromagnet when said valve means is moved to said closed position, and

a coating of nonmagnetic metal on at least one of said surfaces for forming a nonmagnetic gap between said two surfaces and in the path of the magnetic flux fonned by said coil when said valve means is moved to its closed position.

7. The source of claim 6 wherein:

said nonmagnetic metal comprises aluminum which is coated over both of said surfaces.

8. A source for generating acoustic energy for application to the earth for exploratory purposes comprising:

said chamber being formed of rigid wall structure and hav ing a port for the release of pressurized gas,

valve means comprising magnetic structure supported for movement between a closed position and an open position for closing and opening said port, respectively,

electromagnet means comprising a coil and magnetic structure for forming a magnetic flux for holding said valve means in its closed position for confining pressurized gas in said chamber and for releasing said valve means for movement to its open position for releasing pressurized gas from said chamber to generate an acoustic pulse,

said electromagnet means being annular in shape and having a surface facing in the direction of said valve means,

said port being circular in cross section and extending through said electromagnet means,

said valve means having a surface facing in the direction of said electromagnet means,

said valve means being supported to move adjacent said electromagnet means when said valve means is moved to said closed position,

a circular-shaped resilient means coupled to said surface of said valve means and located centrally thereof,

the diameter of said resilient means being larger than the diameter of said port, and

a rigid annular ridge coupled to said electromagnet means around said port and extending beyond said surface of said electromagnet means for contacting said resilient member for forming a fluidtight seal for sealing said port when said valve means is in said closed position.

Claims

1. In a system for carrying out marine seismic operations useful in the investigation of underwater formations comprising: an acoustic source comprising a gas pressure chamber formed of rigid wall structure having an outlet port for releasing highgas pressure rapidly from said chamber to generate an acoustic pulse in water; valve means movable to open and closed positions for opening and closing said outlet port, respectively; an electromagnet comprising an electrical coil for producing a magnetic holding force for application to said valve means; and valve retract means for moving said valve to said closed position following the generation of an acoustic pulse, a source of electrical energy to be coupled to said electrical coil by way of electrical conductors, control means for controlling the application of electrical energy from said source to said coil, said control means normally allowing electrical current of a predetermined value to flow from said source through said coil for energizing said coil to produce said magnetic holding force to hold said valve means closed for confining said gas under pressure in said chamber, and means for controlling said control means to interrupt the flow of electrical current from said source through said coil to decrease said magnetic force to allow said gas pressure to move said valve means to its open position to release pressurized gas from said chamber to generate an acoustic pulse, said valve means upon movement toward said open position resulting in the generation of a large transient current in said coil prior to the decay of magnetic flux in said electromagnet following the interruption of the flow of electrical current through said coil, the combination therewith of: sensing means for sensing the current flowing through said coil, and signal-producing means responsive to said sensing means for producing a signal representative of the time that current flow through said coil changes to a value different from said predetermined value.

2. The combination of claim 1 wherein: said signal-producing means produces a signal representative of the time that current flow through said coil is interrupted.

3. The combination of claim 1 wherein: said signal-producing means produces a signal representative of the time that said valve begins to move to its open position.

4. The combination of claim 1 wherein said signal-producing means comprises: a first means for producing a first signal representative of the time that current flow through said coil is interrupted, and second means for producing a second signal representative of the time that said valve begins to move to its open position.

5. The combination of claim 4 comprising: means responsive to said first and second signals for producing an indicatioN of the time between said first and second signals.

6. A source for generating acoustic energy for application to the earth for exploratory purposes comprising: a chamber for receiving gas and holding gas under pressure to be released for the generation of an acoustic pulse, said chamber being formed of rigid wall structure and having a port for releasing pressurized gas, valve means supported for movement between a closed position and an open position for closing and opening said outlet port means, respectively, electromagnet means including a coil for forming a magnetic flux for holding said valve means in its closed position for confining pressurized gas in said chamber and for releasing said valve means for movement to its open position for releasing pressurized gas from said chamber to generate an acoustic pulse, said electromagnet means comprises ferromagnetic structure having a surface facing in the direction of said valve means, said valve means comprises ferromagnetic structure having a surface facing in the direction of said electromagnet means, said valve means being supported to move adjacent said electromagnet when said valve means is moved to said closed position, and a coating of nonmagnetic metal on at least one of said surfaces for forming a nonmagnetic gap between said two surfaces and in the path of the magnetic flux formed by said coil when said valve means is moved to its closed position.

7. The source of claim 6 wherein: said nonmagnetic metal comprises aluminum which is coated over both of said surfaces.

8. A source for generating acoustic energy for application to the earth for exploratory purposes comprising: a chamber for receiving gas and holding gas under pressure to be released for the generation of an acoustic pulse, said chamber being formed of rigid wall structure and having a port for the release of pressurized gas, valve means comprising magnetic structure supported for movement between a closed position and an open position for closing and opening said port, respectively, electromagnet means comprising a coil and magnetic structure for forming a magnetic flux for holding said valve means in its closed position for confining pressurized gas in said chamber and for releasing said valve means for movement to its open position for releasing pressurized gas from said chamber to generate an acoustic pulse, said electromagnet means being annular in shape and having a surface facing in the direction of said valve means, said port being circular in cross section and extending through said electromagnet means, said valve means having a surface facing in the direction of said electromagnet means, said valve means being supported to move adjacent said electromagnet means when said valve means is moved to said closed position, a circular-shaped resilient means coupled to said surface of said valve means and located centrally thereof, the diameter of said resilient means being larger than the diameter of said port, and a rigid annular ridge coupled to said electromagnet means around said port and extending beyond said surface of said electromagnet means for contacting said resilient member for forming a fluidtight seal for sealing said port when said valve means is in said closed position.