GB2141824A - Subsea seismic source - Google Patents

Abstract

A subsea seismic source comprises a gas gun using a compressed gas such as air to produce an underwater pulse during seismic prospecting for gas or oil. The gun is designed so that the size of gas holding chamber (40) contained therein may be varied from a remote location such as the seismic vessel (10). The source desirably uses a hydraulic servo valve (62) under electrical control (stepper motor 74) having mechanical feedback to a piston (60) on the gas holding chamber (40) to control the volume of the gas holding chamber. <IMAGE>

Description

SPECIFICATION Subsea seismic source This invention relates to a subsea seismic source used to create pulses or shock waves in a liquid medium such as water.

In prospecting in subsea and other areas underlying a body of water, it is desirable to provide a source of energy for propagating sonic pulses or shock waves into the water. Since water is a good conductor of sound, it normally is not necessary to generate pulses nearthefloorofthewaterbody; they can be, and desirably are, produced near the water's surface. These pulses propagate down through the water, across the water-floor interface, into the subfloor geologic formations and are, to some extend, reflected back across the same path to a collection of hydrophones waiting near the surface of the water. Analysis of the signals produced by hydrophones can provide information concerning the structure of the subfloor geological formations and attendant petroleum accumulation in those formations.

The term "water" as used herein is meant to include a swampwater, mud, marshwater and any other liquid containing sufficient water to enable operation of the invention.

There are many ways of generating a sonic pulse in a liquid. For instance, explosives introduce strong pulses into the water and accordingly achieve substantial penetration into subfloorformations. Certain obvious drawbacks exist in their use: They are dangerous to store, handle, and use. When used in open water they kill marine life. In crowded areas such as harbors, explosives cannot be used at all.

Explosives are orders of magnitude more expensive to use, on a per-shot basis, than are gas guns.

Modification of the explosive source's sonic signature to achieve an acceptable spectrum distribution is difficult.

Another method of generating a sonic pulse is by discharge of a bank of capacitors through a subsurface electrode to produce a quickly collapsing gaseous bubble. However, the efficiency of this method is quite low in that only a few percent of the energy charged to the capacitors is found in the shock wave produced on discharge.

Apparatus using explosive gas mixtures, e.g., propane and oxygen, to produce the sonic pulse have gained wide acceptance. The two major types of explosive gas guns are those which operate by exploding a gas mixture behind a flexible membrane which in turn is in contact with the water and those which operate by allowing the abrupt bubble from the gas explosion to pass directly into the water. An example of the former apparatus can be found in U.S. Patent No.3,658,149; and example of the latter can be found in U.S. Patent No.4,193,472.

Air guns using high pressure compressed gases, instead of an explosive mixture, have achieved a wide acceptance in the industry. Typical designs for open-ported compressed gas guns are found in U.S.

Patent No. 3,653,460 to Chelminski and U.S. Patent No.4,141,431 to Baird. These guns employ two pressurized chambers, i.e., a control chamber and a gas-holding chamber, which are sealed by a spoolshaped valve or shuttle. The gun is fired by abruptly releasing air from the control chamber. The gas in the gas-holding chamber forces the shuttle into the control chamber and thereby simultaneously exposes the exhaust ports. These ports allow the gas stored in the gas-holding chamber to exit explosive ly into the water. The control chamber is then repressurized, thereby moving the shuttle back into a position sealing the gas-holding chamber.The gun is again ready to "fire." Other patents disclosing various improvements or variations in the open port gun include 3,653,460 to Chelminski, issued April 4,1972; 4,034,827 to Leerskov et al, issued July 1977; 4,219,097 to Harrison et al, issued August 1980; 4,219,098 to Thomson et al, issued August26, 1980; 4,225,009 to Harrison petal, issued September 1980; 4,230,201 to Bays, issued October 28, 1980; 4,246,979 to Thomson et al, issued January 1981; and 4,271,924 to Chelminski, issued June 9, 1981.

These seismic sources are typicaily deployed in an array towed behind a seismic boat. A streamer cable, which may be miles in length and contain a large number of hydrophones, will also be towed behind the boat. Although a single seismic source may be used in certain instances, the more common situation involves an array of four to twelve sources. The sources, when deployed in arrays, usually have differing gas chamber sizes to provide a better signal at the hydrophones. The source chamber sizes may be determined by depth of penetration, type of subseafloor material, or amount of detail required in a particular prospecting line.

Chelminski '460 and Thomson et al '098, mentioned above discuss changing, respectively, the effective size or the absolute size of the pressurized gas chambers to change the characteristics of the emitted seismic pulse.

Chelminski '460 suggests using a number of spacing sleeves within the chamber and fitted along its wall to support a barrier disc and thereby form a movable end in the chamber. Chelminski also suggests the use of a barrier disc having threads on its edge which mesh with similar threads on the inside surface of the chamber. Turning the disc will cause it to recede into the chamber thereby making the effective chamber volume larger. Turning the disc in the other direction will have the opposite result. In either variation, the gas gun must be pulled from the water to change the volume of the chamber.

Similarly, Thomson et al '098 suggests using a clamp adapted to allow the easy replacement of the chamber with one of a different size. This arrangement seems to be the most common in this art.

Again, the gas gun must be pulled from the water to effect any change in the chamber size.

According to the invention from one aspect there is provided a subsea seismic source comprising a housing containing a gas storage chamber, a shuttle which can be held in one position for maintaining compressed gas in said gas storage chamber, at least one exhaust port for releasing compressed gas from said gas storage chamber when said shuttle is released from said one position, and a piston slidably disposed within said gas storage chamber and attached to a hydraulic actuator operable for slidably adjusting the piston between selected fixed positions whereby the volume of said gas 25 storage chamber may be varied.

According to the invention from another aspect there is provided a subsea seismic source comprising: a housing defining at least one exhaust port and a gas storage chamber having a remotely variable volume for storing compressed air, and containing a shuttle having a first piston thereon adapted to close said gas storage chamber when the shuttle is held in one position, and means for releasing said shuttle from said one position whereby to open said gas storage chamber and release compressed air from said gas storage chamber through said at least one exhaust port and thereby produce a seismic pulse.

In one preferred embodiment, there is provided a subsea seismic source comprising: a housing defining at least one exhaust port, a generally cylindrical gas storage chamber having a piston therein slidably adjustable between selected fixed positions for defining a variable volume whereby said volume may be varied by sliding said piston, and a control chamber.

and containing a shuttle having a first piston for closing said control chamber and a second piston for closing said gas storage chamber, said first and second pistons being generally parallel to each other at opposite ends of a shuttle shaft having a passageway therethrough, a first air supply means for providing compressed air to said gas storage chamber through said control chamber and said shuttle shaft passageway, and a second air supply means controlled by actuating means operable for supplying air to said first piston to cause said shuttle to move and thereby opening said gas storage chamber and releasing compressed air through said at least one exhaust port and thereby produce a seismic pulse.

In another preferred embodiment, there is provided a subsea seismic source comprising: a housing defining at least one exhaust port a generally cylindrical gas storage chamber having a piston therein slidably adjustable between selected fixed positions for defining a variable volume, said piston being attached by a piston rod to a double-acting hydraulic piston situated between two hydraulic volumes in communication with hydraulic control means whereby said hydraulic control means may vary the amount of a hydraulic fluid in said two hydraulic volumes, move said hydraulic piston and thereby move said piston rod and slidable piston to vary said gas chamber volume, and a control chamber, and containing a shuttle having a first piston for closing said control chamber and a second piston for closing said gas storage chamber, said first and second pistons being generally parallel to each other at opposite ends of a shuttle shaft having a passageway therethrough, a first air supply means for providing compressed air to said gas storage chamber through said control chamber and said shuttle shaft passageway, and a second air supply means controlled by actuating means for supplying air to said first piston to cause said shuttle to move and thereby opening said gas storage chamber and releasing compressed air through said at least one exhaust port and thereby produce a seismic pulse.

The gas storage chamber may be remotely varied in size using hydraulic, electrical, or electrohydraulic actuators. Desirably, the variable gas storage chamber assembly replaces the chamber in a device such as those disclosed in Chelminski '460 or Thomson et al '098 as discussed above.

The variable chamber assembly desirably uses a hydraulic servo valve taking rotary input from an electrical stepper motor and employing mechanical feedback from a threaded shaft to control movement of the piston forming the movable end of the variable chamber. The servo valve controls the amount of high pressure hydraulic fluid introduced to one side of a slave piston and liquid withdrawn from the other side of the slave piston. The slave piston is desirably on the same shaft as the movable end of the variable chamber. The electrical stepper motor may be controlled from the boat. Consequently the volume of the air gun's chamber may be varied without pulling the source array from the water.

Use of the invention allows a seismic prospector to change the source array bond width without pulling the array in from the water into the boat. Use of the variable chamber gun allows a boat operator to carry a modest set of spare air gun parts.

Moreover, the signature and frequency spectrum of the equipment can easily be modified.

The invention will be better understood from the following description, given by way of example and with reference to the accompanying drawings, in which: Figure 1 shows a schematic view of a seismic boat towing a multiple gun array and a hydrophone streamer cable.

Figure 2 shows an idealised group of time versus amplitude charts depicting the signals from a multiple air gun array.

Figure 3A shows one preferred embodiment, shown as an air gun in side section partial cutaway.

The shuttle is in firing position and the cylinder ram head is in the minimum displacement position.

Figure 3B shows the air gun shown in Figure 3A in side section partial cutaway. The shuttle is in the fired position and the cylinder ram head is in the maximum displacement position.

Figure 3C shows a detailed cutaway of the servo valve used in Figures 3A and 3B.

Referring to Figure 1 which shows a typical offshore prospecting arrangement using air guns such as those disclosed herein, a seismic boat 10 is shown pulling four air guns 12 and a hydrophone streamer cable 14. The streamer cable 14 may be made up of a number of hollow sections containing an oil to provide the cable with near neutral buoyan cy. Each cable section will typically contain several hydrophones. The air guns 12, on the other hand, are not self-buoyant and are supported by buoys 16.

The four air guns 12 may be of different chamber volumes and are fired simultaneously. The pressure waves emanate from the ports 18 in each of the air guns, proceed down through the water, through the seafloor 20, reflect off the interface between two geologic layers having dissimilar densities, and proceed back across a similar path to be detected by the hydrophones in the streamer cable 14 and recorded in the seismic boat 10.

The signals emanating from each of the guns are depicted on the pressure-time graph in Figure 2. The guns are fired simultaneously at toy In this example, Gun No.1 hasthe largest chambervolume, Gun No.

2 has a smaller volume, Gun No. 3 has a smaller volume than Gun No. 2, and Gun No.4 is the smallest.

Each of the guns emits two distinct signals. The first occurs as the pressurized air is discharged out into the water. This pulse is shown as the downwardly extending projection or "first onset" 22 in each of the graphs. The energy the pulse places in the water is proportional to the cross-hatched area shown under first onset 22. The gun causes a secondary pulse 24 which occurs as the bubble collapses. The frequency of the source is equal to the inverse of the time lapse between the first pulse 22 and the bubble pulse 24. It should be apparent that when these signals are all summed that the difference between the sum of the four bubble pulses 22 and the secondary between the sum of the four bubble pulses 22 and the secondary pulses 24 would be significant. Indeed, it may be difficult even to tell secondary pulses 24 from other background noises.

The guns are sized differently not only to permit sequential reception of the secondary pulses, but also to permit good resolution of small subsurface features as by gun 4 and deep penetration by the more powerful low frequency of gun 1.

Figures 3A shows a preferred form of gun in accordance with the invention which may be used as any one of guns 1 through 4 shown in Figures 1 and 2. The air gun is made up of two major sections. The first portion, which is to the left of clamp 26 in Figure 3A, is the control and port assembly 28. The other portion, to the right of clamp 26, is the variable chamber assembly 30.

The control and port assembly 28 may be conventional in design and made up of any of those discussed above in the background of the invention.

High pressure air, having a pressure from 200 to 5,000 Ibs. per square inch, is fed into the air gun through high pressure line 32 into control chamber 34 and then through an axial passage 36 in shuttle 38 into variable chamber 40.

Shuttle 38 has a first piston 42 which sits against a seal means 44, for instance an O-ring which is capable of maintaining the pressurized gas in control chamber 34. Shuttle 38 has at its opposite end a second piston 46 which engages with seal means 48 and function to maintain the pressure of the gas included in variable chamber 40. The pressure of the gas in control chamber 34 acting against piston 42 serves to hold piston 46 against seal 48.

When the air gun is actuated by feeding an electrical signal to solenoid valve 50, the solenoid valve 50 quickly opens to allow the pressurized gas in chamber 34 to pass through a passage 52 into a passage 54 which leads to the surface of piston 42 opposite that facing control chamber 34. The force of the gas in control chamber 34 is therefore offset and the pressurized gas in variable chamber 40 presses shuttle 38 via second piston 36 into control chamber 34 thereby opening discharge ports 18 so that the pressurized gas in variable chamber 40 may pass into the surrounding water. Shuttle 38 is shown in Figure 3B in the fired position. Port 18 is open between variable chamber 40 and the surrounding water.

In order to return shuttle 38 to the position shown in Figure 3A solenoid valve 50 is closed thereby separating passage 52 from passage 54. The high pressure air in line 32 then presses on first piston 42 and returns the shuttle 38 to the position shown in Figure 3A. As soon as control chamber 34 and variable chamber 40 are pressurized, the gun is then ready for refiring.

The control and port assembly 28 may be made up of a single machine casting 56 and held to the housing 58 of the variable chamber assembly 30 by clamp 26.

In the other major portion of the seismic source, the variable chamber assembly 30, the volume of the enclosed variable chamber 40 is changed by piston 60. Piston 60 is shown in Figure 3A in the position to produce a minimum displacement in variable chamber 40. In Figure 3B piston 60 is shown in a position to produce a maximum displacement in variable chamber 40. Piston 60 has at the end opposite variable chamber 40 a double acting hydraulic piston 62. Piston 60 has a small hole 61 therethrough to allow fluids behind the piston head to enter or escape as it is moved. Piston 62 is surrounded by two hydraulic volumes 64 and 66. The hydraulic control assembly 67 varies the volume of high pressure hydraulic fluid between volume 64 and 66 to move piston 60 back and forth in its bore and therefore control the volume of variable chamber 40.

In this variation, piston 66 is equipped with a feedback apparatus made up of ball screw 68 which is fixedly attached to piston 60 and screw 70 which rotates within ball screw 68 as piston 60 is moved back and forth. Piston 60 does not rotate within housing 58 because of rod 72. Hydraulic control assembly 67 is desirably of the type found in U.S.

Patent 3,695,295 to Olsen et al, issued October 3, 1972. This variation of hydraulic control assembly 67 uses a stepper motor 74. Stepper motor 74, which moves in small increments of, e.g., one degree revolutions, moves shaft 76. Shaft 76, which may be seen more clearly in Figure 3C in turn is connected to cup 78. Within cup 78 is a pin 80 which slidingly engages slot 82 in valve spool 84. Shaft 76 may ride in bearings 84. Slot 82 is cut in such a way that as pin 80 slides within, the spool valve attached thereto moves in a linear fashion. In this way the rotary motion of stepper motor 74 is converted into axial linear motion for spool valve 84. High pressure hydraulic fluid through line 86 is introduced into either of passages 88 or 90 depending upon the direction of axial movement of spool valve 84.For illustration, if the spool valve 84 is moved to the right in Figure 3C, high pressure fluid will flow through high pressure hydraulic line 86 past spool valve 84 into passageway 90 and thence into hydraulic volume 64. Simultaneously, the hydraulicfluid in hydraulic volume 66 will flow out through passage way 88 past spool valve 84 and into return hydraulic line 92. The movement of piston 60 will cause screw 70 to turn within ball screw 68 and will turn flange 94 attached at its end. Flange 94 has a hole 96 therethrough. Hole 96 is engaged by a pin 98 mounted on a rotor 100. Rotor 100 is fixedly attached to spool valve 84. As screw 70 turns and rotates flange 94 and thence rotor 100, spool valve 84 rotates and moves axially within its bore 102 and, because of the relationship between pin 80 and slot 82, tends to return to its original or balanced position.

Other methods and apparatus suitable for moving a piston such as 60 back and forth in a bore are well known in the art. Certainly the hydraulic control apparatus 67 is not required, and other means are known and will be readily apparent to one having ordinary skill upon reading this specification.

Claims (14)

1. A subsea seismic source comprising a housing containing a gas storage chamber, a shuttle which can be held in one position for maintaining compressed gas in said gas storage chamber, at least one exhaust port for releasing compressed gas from said gas storage chamber when said shuttle is released from said one position, and a piston slidably disposed within said gas storage chamber and attached to a hydraulic actuator operable for slidably adjusting the piston between selected fixed positions whereby the volume of said gas storage chamber may be varied.

2. A subsea seismic source comprising: a housing defining at least one exhaust port and a gas storage chamber having a remotely variable volume for storing compressed air, and containing a shuttle having a first piston thereon adapted to close said gas storage chamber when the shuttle is held in one position, and means for releasing said shuttle from said one position whereby to open said gas storage chamber and release compressed air from said gas storage chamber through said at least one exhaust port and thereby produce a seismic pulse.

3. A subsea source according to claim 1 or 2, wherein said housing also comprises a control chamber and said shuttle also comprises a control piston for closing said control chamber when said first piston closes the gas storage chamber, said control chamber being pressurisable for holding said shuttle in said one position and there being means operable for applying the control chamber pressure to an opposite face of said control piston for releasing said shuttle.

4. A subsea source according to claim 1,2 or 3, also comprising a first air supply for supplying compressed air to said gas storage chamber.

5. A subsea source according to claim 3 or claim 4 as appended to claim 3, also comprising a second air supply controlled by actuating means operable for supplying air to said opposite face of said control piston to move said shuttle and act as said means for releasing said shuttle.

6. A subsea source according to claim 5, wherein said actuating means comprises a solenoid.

7. A subsea seismic source comprising: a housing defining at least one exhaust port, a generally cylindrical gas storage chamber having a piston therein slidably adjustable between selected fixed positions for defining a variable volume whereby said volume may be varied by sliding said piston, and a control chamber, and containing a shuttle having a first piston for closing said control chamber and a second piston for closing said gas storage chamber, said first and second pistons being generally parallel to each other at opposite ends of a shuttle shaft having a passageway therethrough, - a first air supply means for providing compressed air to said gas storage chamber through said control chamber and said shuttle shaft passageway, and - a second air supply means controlled by actuating means operable for supplying air to said first piston to cause said shuttle to move and thereby opening said gas storage chamber and releasing compressed air through said at least one exhaust port and thereby produce a seismic pulse.

8. A subsea source according to claim 7, wherein the actuating means comprise a solenoid.

9. A subsea seismic source comprising: a housing defining at least one exhaust port a generally cylindrical gas storage chamber having a piston therein slidably adjustable between selected fixed positions for defining a variable volume, said piston being attached by a piston rod to a double-acting hydraulic piston situated between two hydraulic volumes in communication with hydraulic control means whereby said hydraulic control means may vary the amount of a hydraulic fluid in said two hydraulic volumes, move said hydraulic piston and thereby move said piston rod and slidable piston to vary said gas chamber volume, and a control chamber, and containing a shuttle having a first piston for closing said control chamber and a second piston for closing said gas storage chamber, said first and second pistons being generally parallel to each other at opposite ends of a shuttle shaft having a passageway therethrough, a first air supply means for providing compressed airto said gas storage chamber through said control chamber and said shuttle shaft passageway, and a second air supply means controlled by actuat ing means for supplying air to said first piston to cause said shuttle to move and thereby opening said gas storage chamber and releasing compressed air through said at least one exhaust port and thereby produce a seismic pulse.

10. A subsea source according to claim 9, wherein said hydraulic control means is a hydraulic servo valve.

11. A subsea source according to claim 10, wherein said hydraulic control means is in mechanical feedback with the position of said doubleacting hydraulic piston.

12. A subsea source according to claim 11, wherein said mechanical feedback comprises a ball screw fixedly attached to said double-acting hydraulic piston and a screw within said ball screw, said screw controlling feedback to said servo value.

13. A subsea source according to claim 10, wherein said servo valve is directly controlled by a stepper motor.

14. A subsea seismic source substantially as hereinbefore described with reference to the accompanying drawings.