GB2461298A

GB2461298A - Marine seismic source

Info

Publication number: GB2461298A
Application number: GB0811764A
Authority: GB
Inventors: Mark Francis Lucien Harper
Original assignee: Mecon Ltd
Current assignee: Mecon Ltd
Priority date: 2008-06-27
Filing date: 2008-06-27
Publication date: 2009-12-30
Also published as: GB0811764D0

Abstract

A low frequency sound source has a radiating piston 3 of the order of a few metres across backed by a gas spring 13, 15 containing a fixed mass of gas. The gas pressure in the spring is kept at levels for which the natural frequency of the piston 3 loaded by the fluid 41 lies in the seismic band and may be as low as 0.5Hz. The piston 3 is given an initial displacement and begins to oscillate. Its oscillations are sustained by an actuator 27, 29 whose drive signal is derived froth the velocity of the piston 5 via a velocity or displacement sensor. The sound source is caused to perform a frequency sweep by gradually compressing the gas in the gas spring 13, 15 so that the spring becomes stiffer both because of the rising pressure and because of the reducing volume of the gas spring spaces 13, 15. This double effect allows large changes in stiffness to be produced and hence allows the source to operate over at least three octaves of frequency.

Description

Marine Seismic Source This invention relates to generating acoustic signals in water for marine seismic surveys down to very low frequencies, down to 0.5Hz or lower.

During the course of seismic exploration there can arise a need to monitor seismic response down to very low frequencies. For example, the accuracy with which seismic data can be inverted to estimate acoustic impedance as a function of depth or traveltime is much improved by including very low frequencies in the inversion.

A number of different methods have previously been proposed, mainly but not exclusively relating to seismic surveys undertaken at sea (marine surveys). They include an underwater trombone ("Suggestions for the development of controlled frequency marine seismic source", O'Brien iT, Geophysics NOVEMBER 1986); hydraulically driven sources; ("Development of a hydraulic transducer for marine seismics", Bird J.M., Peacock J.H., Walker L.J. presented at SEG, Atlanta 1984; Cole, US3394775, July 30 1968; Ross, US3578102, May 11 1971; Mifsud, US4483411, Nov 20, 1984; Mifsud, US4557348, Dec 10, 1985; Gram, US5491306, Feb 13, 1996); devices based on giant magnetostriction ("Development of a New Improved Marine Vibrator Based on Terfenol-D", Bjflrn &sin-Helm, Saga Petroleum: Rune Tengham, EB Seatech; Ragnar Fritsvold, Norsk Hydro; and Per Anders Osterholt, Master Surveys, Norway, SEG Atlanta 1984); and an electrical vibrator (Newsletter of PGS Geophysical mc, November 2005).

However none of the proposed methods is described as being capable of producing useful seismic signals at frequencies as low as 0.5Hz.

According to the present invention there is provided a means of producing a swept-sine signal that will contain sufficient energy for a marine seismic survey down to 0.5Hz and extending to higher frequencies over a band of at least three octaves, by causing a radiating piston of order a few metres diameter loaded by the mass of surrounding water and backed by a gas spring containing a fixed mass of gas which is compressed by a secondary piston to oscillate with large enough amplitude to radiate the desired acoustic signal. Because the spring-piston system resonates, the forces required to excite the required amplitudes of motion are much smaller than would be the case if the radiating piston were directly driven by an actuator. These forces would otherwise be very large.

The stiffness of the gas spring is varied by compressing the gas it contains to produce signals over a range of frequencies which can extend as low as 0.5Hz. By compressing a fixed mass of gas the gas spring becomes stiffer both because of the increased pressure of the gas and because of its reduced volume. This allows the stiffness of the gas spring to be varied over a very wide range and hence allows the source to produce a very broadband acoustic signal in the surrounding water, extending over at least three octaves of frequency.

The invention will now be described by way of example only with reference to the accompanying drawings in which:-Figure 1 shows an embodiment of the invention; Figure 2 shows a suitable control system.

In Figure 1 an embodiment of the invention is immersed in water 41 which may for example be seawater. In Figure 1, the device is contained within a gas-filled cylindrical shell 1 made of steel or other strong material. A radiating piston 3 of the order of a few metres in diameter is fixed to a rod 5 supported by linear bearings 7,9 so that the rod S is free to move vertically. A circumferential sliding seal (not shown) prevents water ingress between shell I and radiating piston 3. The lower bearing 7 is supported from the shell 1 via three webs one of which 21 is shown shaded. The upper bearing 9 is supported by a gas-tight end-cap 25 which closes the lower end of the gas-spring cylinder 17. A gas-tight sliding seal between end cap 25 and rod 5 (not shown) prevents escape of gas from the cylinder 17. The cylinder 17 is supported from the shell I by three webs one of which 19 is shown shaded. Behind the piston 3 a secondary piston 11 is also fixed to the rod 5 and has a gas-tight sliding seal (not shown) around its outer circumference. The secondary piston 11 divides the gas within the gas spring cylinder 17 into two volumes 13,15. The mass of gas in the spaces 13, 15 is fixed during operation. The upper end of the space 13 is closed by a tertiary piston 23 which has a gas-tight sliding seal (not shown) around its -3-.

outer circumference and a second gas-tight seal between piston 23 and rod 5. The tertiary piston 23 is attached to a yoke 26 which is in turn attached to a powerful linear actuator 33 which presents a very high mechanical impedance compared to the mechanical impedance of the gas spring 13, 15. The actuator 33 incorporates a position sensor (not shown) which allows a control system to measure the position of the actuator 33 at any time in a manner well known to those versed in the art of industrial contril systems and illustrated in Figure 2. Thus the position of the tertiary piston 23 will be controlled by the actuator 33 and will not be affected by changes of pressure in the gas spring volume 13 resulting from oscillations of the radiating piston 3. The actuator 33 is driven by an electric motor 34 via a drive band 36. The yoke may be guided within the gas spring cylinder 17 by a collar 28 which is attached to the yoke 26 and is a sliding fit in the cylinder 17. The actuator 33 is supported by the upper end-cap 35 of the shell 1 and from the wall of the shell I by three webs one of which 37 is shown shaded. Within the yoke 26 and supported from it is an electric linear motor coil 27. A magnet rod 29 is guided through the coil 27 by plain bearings (not shown) and is attached to the upper end of the rod 5. An electric linear motor may be used because it has low mechanical impedance and does not limit or impede the motion of the moving assembly 3, 5, 11, 29. The linear motor incorporates a position sensor (not shown) which allows a control system to measure the relative position of the actuator rod 29 and coil 27 at any time in a manner well known to those versed in the art of industrial contril systems and illustrated in Figure 2.

Before operation it should be ensured that the mass of gas in the volume 2 enclosed by the shell 1 is such that when stationary the piston 3 remains near the midpoint of its possible range of motion between the bearings 7,9 and is not close to either bearing 7, 9.

This can be accomplished by pumping gas (for example air) into or out of the space 2 using a gas pump 43 via a tube 45. The pump 43 may be located on a tender vessel (not shown). During operation the actuator 33 is first retracted so that the tertiary piston 23 moves towards the top of the gas spring cylinder 17 and volumes 13, 15 are large and the gas pressure within them is low. The linear motor 27, 29 is then used to excite the rod 5 and pistons 3, 11 into motion so that sound is radiated from the piston 3 into the surrounding water 41. This may be done simply by producing a large initial displacement and then allowing the piston 3 to oscillate freely, or by applying a force proportional to the velocity of the piston 3 so that it becomes negatively damped and oscillates with increasing amplitude until the radiated energy causes sufficient loss of energy to limit the oscillations, or by applying a force proportional to the sign of the velocity of the piston 3 so that it becomes similarly negatively damped, or by a combination of these methods.

Applying a velocity-dependent force may be accomplished using a simple feedback system from a velocity or position sensor attached to any part of the piston 3 or rod 5 in a manner well known to those versed in the art. The initial displacement of the piston 5 may be produced by applying a force with the linear motor 27, 29 or by temporarily withdrawing gas from the volume 2, latching the piston in its displaced position using a solenoid latch (not shown), returning the gas to the volume 2 and then releasing the latch.

Alternatively it may be produced by raising the tertiary piston 23 so that the secondary piston 11 and hence the radiating piston 3 are displaced, latching the piston 3 as previously described, returning the piston 23 to its former position and then releasing the latch.

The piston 3 will be loaded by the water 41 in the manner well known to those versed in the art of acoustics and the effective moving mass will typically be several times the mass of the piston 3 together with the rod 5 the secondary piston 11 and the magnet rod 29. The angular frequency of oscillation of the rod 5 and pistons 5, 11 will be given with reasonable accuracy by the formula /k13 +k15 +k0 Where m is the effective moving mass including the mass of the pistons 3, 11 and rods 5, 29 and the mass loading produced by the fluid 41, k13 and k15 are the stiffnesses of the gas volumes 13, 15, and k0 is the stiffness of the gas contained in the space 2 within the shell 1 when compressed or rarified by the motions of the radiating piston 5. These are given to reasonable accuracy by the equation In which yis the ratio of specific heats of the gas, p is the mean pressure of the gas, A is the area of the gas space normal to the axis of motion and / is the length of the gas space parallel to the axis of motion. The masses of gas in the spaces 13, 15 may be identical but their pressures and hence lengths will differ in that the pressure in the lower space 15 will be higher because of the weight of the rod 5 pistons 3 and 11 and magnet rod 29.

It is an important aspect of the design that the volume 2 of gas within the shell I is sufficiently large so that the stiffness that it presents to the piston 3 is not high enough to prevent the system oscillating at the lowest desired frequency.

Once the piston 3 has achieved the desired amplitude of motion, the actuator 33 is gradually extended pushing the tertiary piston downwards so that the pressure of the gas in the spaces 13, 15 is gradually increased and the length of the spaces 13, 15 is gradually reduced. It is evident from the second of the above equations that this simultaneous reduction in / and increase in p will lead to large changes in the stiffnesses k13, k15. This in turn will lead to substantial changes in the frequency of oscillation of the piston 5 so that the device is able to operate over a broad band of frequencies. Very low frequencies, of order 0.5Hz, can be achieved by having the pressure in the gas volumes 13, 15 low at the beginning of operation and by having the lengths of the spaces 13, 15 long.

A suitable control system is illustrated in Figure 2. A PC 51 communicates with two single-axis servo-controllers 53, 57 via serial communications links 59, 61. The controller 53 controls the high impedance actuator 33, 34, 36 shown here as block 55 via motor drive lines 63. The extension of the actuator 55 is fed back from an internal sensor (not shown, but for example a shaft encoder on its motor) to the servo-controller 53 via feedback cable 65. Likewise the controller 57 controls the low impedance actuator 27, 29 shown here as block 59 via motor drive lines 69. The position of the radiating piston is fed back from a position sensor 67 attached to the radiating piston 3 which may for example be a linear variable differential transformer to the controller 57.

The PC 51 is used to download control programs to the servo-controllers 53, 57. The program downloaded to the servo-controller 53, when initiated, may cause the high-impedance actuator 55 to execute a desired extension which may for example consist of an initial short high-acceleration phase, a phase of extension at constant velocity, a subsequent short deceleration phase in which the actuator 55 is brought to rest, a delay which may for example be ten seconds long, and a return-to-start phase in which the actuator extension is returned to its initial value and the program terminates execution.

The total time interval between initiation and return to the initial extension value may be fixed and may be denoted by T. The program downloaded to the servo-controller 57, when initiated, may cause the low-impedance actuator 59 to deliver an initial impulsive force to the piston rod 5 and may thereafter monitor the motion of the radiating piston 3 via the sensor 67 and cause the actuator 59 to apply a fixed level of force in the direction of the motion detected by the feedback 67 thereby effecting bang-bang control in a manner well known to those versed in the art of control system design. The force may then be reduced to zero and the program may terminate after a time interval after initiation also equal to T. In normal operation the PC may cause the programs downloaded to the controllers 53, 57 to begin execution simultaneously. The combined effect will then be to cause the system to execute a frequency sweep as hereinbefore described.

Claims

CLAIMS1. A marine seismic source suitable for producing seismic signals over a band of frequencies the source comprising: a radiating piston; a gas spring of variable volume acting on the radiating piston and containing a fixed mass of gas; means for varying the stiffness of the gas spring by varying its volume so that the stiffness of the gas spring changes both by virtue of the change in volume and the consequent change in gas pressure; means for causing the piston to vibrate at a frequency dependent on the stiffness of the gas spring; and means for varying the stiffness during operation by varying the volume and thereby also the pressure of the body of gas.
2. A marine seismic source according to claim 1 designed to produce seismic signals over a band of frequencies covering at least three octaves.
3. A marine seismic source according to claim 1 or 2 in which the band of frequencies includes 0.5 Hz.
4. A marine seismic source according to any preceding claim in which the piston has a diameter of at least one metre.
5. A marine seismic sound source according to any of the preceding claims in which the volume of the gas spring is varied by varying its length.
6. A marine seismic sound source according to any of the preceding claims in which the means for varying the volume of the gas spring includes a powerful actuator having a high mechanical impedance.
7. A marine seismic source according to any preceding claim in which the means for varying the stiffness is designed to do so continuously thus enabling the source to emit a swept-frequency acoustic signal.
8. A marine seismic sound source according to any preceding claim in which the said spring consists of two gas volumes arranged to act on opposite faces of a secondary piston rigidly attached to the radiating piston via a rod supported on.linear bearings and free to move along its axis.
9. A marine seismic sound source according to any preceding claim in which the gas spring is contained within a gas-filled shell one wall of which is formed by the rear face of the radiating piston.
10. A marine seismic source according to claim 9 in which the shell forms an outer container of the source and is sufficiently large for the stiffness of the gas contained within it not to prevent the source from oscillating at the lowest desired frequency.
11. A marine seismic sound source according to any of the preceding claims in which a displacement or velocity sensor is attached to the radiating piston and including means for using an output signal of the velocity sensor directly or indirectly as an input signal for an actuator having low mechanical impedance acting on the radiating piston.
12. A marine seismic sound source according to claim 11 in which the said actuator is caused to apply a force to the radiating piston that is proportional to and in the same direction as the velocity of the radiating piston.
13. A marine seismic sound source according to claim 11 in which the said actuator is caused to apply a force to the radiating piston that is of constant magnitude and in the same direction as the velocity of the radiating piston.
14. A marine seismic sound source according to any of claims 11, 12 or 13 in which the said actuator is a linear electric motor.
15. A sound source according to any of the preceding claims including means for initially displacing the radiating piston from an equilibrium position and then releasing it.
16. A sound source according to claim 15 in which the means for initially displacing the radiating piston includes a mechanical actuator for applying a force to the radiating piston.
17. A sound source according to claim 15 when dependant on claim 9 in which the means for initially displacing the radiating piston includes means for causing gas to flow into or out of the said gas-filled shell.
18. A sound source according to claim 15 including means for producing the initial displacement by causing gas to flow into or out of a volume one of whose walls is the gas spring piston.
19. A sound source according to any of claims 15 to 18 in which the displaced radiating piston is adapted to be held in place by a remotely-operated latch and including means for releasing the latch when the sound source is desired to start radiating sound.
20. A sound source substantially as described herein above and illustrated in the accompanying drawings.