FR3005749A1 - MARINE ACOUSTIC VIBRATOR WITH GAS SPRING COMPENSATION - Google Patents

Abstract

The present invention relates to the limitation of gas flow in a marine acoustic vibrator (100) to compensate for gas spring effects. One embodiment provides a marine acoustic vibrator (100), comprising: an outer casing (104); and a variable gas flow limiter (102) disposed within the outer casing (104), the marine acoustic vibrator (100) having a resonant frequency selectable at least in part based on the limiter variable gas flow (102).

Description

The present invention relates generally to acoustic vibrators for marine seismic research. More particularly, embodiments relate to limiting the flow of gas in a marine acoustic vibrator to compensate for gas spring effects. Sound sources are generally devices that generate acoustic energy. Sound sources are used in marine seismic research in which sound sources can be used to generate acoustic energy that moves downward through water and into rock below the surface. After interacting with the rock below the surface, for example at the boundaries between different layers below the surface, some of the acoustic energy can be returned to the surface of the water and be detected by specialized sensors. The detected energy can be used to derive certain properties of the rock beneath the surface, such as structure, mineral composition and fluid content, thereby providing useful information in hydrocarbon recovery. Most of the sound sources used today in marine seismic research are pulse type, in which efforts are made to generate as much energy as possible over as short a time as possible. The most commonly used of these pulse type sources are pneumatic guns which typically use compressed air to generate a sound wave. Other examples of pulse type sources include explosives and weight loss pulse sources. Another type of sound source that can be used in seismic research includes marine acoustic vibrators, such as hydraulically controlled sources, electromechanical vibrators, electric marine acoustic vibrators, and sources using a piezoelectric or magnetostrictive material. Vibrator sources typically generate vibrations over a frequency range in a configuration known as "sweep" or "chirp" ("compression-expansion"). Earlier sound sources for use in marine seismic research have been typically designed for relatively high-frequency operation (eg, above 10 Hz). However, it is well known that, as sound waves move through water and through sub-surface geological structures, higher frequency sound waves can attenuate faster than lower frequency sound waves. and, therefore, lower frequency sound waves can be transmitted over longer distances through water and geological structures than higher frequency sound waves. Thus, efforts have been made to develop sound sources that can operate at low frequencies. Marine acoustic vibrators that can have at least one resonance frequency of about 10 Hz or less have been developed. In order to obtain a given level of output in the water, these marine acoustic vibrators must typically undergo a change in volume. In order to operate in depth while minimizing the structural weight, the marine acoustic vibrator can be pressure balanced with external hydrostatic pressure. When the internal gas (for example, air) in the source increases in pressure, the compressibility modulus of the internal gas also increases. This increase in compressibility modulus or "gas spring" thus tends to make the stiffness of the internal gas a function of the operating depth of the source. In addition, the stiffness of the structure and the internal gas are major determinants in the frequency of the resonance of the source. Therefore, the resonance of the marine acoustic vibrator can vary with depth, particularly in vibrators where the interior volume of the source can be pressure balanced with the external hydrostatic pressure. In order to solve the problems of the prior art, the present invention provides, in a first aspect, a marine acoustic vibrator which comprises an outer casing, a variable gas flow limiter disposed inside the outer casing, the marine acoustic vibrator having a resonant frequency selectable at least in part based on the variable gas flow limiter. The marine acoustic vibrator may further comprise a driving device disposed at least partially inside the outer casing and connected thereto, which driving device may comprise an electrodynamic drive device. The marine acoustic vibrator can have at least two resonant frequencies of about 10 Hz or less when submerged in water at a depth of about 0 meters to about 300 meters. The variable gas flow restrictor may include a first plate having holes, and a second plate having holes, the second plate being movable to at least partially cover the holes in the first plate. The variable gas flow limiter may have an open position and a closed position, the holes in the first plate being at least partially obstructed by the second plate in the closed position, and the holes in the first plate and the holes in the second plate being aligned in the open position for maximum gas flow through the variable gas flow restrictor. The variable gas flow limiter can be fixed on a frame in the marine acoustic vibrator, the frame being connected to the outer casing. The marine acoustic vibrator may further comprise a frame connected to the flexion-extension envelope, the driving device having a first end fixed to the outer casing and a second end fixed to the frame. It may further comprise a spring connected to the outer casing, and masses fixed to the spring. According to a second aspect, the present invention provides a marine acoustic vibrator which comprises an outer flexion-extension envelope, a frame connected to the outer flexion-extension envelope, a driving device having a first end and a second end, the first end being fixed to the outer bending-extension shell, and the second end being fixed to the frame, a gas spring being provided with a mass to generate a first resonance frequency, and the value of the gas spring being changed by a limitation of the gas flow in the marine acoustic vibrator so as to thereby control the first resonant frequency at depth. The marine acoustic vibrator can have at least two resonant frequencies of about 10 Hz or less when submerged in water at a depth of about 0 meters to about 300 meters. It may further comprise a gas flow limiter disposed inside the outer casing. The variable gas flow restrictor may include a first plate having holes, and a second plate having holes, the second plate being movable to at least partially cover the holes in the first plate. The variable gas flow limiter may have an open position and a closed position, the holes in the first plate being at least partially obstructed by the second plate in the closed position, and the holes in the first plate and the holes in the second plate being aligned in the open position for maximum gas flow through the variable gas flow restrictor. The variable gas flow limiter can be fixed on the frame. Finally, the present invention provides, in a third aspect, a method which comprises towing an acoustic vibrator into a body of water, to trigger the acoustic vibrator to generate acoustic energy in the body of water , limiting a flow of gas in the acoustic vibrator to control a first resonance frequency of the acoustic vibrator, and detecting acoustic energy from the acoustic vibrator. The acoustic vibrator can be towed to a first depth of about 0 meters to about 300 meters. The method may further include towing the acoustic vibrator to a second depth, the gas flow being limited in the acoustic vibrator when it is towed to the first depth so that the first resonance frequency of the acoustic vibrator is substantially constant when a towing depth varies from the first depth to the second depth. Restricting gas flow in the acoustic vibrator may include moving a plate to at least partially obstruct holes in another plate.

The method may further include opening a variable gas flow restrictor to allow increased gas flow into the acoustic vibrator when the acoustic vibrator is towed into the body of water. It may further include increasing a gas pressure within the envelope of the acoustic vibrator to equalize the gas pressure within the envelope with the water pressure at depth. It may also include generating a geophysical data product from the detected acoustic energy indicative of certain rock properties below the surface below the water body.

The accompanying drawings illustrate certain aspects of certain embodiments of the present invention and should not be used to limit or define the invention. Figures 1 and 2 illustrate the effect of the gas spring when the marine acoustic vibrator is towed deeper according to exemplary embodiments. Fig. 3 is a simulated amplitude spectrum showing the expected effect of the compressed gas that generates a gas spring when the marine acoustic vibrator is towed deeper according to exemplary embodiments. Figure 4 illustrates an exemplary embodiment of a marine acoustic vibrator with a variable gas flow limiter. Figure 5 illustrates an exemplary embodiment of a variable gas flow restrictor for use with a marine acoustic vibrator.

Figure 6 illustrates an exemplary embodiment of a marine acoustic vibrator with a variable section gas flow limiter. Figure 7 illustrates another exemplary embodiment of a marine acoustic vibrator with a variable section gas flow limiter. Figure 8 illustrates yet another exemplary embodiment of a marine acoustic vibrator with a variable section gas flow limiter.

Figure 9 is a top view of the marine acoustic vibrator of Figure 8 according to exemplary embodiments. Figures 10 and 11 are frequency-amplitude spectrum plots for an exemplary marine acoustic vibrator at 10 meters and 100 meters, respectively, according to exemplary embodiments. Fig. 12 illustrates an exemplary embodiment of a marine acoustic vibrator according to exemplary embodiments.

Fig. 13 is an exemplary embodiment of a marine seismic survey system using an acoustic vibrator. It is obvious that the present description is not limited to particular devices or methods, which of course may vary. It is also evident that the terminology used herein is intended to describe particular embodiments only, and is not intended to be limiting. All numbers and ranges described here may vary to some extent. Whenever a numeric range with a lower limit and an upper limit is described, any number and any included range falling into the range are specifically disclosed. Although individual embodiments are discussed, the invention covers all combinations of all these embodiments. As used herein, the singular forms "a", "an", "the" and "the" refer to configurations where one or more of the subject elements are present, unless the content clearly dictates otherwise . The term "may" is used in this description in a broad sense (i.e., having the potential of, being able to) and not a strict sense (i.e., must). The term "includes" and its derivatives means "including, but not limited to". The term "connected" means connected directly or indirectly. If there is any conflict in the use of a word or term in this description, the definitions which are relevant to that description must be adopted for the understanding of this invention. Embodiments generally relate to acoustic vibrators for marine seismic searches. More particularly, in one or more embodiments, a gas flow may be limited in a marine acoustic vibrator to compensate for gas spring effects. As discussed in more detail below, the flow of gas in the marine acoustic vibrator can be limited to make the gas spring more or less stiff to thereby control the first resonance frequency at depth. Acoustic vibrators can be used in marine seismic research to generate acoustic energy that moves down through the water and down into the earth. Embodiments of marine acoustic vibrators may include an outer shell that contains a gas under pressure. By way of example, a marine acoustic vibrator may comprise an outer envelope which defines an internal volume in which a gas may be arranged. The gas may be any gas or a combination of gases (by air, oxygen, nitrogen, carbon dioxide, etc.) which is selected on the basis of the expected operational requirements of the device. One skilled in the art having the benefit of this description is able to choose a gas or a combination of gases suitable for use in the marine acoustic vibrator. Examples of marine acoustic vibrators may include hydraulically operated vibrators, electromechanical vibrators, electric marine acoustic vibrators, and vibrators using a piezoelectric or magnetostrictive material. In some embodiments, the marine acoustic vibrator may be a flexion-extension envelope type source. Flexion-extension devices comprising actuators and transducers act as mechanical transformers, which transform and amplify the displacement and force generated in the active element to meet the demands of different applications. Bending-extension envelope type sources are generally marine acoustic vibrators having an outer shell that vibrates and flexes to generate acoustic energy. Examples of flexion-extension envelope type sources can be found in U.S. Patent No. 8,446,798. In some embodiments, the marine acoustic vibrator may have a pressure compensation system. The pressure compensation system may be used, for example, to equalize the internal gas pressure of the outer shell of the marine acoustic vibrator with the external pressure. The internal gas pressure of the outer shell of the marine acoustic vibrator is hereinafter referred to as "gas pressure inside the shell". The pressure compensation can be used, for example, with marine acoustic vibrators, where the source must undergo a change of volume to obtain a given output level. When the depth of the marine acoustic vibrator, the gas pressure inside the envelope can be increased in order to equalize a pressure with the water pressure which increases due to the depth. Air or other suitable gas may be introduced into the outer casing of the vibrator, for example, to increase the internal gas pressure. However, increasing the gas pressure inside the shell can create a "gas spring" effect that affects the resonant frequency of the marine acoustic vibrator. In particular, the resonance frequency may increase as the gas pressure inside the envelope increases. The pressurized gas within a marine acoustic vibrator may be stiffer than the outer envelope of the sound source in some embodiments. Those skilled in the art, with the benefit of this description, must appreciate the fact that an increase in the gas pressure inside the casing may also result in an increase in the compressibility modulus (stiffness) of the casing. gas (eg air) in the outer shell. Since the resonant frequency of the marine acoustic vibrator is based at least on the combination of the stiffness of the outer shell and the stiffness of the gas in the outer shell, this increase in compressibility modulus affects the resonant frequency. Thus, the resonance frequency of the marine acoustic vibrator can increase when the vibrator is towed to a greater depth.

Figures 1 and 2 illustrate the effect of a gas spring (eg compressed air) on a marine acoustic vibrator in depth according to exemplary embodiments. In FIG. 1, the internal shell gas is represented by reference numeral 2. In order to illustrate the gas spring, the internal shell gas 2 is represented in the neutral state at 4, is in compression at 6, and is 8. With regard to FIG. 2, the curve 10 is a hypothetical representation of the output of a marine acoustic vibrator at D meters without pressure compensation, while the curve represented at 12 represents the output of the vibrator. Marine acoustics at D + x meters with pressure compensation. Pressure compensation means an increase in pressure and a resulting increase in the stiffness of the gas spring. As illustrated, the resonance of the marine acoustic vibrator moves upward with pressure compensation, thus showing how a stiff gas spring can result in a higher resonant frequency. Figure 3 is a simulated amplitude spectrum from a finite element simulation showing the effect of the gas spring as a function of depth. The curves of FIG. 3 represent the output of a marine acoustic vibrator towed to a variable depth with pressure compensation. In particular, the curves in Figure 3 represent the output of the marine acoustic vibrator towed at 0 meters, 50 meters, 100 meters, and 120 meters, respectively, represented in 14, 16, 18, 30, and 20 in Figure 3. As this is illustrated, the increase of the resonance frequency can be more pronounced at greater depths, thus indicating that the resonant frequency increases when the gas spring is made steeper.

According to the present embodiments, the gas spring can be controlled by limiting a flow of gas in the marine acoustic vibrator. For example, a variable gas flow limiter may be disposed within the marine acoustic vibrator and may change the internal volume of gas to make the gas spring more or less stiff. Because the stiffness of the gas spring affects the resonant frequency, the gas spring can be changed to thereby control the resonance frequency. This may be particularly desirable with a marine acoustic vibrator that can be towed at different depths. In some embodiments, it may be desirable to have the resonant frequency that remains substantially constant (e.g., which varies by no more than 5%) regardless of the depth. However, as previously described, when the marine acoustic vibrator can be lowered into the water, the gas can be compressed by the pressure compensation system such that the gas spring can become steeper at increasing depths. . For example, a marine acoustic vibrator with a resonance of 2.5 Hz to 120 meters may have a much lower resonance at 50 meters. To compensate for this gas spring effect, the flow of gas in the marine acoustic vibrator can be limited to lower depths to make the gas spring stiffer, thereby increasing the resonant frequency. In some embodiments, the marine acoustic vibrator may have at least one resonant frequency (when immersed in water at a depth of about 0 meters to about 300 meters) between about 1 Hz and about 200 Hz. In alternative embodiments, the marine acoustic vibrator may have at least one resonant frequency (when immersed in water) between about 0.1 Hz and about 100 Hz, alternatively between about 0, 1 Hz and about 10 Hz, and alternatively, between about 0.1 Hz and about 5 Hz. In one embodiment, the marine acoustic vibrator may have at least two resonance frequencies of about 10 Hz or less (when it is immersed in water). In some embodiments, the first resonance frequency can be controlled by limiting a flow of gas in the marine acoustic vibrator. In particular embodiments, the first resonance frequency can be increased by limiting the gas flow in the marine acoustic vibrator. For example, the first resonance frequency can be controlled to be substantially constant regardless of the depth. Figure 4 illustrates an exemplary embodiment of a marine acoustic vibrator 100 which includes a variable gas limiter 102, for example, to limit a gas flow, and thereby compensate for gas spring effects. In the illustrated embodiment, marine acoustic vibrator 100 is a flex-extension envelope type source. As illustrated, the marine acoustic vibrator 100 may include an outer casing 104, which may be closed, for example, by two casing side portions 106a, 106b. While not shown in FIG. 4, the casing side portions 106a, 106b may be connected at or near the ends of their longer major shafts by means of a suitable connecting mechanism, such as hinges. . As illustrated, the marine acoustic vibrator 100 may further include one or more driving devices 108, which may be an electrodynamic drive, for example. The outer casing 104 and the drive devices 108 may operate to determine a first resonant frequency for the marine acoustic vibrator. The drive devices 108 may be connected to the face of the two envelope side portions 106a, 106b. As illustrated, the marine acoustic vibrator 100 may further include a frame 109 capable of suspending the drive devices 108 in the outer casing 104. In the illustrative embodiment, the frame 109 may be in the form of a frame. In the cross-sectional illustration of FIG. 4, the variable gas limiter 102 is disposed within the outer shell 104. As illustrated, the variable gas limiter 102 may be attached to the frame 109. In In exemplary embodiments, the variable gas limiter 102 has a sliding plate structure that is movable between a closed position and an open position. In the closed or partially closed position, the variable gas limiter 102 may be used to limit the flow of gas into the outer casing 104. In some embodiments, the variable gas restrictor 102 may completely isolate a portion of the volume. Thus, the flow of gas may be limited when it is desired to make the gas spring stiffer, which may be desired in some embodiments. For example, it may be desired to make the gas spring stiffer and thus increase the first resonant frequency to shallow depths. This type of gas spring compensation can be realized, for example, when a substantially constant resonant frequency is desired regardless of the depth. Without gas spring compensation, the gas spring can stiffen when the marine acoustic vibrator 100 is lowered into the water, thereby causing the first resonant frequency to vary with the depth. However, the present embodiments can provide a resonant frequency for the marine acoustic vibrator 100 selected based at least in part on the variable gas limiter 102 so that the marine acoustic vibrator 100 can have a resonant frequency substantially constant regardless of depth. Referring now to FIG. 5, an exemplary embodiment of a variable gas limiter 102 will now be described in more detail. As illustrated, the variable gas limiter 102 may have a sliding plate structure that includes a first plate 110 and a second plate 112. The first plate 110 may include holes 114, and the second plate 112 may also include The first plate 112 and the second plate 110 as illustrated may each be of a generally rectangular shape in some embodiments, but other plate configurations may be suitable including square, circular, elliptical, or irregularly shaped. The number of holes 114 in the first plate 110 and holes 116 in the second plate 112 may be selected to provide the desired amount of gas flow. Each of the holes 114 and holes 116 may have a selected diameter and spacing based on the desired amount of gas flow and a desired resonant frequency, among others. For example, the hole size 30 can be reduced with increased spacing if less gas flow is desired while the hole size can be increased with reduced spacing if more gas flow is desired.

The variable gas limiter 102 may be movable from (or to) a closed or partially closed position (e.g., left side of Fig. 5) to (or from) an open position (e.g., right side of Fig. 5). In the open position, the holes 114 in the first plate 110 may be aligned with the holes 116 in the second plate 112 such that through holes 118 are formed in the variable gas limiter 102 allowing maximum gas flow.

In the closed position, the holes 114 in the first plate 110 may be at least partially limited by the second plate 112 thereby limiting gas flow into through-holes 118. Due to a movement of the second plate 112, the size of hole of the through holes 118 can be reduced by limiting a flow of gas. In other words, the second plate 112 can be positioned to effectively limit the size of the through holes 118. In some embodiments as shown in Fig. 5, the second plate 112 can be positioned to partially close the variable gas limiter 102 so that the holes 114 in the first plate 110 are substantially blocked. An electric drive, pneumatic drive, hydraulic drive, or other suitable drive may be used in the control of the variable gas limiter 102. A mechanism (not shown) may connect the variable gas limiter 102 to a control system which can operate to control the position of the second plate 112 and thus the flow of gas. The variable gas limiter 102 may be controlled, for example, to maintain a substantially constant resonant frequency as the depth of the marine acoustic vibrator 100 changes. For example, the variable gas limiter 102 may be closed as the frequency increases to maintain a substantially constant resonant frequency. In some embodiments, the variable gas limiter 102 may be driven passively, for example on the basis of a pressure sensor. In some embodiments, the variable gas limiter 102 may be remotely controlled from the towing vessel or work boat (eg, research vessel 200 of Fig. 13). In some embodiments, the variable gas limiter 102 may be fixed in place in some operations. It is obvious that the first plate 110 may be provided with slots or another suitable cowling (for example a flap, a guillotine device, etc.) that can be controlled in order to allow or limit a flow of gas through the holes 114. Although FIG. 5 illustrates the variable gas limiter 102 in the form of a sliding plate structure, other suitable mechanisms for limiting the flow of gas in the marine acoustic vibrator 100 can be used according to the invention. exemplary embodiments, including hinged doors, rolled doors, and the like. For example, a device (eg, plate, door, etc.) may be used to isolate a portion of the internal volume available for the gas spring. Figure 6 illustrates a marine acoustic vibrator 100 which includes a variable gas limiter 102. The marine acoustic vibrator 100 of Figure 6 is shown in section. As illustrated, the marine acoustic vibrator 100 comprises an outer casing 104, which may be made of spring steel or a similar elastic metal, and which may be a Class V flexural-extension transducer. In the illustrated embodiment the shape of the outer shell 104 may be generally referred to as bending-extension. As illustrated, the outer casing 104 may be formed, for example, by two casing side portions 106a, 106b connected at or near the ends of their longer major shafts, by respective hinges 120 into shapes. particular embodiments. In particular embodiments, the outer shell 104 may act as a spring having a first spring constant to generate a first resonance frequency. As is apparent to one skilled in the art with the benefit of this description, the spring constant of the outer shell 104 can be determined by its dimensions, material composition, and shape in the relaxed state, by example. Although FIG. 6 shows a substantially semi-elliptical shaped flexion-extension envelope, flexural-extension envelopes of other shapes, including convex, concave, flat, or combinations thereof, may also be suitable. In some embodiments, the dimensions, material composition, and shape of the outer shell 104 may be selected to provide a soft spring constant for vibrations of between about 1 and 10 Hz when the marine acoustic vibrator is immersed. in water at a depth of about 0 meters to about 300 meters. As illustrated, the marine acoustic vibrator 100 may further include a drive device 108, which may be an electrodynamic drive. The outer casing 104 and the drive device 108 may operate to determine a first resonant frequency of the marine acoustic vibrator 100. In some embodiments, the drive device 108 may be a training device. voice coil "or" voice coil ", which can provide the ability to generate very large amplitudes of acoustic energy. Although the particular embodiment described herein shows a bi-directional driving device, embodiments with one or more unidirectional driving devices or in which a plurality of driving devices are used in parallel, are within the range of the invention. The drive device 108 may be connected to the face of the two side shell parts 106a, 106b. For example, as shown in FIG. 6, the drive device 108 may be connected approximately to the vertical midpoint of the face of the outer envelope 104, near the ends of the shorter, shorter axes of the side portions of the outer portion. envelope 106a, 106b. In some embodiments, the marine acoustic vibrator 100 may further include a frame 109 capable of suspending a driving device 108 within the outer casing 104. For example, in the illustrated embodiment, the frame 109 extends along the major axis of the outer casing 104 and may be connected to the outer casing 104 with linear bearings 122. In some embodiments, the frame 109 may be of circular section and may be mounted on the hinges 120 using the linear bearings 122. This support can allow contraction of the major axis of the outer casing 104 when the minor axis is enlarged by the movement of the drive device 108.

As illustrated, the drive device 108 may include a moving coil bidirectional drive having two sets of electrical coil 124, transmission element 126, and magnetic circuit 128, which are capable of generating a magnetic field. As illustrated, the magnetic circuit 128 can be connected to the frame 109, while the transmission element 126 can be connected to the outer casing 104. In some embodiments (not shown), this arrangement can be reversed ( that is to say, the magnetic circuit 128 is connected to the outer casing 104, while the transmission element 126 is connected to the frame 109). By attaching the heavier portion (magnetic circuit 128) of the drive device 108 to the outer casing 104, it may be easier to generate low frequencies without having to make the outer casing 104 too small to allow a constant of soft spring. As illustrated, each transmission element 126 can transfer the movement of the electrical coil 124 to the inner surface of the outer shell 104 near its minor axis. When an electric current I is applied to the electrical coil 124, a force F acting on the electrical coil 124 can be generated as follows: F = IlB (Equation 1) where I is the current, 1 is the length of the conductor in the coil electrical 124, and B is the magnetic flux generated by the magnetic circuit 128. By varying the magnitude of the electric current and therefore the magnitude of the force acting on the electrical coil 124, the length of the device travel of training must vary. The training device 108 may provide stroke lengths of several centimeters (up to and including about 25.4 cm (10 inches)), which may allow the marine acoustic vibrator 100 to generate an amplitude acoustic output. improved in low frequency ranges, for example, between about 1 Hz and about 100 Hz, and more particularly between about 1 and 10 Hz when the marine acoustic vibrator 100 is immersed in water at a depth of about 0 meters 300 meters away. Magnetic circuit 128 may often include permanent magnets, although any device capable of generating a magnetic flux may be incorporated. In the illustrated embodiment, the marine acoustic vibrator 100 further comprises the variable gas limiter 102 disposed within the outer casing 104. As illustrated, the variable gas limiter 102 may be attached to the frame As previously described, the variable gas limiter 102 can be movable between an open position and a closed position to limit a flow of gas within the outer casing 104. By way of example, a limitation of the gas flow can be used to increase the first resonance frequency by stiffening the gas spring. As is obvious to one skilled in the art, the total impedance that can be supported by a marine acoustic vibrator 100 can be expressed as: Zr = Rr + jX, (Equation 2) where Zr is the total impedance , Rr is the radiation impedance, and Xr is the reactive impedance.

In an energy transfer analysis of the marine acoustic vibrator 100, the system can approach a baffled piston. In the expression of the total impedance that is supported, the radiation impedance Rr of a baffled piston can be: Rr = na2pocR1 (x) (Equation 3) and the reactive impedance can be: Xr = na2pocX1 ( x) (Equation 4) where x = 2ka = (47-ca / X) = (2coa / c) (Equation 5) and where R1 (x) = 1 - (2 / x) J1 (x) and (Equation 6 ) Where n is the density of water, w = radial frequency, piston radius, c = sound velocity, X = wavelength, and J1 = first order Bessel function. Using the Taylor series development on the equations above gives the following: x2 X4 R1 (x) = 221! 2! 242131 + (Equation 8) 4 x x3 x5 X1 (x) = -3 55 +32527 - ... (Equation 9) For low frequencies, when x = 2ka is much smaller than 1, the real and imaginary part of the expression of total impedance can be approximated with the first term of the Taylor expression. The expressions for low frequencies, when the wavelength is much greater than the radius of the piston become: R1 (x) = (1/2) (ka) 2 (Equation 10) Xl (x) -> (8ka ) / (3n) (Equation 11) It follows that for low frequencies, R is a small number compared to X, suggesting a very low signal generation. However, embodiments may introduce resonance into the lower portion of the frequency spectrum so that low frequency acoustic energy can be generated more efficiently. At resonance, the imaginary (reactive) part of the impedance is canceled, and the marine acoustic vibrator 100 may be able to efficiently transmit acoustic energy into the body of water.

In some embodiments, the marine acoustic vibrator 100 may have two resonance frequencies (when immersed in water at a depth of about 0 meters to about 300 meters) in the seismic frequency range of interest, for example, between about 1 Hz and about 200 Hz. In particular embodiments, marine acoustic vibrator 100 may have two resonant frequencies (when immersed in water) between about 0.1 Hz and about 10 Hz. Hz, and alternatively between about 0.1 Hz and about 5 Hz. As previously described, the first resonance frequency can be controlled by limiting a gas flow in the marine acoustic vibrator 100. In particular embodiments , the first resonant frequency can be increased by a limitation of the gas flow in the marine acoustic vibrator 100. For example, the first resonant frequency p could be ordered to be substantially constant regardless of depth.

FIG. 7 illustrates another embodiment of a marine acoustic vibrator 100 comprising a variable gas limiter 102. In the illustrated embodiment, the marine acoustic vibrator 100 further comprises a spring 130 within the envelope. 104 with masses 132 secured thereto along the ends of the major axis and slidably supported on the frame 109 using a linear bearing 134. As illustrated, the spring 130 may be of a generally elliptical shape. The spring 130 can be connected to the outer casing 104 near the minor axis. In the illustrated embodiment, the driving device 108 may be connected to the outer casing 104. The spring 130 with the masses 132 may cause a second system resonant frequency when the marine acoustic vibrator 100 is immersed in the water at a depth of about 0 meters to about 300 meters in the seismic frequency range of interest (for example between about 1 Hz and about 10 Hz). Although a marine acoustic vibrator 100 as shown in Fig. 6 which comprises only the outer casing 104 acting as a spring typically has a second resonance frequency, for systems having a size suitable for use in geophysical exploration, the second resonant frequency when the marine acoustic vibrator 100 is immersed in water is typically much higher than the seismic frequency range of interest. FIG. 8 illustrates yet another embodiment of a marine acoustic vibrator 100 having a variable gas limiter 102. In the illustrated embodiment, the major axis ends of the spring 130 can be connected to the major axis ends of the outer casing 104 at the hinges 120.

The masses 132 may be attached to the spring 130 near its minor axis. As illustrated in FIG. 9, the spring 130 can be vertically divided into two springs 130a, 130b, each with added masses 132. In the illustrated embodiment, a spring 130a is disposed above the driving device. 108, while the other inner spring 130b is disposed below the drive device 108, and the drive device 108 remains connected to the outer casing 104, as shown in FIG. 8.

In an evaluation of gas spring effects, finite element analysis can be used as known to those skilled in the art. In such an analysis, the following principles may be relevant. If the outer casing 104 of the marine acoustic vibrator 100 is approximated as a piston, then, for low frequencies, the mass load, or the equivalent fluid mass acting on the casing, may be: Mshell = Po (8a3 / 3) (Equation 12) Where Mshell is the mass load of the outer shell 104, po is the density of the water, and a is the equivalent radius for a piston which corresponds to the size of the shell 104. The outer casing 104 may also have a spring constant, for example, in the direction of the moving electrical coils of the marine acoustic vibrator 100. The stiffness of the entrained gas (gas spring) may be described by the following general formula : Kvariablegasspring = AVolume / Volume * P * y (Equation 13) WHERE: Kvariablegasspring is the gas spring value, Volume is the internal volume of the marine acoustic vibrator 100, AVolume is the volume change due to the action of the acoustic vibratorMarine 100, P is the absolute pressure of the gas inside the marine acoustic vibrator 100, and y is the adiabatic constant which is a unique property depending on the chemical composition of the gas. Therefore, taking into account gas spring effects, the first resonance frequency, resonance-1 due to the interaction of the spring-acting outer casing 104, can be substantially determined by the following spring mass relationship: F resonance-1 27t V Mshell + Mdriver Kshell + Kvariableairsprinq (Equation 14) is the spring constant of outer casing 104, Kvariablegasspring is the gas spring value determined by the change in gas volume using, for example, Equation 13 above, Mariver is the mass load of the driver, and Mshell OÙ Kshell is the mass load of the outer casing 104. Therefore, it may be possible, as shown below. above, to adjust the first resonance frequency by compensating the gas spring. By limiting the flow of gas in the outer casing 104, the effective volume of gas can be changed, resulting in a change in the gas spring value. The first resonance frequency can also change because the gas spring value has also changed. For example, a steeper gas spring due to a pressure increase or gas base volume reduction has a higher gas spring value thereby causing a corresponding increase in the first resonance frequency.

To achieve efficient energy transmission in the seismic frequency range of interest, it may be desirable to obtain a second resonant frequency in the seismic frequency range of interest. In the absence of the spring 130 with its added masses 132, the second resonant frequency occurs when the outer envelope 104 has its own second mode. This resonant frequency, however, is normally much higher than the first resonant frequency, and therefore typically is outside the seismic frequency range of interest. As is evident from the above equation, the resonant frequency is reduced if the mass load on the outer shell 104 is increased. However, in order to add a mass sufficient to obtain a second resonant frequency in the seismic frequency range of interest, the amount of mass that may need to be added to the outer casing 104 may make this system less convenient for use in marine seismic operations.

In some embodiments, the spring 130 is included within the outer casing 104 with added masses 132 on the side of the spring 130. The spring 130 may have a Tspring transformation factor between the major and minor axes its ellipse, so that the displacement of the two lateral parts has a greater amplitude than the deflection of the end fixed on the outer casing 104 and the drive device 108.

The effect of these added masses 132 may be equivalent to adding a mass to the end of the drive device 108 where it is attached to the outer envelope 104. Mspring = (Tspring) 2 - Madded (Equation 15 ) Where Mspring is the mass of the spring, Tspring is the spring transformation factor, and Madded is the mass of the added mass 132. The use of the spring 130, with the added masses 132, can allow the second resonance frequency the system is tuned so that the second resonant frequency is in the seismic frequency range of interest, thereby improving the effectiveness of the marine acoustic vibrator 100 in the seismic band. 1 Kspring + Kshell Tspring) 2 - Madded + Mshell (Equation 16) f resonance2 = -27c Where Kspring is the spring constant of the spring 130, Kshell is the spring constant of the outer shell 104, Tspring is the transformation factor of the spring, Madded is the mass of the added mass 132, and M -shell is the mass load on the outer shell 104. Therefore, it may be possible, as shown above, to choose the mass added 132 on the spring 130 to tune the second resonance frequency. It may also be possible to choose the magnitude of influence that the second resonant frequency must have on the system. For example, if the spring 130 has a low spring constant compared to the outer casing 104, and a corresponding mass 132 is added to the spring 130, the spring 130 with its mass 132 operates relatively independently of the outer casing 104. In this case, the second resonance frequency can be as follows: f 1 Kspring11 resonance2 (Tspring) -2 - Madded cation 17) 2n In the same way, it may also be possible in some embodiments to make the second very high resonance frequency by choosing a high spring constant for the spring 130 with a corresponding mass 132 so that the second resonance frequency has an amplitude greater than the first resonance frequency. In some embodiments, the marine acoustic vibrator 100 can be towed relatively deep, for example, from about 10 meters to as deep as 100 meters or more. Figures 10 and 11 are plots showing the attenuation for a model of a marine acoustic vibrator 100 due to the phantom source. Figure 10 shows the attenuation due to the phantom source at 10 meters. Figure 11 shows the attenuation due to the phantom source at 100 meters. Therefore, the marine acoustic vibrator 100, in particular embodiments, must be towed deeper as shown in Figures 10 and 11 to avoid undesirable attenuation of the signal by the phantom source. The dimensions of the marine acoustic vibrator 100 may vary as necessary for a particular application. Referring to FIG. 12, an exemplary embodiment of a marine acoustic vibrator 100 may have an envelope size as follows: 1) envelope height H1 ranging from about 0, 5 meters at about 4 meters, for example about 1.59 meters; 2) envelope end height H2 of the envelope end extending from about 0.3 meters to about 1 meter; 3) envelope width W1 ranging from about 0.5 meters to about 4 meters, for example, about 1.75 meters, 4) envelope thickness T1 ranging from about 0.2 meters to about 3 meters, for example, about 2.5 meters. As illustrated, the envelope height H1 is the height of the outer shell 104 at or near its center line while the shell end height H2 is the height of the outer shell 104 at level of its longitudinal end. In particular embodiments, the marine acoustic vibrator 100 may have an envelope size as follows: 1) H1 envelope height of 1.59 meters; 2) H2 envelope end height of 1.0 meter; 3) W1 envelope width of 1.75 meters; 4) T1 shell thickness of 2.5 meters. Figure 13 illustrates an exemplary technique for acquiring marine seismic data that can be used with embodiments of the present techniques. In the illustrated embodiment, a search vessel 200 travels along the surface of a body of water 202, such as a lake or an ocean. The search vessel 200 may comprise equipment, shown generally at 204 and collectively referred to hereinafter as "registration system". The recording system 204 may comprise devices (none of which are shown separately) for detecting and performing a time-indexed recording of the signals generated by each of the seismic sensors 206 (explained further below) and intended for operating one or more seismic sources (as illustrated, a marine acoustic vibrator 100) at selected times. The recording system 204 may also include devices (none shown separately) for determining the geodesic position of the search vessel 200 and the different seismic sensors 206.

As illustrated, the research vessel 200 (or a different vessel) can tow the marine acoustic vibrator 100 into the body of water 202. A source cable 208 can connect the marine acoustic vibrator 100 to the research vessel 200. The marine acoustic vibrator 100 can be towed in the body of water 202 to a depth ranging from 0 meters to about 300 meters, for example. Although a single marine acoustic vibrator 100 is shown in Figure 13, it is intended that the embodiments may include more than one seismic source (eg, marine acoustic vibrators or pneumatic guns) towed by the research vessel 200 or a different ship. In some embodiments, one or more rows of seismic sources may be used. At selected times, the marine acoustic vibrator 100 may be triggered, for example, by the recording system 204, to generate acoustic energy. The research vessel 200 (or a different vessel) may further tow at least one sensor marine stream 210 to detect acoustic energy that originates from the marine acoustic vibrator 100 once it has interacted, for example, with the rock formations 212 below the bottom 214. As illustrated, the marine acoustic vibrator 100 and the sensor marine flute 210 can be towed over the bottom 214. The seismic marine flute 210 can contain seismic sensors 206 in spaced locations. Although not shown, some seismic searches place seismic sensors 206 on ocean bottom cables or nodes in addition to, or instead of, a marine sensor stream 210. The seismic sensors 206 may be any type of seismic sensors known in the field, including hydrophones, geophones, particle velocity sensors, particle displacement sensors, particle acceleration sensors, or pressure gradient sensors , for example. For example, seismic sensors 206 may generate response signals, such as electrical or optical signals, in response to sensed acoustic energy. Signals generated by the seismic sensors 206 may be transmitted to the recording system 204. In some embodiments, more than one marine sensor stream 210 may be towed by the research vessel, which may be laterally, vertically or vertically spaced apart. both laterally and vertically. The detected energy can be used to derive certain properties of the rock beneath the surface, such as structure, mineral composition, and fluid content, thereby providing useful information in hydrocarbon recovery. According to one embodiment of the invention, a geophysical data product indicative of certain properties of the rock below the surface can be produced from the detected energy. The geophysical data product may include processed seismic geophysical data and may be recorded on a non-transitory and tangible machine readable medium. The geophysical data product can be produced at sea (ie by equipment on a ship) or on land (ie in an onshore installation) in the United States or in another country. If the geophysical data product is produced at sea or in another country, it can be imported to land at a facility in the United States. Once on land in the United States, geophysical analysis can be performed on the data product.

Claims (22)

REVENDICATIONS1. Marine acoustic vibrator (100), characterized in that it comprises: an outer casing (104); a variable gas flow limiter (102) disposed within the outer casing (104); the marine acoustic vibrator (100) having a resonant frequency selectable at least in part based on the variable gas flow limiter (102). 2. marine acoustic vibrator (100) according to claim 1, characterized in that it further comprises: a drive device (108) disposed at least partially inside the outer casing (104) and connected to it. 3. marine acoustic vibrator (100) according to claim 2, characterized in that the drive device (108) comprises an electrodynamic drive device (108). 4. marine acoustic vibrator (100) according to any one of claims 1 to 3, characterized in that the marine acoustic vibrator (100) has at least two resonance frequencies of about 10 Hz or less when immersed in the water at a depth of about 0 meters to about 300 meters.30 A marine acoustic vibrator (100) according to any one of claims 1 to 4, characterized in that the variable gas flow restrictor (102) comprises a first plate (110) having holes (114), and a second plate (112) having holes (116), the second plate (112) being movable to at least partially cover the holes (114) in the first plate (110). The marine acoustic vibrator (100) according to claim 5, characterized in that the variable gas flow restrictor (102) has an open position and a closed position, the holes (114) in the first plate (110) being at least partially obstructed by the second plate (112) in the closed position, and the holes (114) in the first plate (110) and the holes (116) in the second plate (112) being aligned in the open position for a maximum gas flow through the variable gas flow restrictor (102). Marine acoustic vibrator (100) according to claim 5 or 6, characterized in that the variable gas flow limiter (102) is fixed on a frame (109) in the marine acoustic vibrator (100), the frame ( 109) being connected to the outer casing (104). 8. marine acoustic vibrator (100) according to any one of claims 1 to 7, characterized in that it further comprises a frame (109) connected to the flexion-extension envelope, the drive device (108). ) having a first end attached to the outer shell (104) and a second end attached to the frame (109). 9. marine acoustic vibrator (100) according to any one of claims 1 to 8, characterized in that it further comprises a spring (130) connected to the outer casing (104), and masses (132) fixed at the spring (130). 10. marine acoustic vibrator (100), characterized in that it comprises: an outer casing (104) flexion-extension; a frame (109) connected to the outer flexion-extension shell (104); a driving device (108) having a first end and a second end, the first end being fixed to the outer flexion-extension shell (104), and the second end attached to the frame (109); a gas spring being provided with a mass to generate a first resonance frequency, and the value of the gas spring being changed by a limitation of the gas flow in the marine acoustic vibrator (100) to thereby control the first resonance frequency in depth. The marine acoustic vibrator (100) according to claim 10, characterized in that the marine acoustic vibrator (100) has at least two resonant frequencies of about 10 Hz or less when immersed in water at a depth from about 0 meters to about 300 meters. 12. marine acoustic vibrator (100) according to claim 10 or 11, characterized in that it further comprises a gas flow limiter disposed within the outer casing (104). Marine acoustic vibrator (100) according to claim 12, characterized in that the variable gas flow restrictor (102) comprises a first plate (110) having holes, and a second plate (112) having holes, the second plate (112) being movable to at least partially cover the holes in the first plate (110). The marine acoustic vibrator (100) according to claim 13, characterized in that the variable gas flow restrictor (102) has an open position and a closed position, the holes in the first plate (110) being at least partially obstructed by the second plate (112) in the closed position, and the holes in the first plate (110) and the holes in the second plate (112) being aligned in the open position for maximum gas flow through the limiter variable gas flow (102). 15. marine acoustic vibrator (100) according to any one of claims 12 to 14, characterized in that the variable gas flow limiter (102) is fixed on the frame (109). 16. A method characterized in that it comprises the fact of: towing an acoustic vibrator (100) in a body of water; triggering the acoustic vibrator (100) to generate acoustic energy in the water body; limiting a flow of gas in the acoustic vibrator (100) to control a first resonance frequency of the acoustic vibrator (100); and detecting acoustic energy from the acoustic vibrator (100). 17. The method of claim 16, characterized in that the acoustic vibrator (100) is towed at a first depth of about 0 meters to about 300 meters. 10 18. A method according to claim 17, further comprising: towing the acoustic vibrator (100) to a second depth, the gas flow being limited in the acoustic vibrator (100) when it is towed to the first depth such that the first resonant frequency of the acoustic vibrator (100) is substantially constant when a towing depth varies from the first depth to the second depth. 20 19. A method according to any one of claims 16 to 18, characterized in that limiting the flow of gas in the acoustic vibrator (100) comprises moving a plate to at least partially obstruct holes in another plate. The method of any one of claims 16 to 19, characterized in that it further comprises opening a variable gas flow restrictor (102) to allow increased gas flow in the acoustic vibrator (100) when the acoustic vibrator (100) is towed into the body of water. 21. A method according to any one of claims 16 to 20, characterized in that it further comprises increasing a gas pressure inside the envelope of the acoustic vibrator (100) to equalize the gas pressure inside the envelope with the water pressure at depth. 22. Method according to any one of claims 16 to 21, characterized in that it further comprises the fact of: producing a product of geophysical data from the detected acoustic energy indicative of certain properties of rock under the surface below the body of water.15