LINEAR MOTOR DRIVING MEANS FOR ACOUSTIC EMITTERS

Abstract

An apparatus for generating acoustic waves in an aquatic environment through the use of linear motors mounted on a frame is disclosed herein. The linear motors can reciprocate at a first frequency which is transmitted through a transmission element and to a sound emitting surface. The structure of the apparatus can generate acoustic waves across a broad frequency to measure subterranean formations.

Claims

1. A drive assembly for acoustic sources, the drive assembly comprising: at least one vibrating surface capable of being set in vibrational motion; a frame comprising at least two or more of linear motors; two or more flexible transmission elements connecting between fastening devices and extending on both sides of the axis, between the two fastening devices; the two or more linear motors connected to the transmission elements, and that each of the linear motor are adapted to provide a controlled oscillating motion; and a sound emitting surface connected to the fastening devices.

2. The assembly of claim 1, wherein the linear motors are centrally positioned on the frame.

3. The assembly of claim 1, wherein the linear motors oscillate at a first frequency.

4. The assembly of claim 4, wherein the flexible transmission elements oscillate at a second frequency.

5. The assembly of claim 5, wherein the first and second frequency are between 1 to 300 Hz.

6. The assembly of claim 1, wherein the transmission elements are made of a material selected from the group consisting of carbon fiber, glass fiber, and spring steel.

7. The assembly of claim 1, comprising at least one spring element.

8. The assembly of claim 7, wherein the at least one spring element is made of a material selected from the group consisting of carbon fiber, glass fiber, and spring steel.

9. A method of generating acoustic waves, the method comprising: reciprocating at least one linear motor at a first frequency; vibrating a transmission element by the at least one linear motor; varying a shape of a sound emitting surface by the movement of the transmission element connected to the sound emitting surface by a fastening device; and emitting an acoustic wave from the sound emitting surface.

10. The method of claim 9, wherein the linear motor reciprocates with a relatively large range of motion.

11. The method of claim 9, where the transmission element vibrates with a relatively small range of motion.

12. The method of claim 9, wherein the transmission element vibrates at a second frequency.

13. The method of claim 12, comprising combining the first frequency and the second frequency to a third frequency.

14. The method of claim 13, wherein at least one spring element modifies the third frequency to a fourth frequency.

15. A drive assembly for acoustic sources, the drive assembly comprising: vibrating surfaces capable of being set in vibrational motion; a frame comprising at least one centrally positioned linear motor, wherein the at least one centrally positioned linear motor is mounted in relation to a sound emitting surfaces and positioned on opposite sides of the frame; two or more flexible transmission elements and two or more spring elements connecting the fastening devices to each other and extending on both sides of the axis; and between the two fastening devices, at least one centrally positioned linear motor connected to the transmission elements and spring elements, wherein at least one centrally positioned linear motor is adapted to provide a controlled oscillating motion.

16. The driver assembly of claim 15, wherein the transmission elements have a resonance mode which is designed to act on an upper part of the frequency band and the spring element will generate the third resonance mode between a first and second resonance modes of the source.

17. The driver assembly of claim 15, wherein the transmission elements and spring element are made of carbon fiber.

18. The driver assembly of claim 15, wherein the transmission elements and spring elements, together with the vibrating surfaces define at least 3 resonances within the frequency range of about 1 to 300 Hz.

19. The driver assembly of claim 15, wherein the transmission elements are flexible plates or rods.

20. The drive assembly of claim 15, wherein the transmission elements are rotatably fastened to the frame by a fastening device.

Description

DESCRIPTION OF THE DRAWINGS

[0021] The invention will be described in detail below, referring to the disclosed drawings:

[0022] FIG. 1 shows the principal of a linear motor with a forcer and a plunger as provided by embodiments of the invention.

[0023] FIGS. 2A and 2B show one embodiment of the flextensional source with linear motors connected to a transmission element.

[0024] FIGS. 3A-3C show resonance modes of the transmission element connected to the linear motor.

[0025] FIGS. 4A-4C show different resonance modes for the flextensional source.

[0026] FIG. 5 shows a 3D view of the embodiment of the flextensional source of FIG. 2B.

[0027] FIGS. 6A and 6B show one embodiment of the flextensional source with linear motors connected to a transmission element and a spring elements connected to the transmission element.

[0028] FIG. 7 shows a 3D view of the embodiment of the flextensional source of FIG. 6B.

[0029] FIG. 8A and 8B show one embodiment of the flextensional source with linear motors connected to a transmission element and to the sound emitting surface.

[0030] FIG. 9 shows a 3D view of one embodiment of the flextensional source of FIG. 8B.

[0031] FIGS. 10A and 10B show one embodiment of the flextensional source with linear motors connected to a transmission element and to the sound emitting surface and a spring elements connected to the transmission element.

[0032] FIG. 11 shows a 3D view of one embodiment of the flextensional source of FIG. 10B.

[0033] FIG. 12 shows an embodiment of a towing vessel towing two flextensional sources.

[0034] FIG. 13 shows an ocean streamer cable marine seismic acquisition system.

DETAILED DESCRIPTION

[0035] The particulars shown herein are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present disclosure only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of various embodiments of the disclosure. In this regard, no attempt is made to show structural details of the disclosure in more detail than is necessary for the fundamental understanding of the disclosure, the description taken with the drawings making apparent to those skilled in the art how the several forms of the disclosure may be embodied in practice.

[0036] When introducing elements of various embodiments of the present invention, the articles a, an, the, and said are intended to mean that there are one or more of the elements. The terms comprising, including, and having are intended to be inclusive and mean that there may be additional elements other than the listed elements. Any examples of operating parameters and/or environmental conditions are not exclusive of other parameters/conditions of the disclosed embodiments. Additionally, it should be understood that references to one embodiment, an embodiment, certain embodiments, or other embodiments of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Furthermore, reference to terms such as above, below, upper, lower, side, front, back, or other terms regarding orientation are made with reference to the illustrated embodiments and are not intended to be limiting or exclude other orientations.

[0037] To understand the importance of resonances for an acoustic source which have a size that is smaller than the wavelength generated, the impedance for a source is provided as:

[00001]$Z_r=R_r+{\mathrm{jX}}_r$ [0038] where Z.sub.r is total impedance, R.sub.r is radiation impedance, and jX.sub.r is reactive impedance.

[0039] In an analysis of the energy transfer of a marine vibrator, the system may be approximated as a baffled piston. In the expression of the total impedance that will be experienced, the radiation impedance R.sub.r of a baffled piston is:

[00002]$R_r=a^2{}_0{\mathrm{cR}}_1\left(x\right)$ [0040] and the reactive impedance is:

[00003]$X_r=a^2{}_0{\mathrm{cX}}_1\left(x\right);$$x=2\mathrm{ka}=\frac{4a}{}=\frac{2a}{c};$$R_1\left(x\right)=1-\frac{2}{x}{J}_1\left(x\right);$$\mathrm{and}$$X_1\left(x\right)=\frac{4}{}{}_0^{\frac{}{2}}\sin \left(x\cos \right){\sin }^{2}d$

[0041] Where .sub.0=density of water, =radial frequency, k=wave number, a=radius of piston, C=sound velocity, =wavelength, and J.sub.1=Bessel function of the first order.

[0042] Using the Taylor series expansion on the above equations yields

[00004]$R_1\left(x\right)=\frac{x^2}{2^{2}1!2!}-\frac{x^4}{2^{4}2!3!}+.Math.$$X_1\left(x\right)=\frac{4}{}\left[\frac{x}{3}-\frac{x^3}{3^{2}5}+\frac{x^5}{3^2{5}^{2}7}-.Math.\right]$

[0043] For low frequencies, when x=2ka is much smaller than 1, the real and imaginary part of the total impedance expression may be approximated with the first term of the Taylor expansion. The expressions for low frequencies, when the wavelength is much larger than the radius of the piston, becomes:

[00005]$R_1\left(x\right).fwdarw.\frac{1}{2}{\left(\mathrm{ka}\right)}^2$$X_1\left(x\right).fwdarw.\frac{8\mathrm{ka}}{3}$

[0044] It follows that for low frequencies R will be a small number compared to X, which suggests a very low efficiency signal generation. However, by introducing a resonance in the lower end of the frequency spectrum, low frequency acoustic energy may be generated more efficiently. At resonance the imaginary (reactive) part of the impedance is cancelled, and the acoustic source can efficiently transmit acoustic energy into the water. To improve the efficiency further, more than one resonance would improve the acoustic output more.

[0045] The present technology, in a preferred embodiment, comprises a marine vibrator that displays at least two resonant frequencies within the seismic frequency range of interest, typically a range between 1 Hz and 300 Hz

[0046] The object of this technology is to provide a simple, flexible, and reliable drive assembly capable of emitting signals within a wide range of frequencies. The drive assembly may be used in several different situations in addition to seismic explorations, such as uses related to submarine sound sources and sonars. The shape of the sound emitting surfaces may vary according to use, and all of the different embodiments mentioned below may be utilized. The aim of the present invention can be to provide a sound apparatus for use in seismic studies capable of emitting signals in a wide range of frequencies. According to the technology, such apparatus comprises a frame with a first driver part for vibrating a sound emitting surface attached to at least two fastening devices positioned on opposite sides of the frame and connected by two or several flexible transmission elements extending on both sides of the axis between the fastening devices or in parallel to the fastening devices. At least two linear motors are connected to the transmission elements for generating thereby controlled relative oscillating motion.

[0047] In FIG. 1 the principal of a linear motor is shown. Linear motors can have a simple design with a minimum of moving parts. Linear motors with high speeds can further be self-contained. The efficiency of linear motors can be about 85%. The forcer 1 can contain electrical coils and other electronics. The plunger 2 can have permanent magnets and can usually be a sleeve made of steel. Linear motors can have an advantage in outdoor or dirty environments in that the plunger 2 and forcer 1 may not need to contact each other. As a result, the electromagnetic drive coils can be waterproofed and sealed against moisture and corrosion, allowing for a very long service life. Linear motors can be used extensively in high performance positioning systems for applications which require various combinations of high velocity, high precision, and high force.

[0048] In FIGS. 2A and 2B an embodiment of the technology is shown in which transmission elements 6 can have a slightly arched shape and the linear motors 7 can be centrally mounted on the frame 5 and the transmission elements 6 respectively. FIG. 2A shows a cross-sectional view of the embodiment along plane A-A of FIG. 2B. The transmission elements 6 may be shaped as flexible plates or rods and can be preferably rotatably fastened to the fastening devices 4. The distance from the central part of the transmission elements 6 to the axis between the fastening devices 4 may be substantially less than the distance from the central part to the fastening devices 4. This way a transmission can be provided in which a large movement of the drive part 7 on the transmission element 6 with a relatively small force, can lead to a small movement of the fastening devices 4 but with a correspondingly larger force.

[0049] The transmission can depend on the curvature of the transmission elements 6. If the transmission elements are essentially straight a force doubling can be obtained compared to the movements of the drive. The fastening devices 4 are shown in FIG. 2B as beams, but the fastening of the transmission elements 6 to the sound emitting surfaces may also be done directly to the sound emitting surfaces 3. The sound emitting surfaces 3 in FIGS. 2A and 2B can be elliptic. When the fastening devices 4 are pulled inwards by the transmission elements 6, the ellipse can widen, creating a pressure wave in the environment. In this way, the movements of the linear motor drives can propagate outwards and result in acoustic waves in the water.

[0050] By varying the eccentricity of the ellipse and the transmission rate in the drive assembly it may be adapted to different situations and frequency contents. As an example, the fastening devices may be fastened directly to pistons, in which a relatively large movement of the drives will provide a small movement of the pistons.

[0051] The sound emitting surface 3 can have a resonance which depends on the equivalent water mass acting on the surface, which is the amount of water oscillating with the vibrating sound emitting surfaces and the stiffness of the sound emitting surfaces K.sub.Shell.

[0052] For a piston with a radius .sub.x the equivalent water mass is:

[00006]$M_x=\frac{8a_x^3}{3}$

[0053] Where is the density. When using an elliptical shell and not a baffled piston, the surface area can be approximated by calculating the length and the width of the elliptical and using reduction factor of 0.5.

[00007]$S_x=\mathrm{length}*\mathrm{width}*0.5$

[0054] The equivalent radius .sub.x is then arrived from:

[00008]$a_x=\sqrt{S_x/}$

[0055] The primary resonance frequency will then be:

[00009]$f=\frac{1}{2}\sqrt{\frac{K_{\mathrm{Shell}}}{M_x}}$

[0056] Having a source with only one resonance would result in a frequency response that is similar to the FIG. 4A. In FIGS. 3A-3C, different resonance modes are shown for the transmission element 6. The first resonance mode shown in FIG. 3A can be approximated with:

[00010]$f_1=1.028\frac{a}{L^2}\sqrt{\frac{Y}{d}}$

[0057] Where =thickness, L=length, Y=Young's modulus and d=density.

[00011]$f_n=0.44{1\left[n+\frac{1}{2}\right]}^2{f}_1$

[0058] The third resonance mode shown in FIG. 3C can be approximated with f=5.40f.sub.1. The second resonance mode (FIG. 3B) may not exist in a forced vibration mode where the linear motor acts on the transmission element 6.

[0059] With the linear motor 7 attached to the transmission element 6, a spring mass system can result. The plunger 2 can add mass, and the spring constant together with the mass will determine the resonance frequency:

[00012]$f_1=\frac{1}{2}\sqrt{\frac{K_1}{M_{\mathrm{plunger}}}}$$K_1={\left(2\right)}^2{f}_1^2{M}_{\mathrm{plunger}}$

[0060] It is obvious that for the third resonance mode (FIG. 3C) the transmission element 6 can have a much larger spring constant

[00013]$K_3={\left(2\right)}^2{\left(5.40f_1\right)}^2{M}_{\mathrm{plunger}}$

[0061] The stiffness for the third resonance mode can make the transmission element 6 nearly 30 times stiffer than for the first resonance mode. This can be used to improve the broadband performance of the source. For example, a source can operate from 25-90 Hz in the first resonance mode (FIG. 3A) as determined by the elliptical sound emitting surface 3 and the mass load from the surrounding water.

[0062] FIG. 4A-4C show the result of the first resonance from equivalent water mass and the sound emitting surface 3. In a first embodiment, the resonance can be around 25 Hz (FIG. 4A). The resonance of the transmission element can be set to 80 Hz (FIG. 4B). Together will the two resonances in FIG. 4A and FIG. 4B give the combined result shown in FIG. 4C. The two resonances in FIG. 4A and FIG. 4B can be set rather independent from each other. This result shows a source that can have a much better broadband performance and efficiency over large band of frequencies. The first resonance of the transmission element 6 can approximately be 80/5.4 which is about 15 Hz. This 15 Hz resonance may not affect the output of the source.

[0063] In FIG. 5 is a 3D view of the source shown in FIG. 2. The linear motors 7 can be positioned along a centerline of the assembly.

[0064] In FIGS. 6A and 6B an embodiment of the invention is shown in which the transmission elements 6 can have a slightly arched shape and the linear motors 7 can be centrally mounted on the frame 5 and the transmission elements 6 respectively. FIG. 6A shows a cross-sectional view of the embodiment along plane A-A of FIG. 6B. The transmission elements 6 may be shaped as flexible plates or rods and are preferably rotatably fastened to the fastening devices 4. The distance from the central part of the transmission elements 6 to the axis between the fastening devices 4 can be substantially less than the distance from the central part to the fastening devices 4. In this way, an apparatus is provided in which a large movement of the drive part 8 on the transmission element 6, but with a relatively small force, leads to a small movement of the fastening devices 4, but with a correspondingly larger force. The transmission can depend on the curvature of the transmission elements 6. If the transmission elements are essentially straight a force doubling can be obtained compared to the movements of the drive. The fastening devices 4 are shown in FIG. 6B as beams, but the fastening of the transmission elements 6 to the sound emitting surfaces may also be done directly to the sound emitting surfaces. The sound emitting surfaces 3 in FIG. 6B are elliptic.

[0065] When the fastening devices 4 are pulled inwards by the transmission elements 6 the ellipse will widen, which can create a pressure wave in the environment. In this way, the movements of the linear motor drives 7 will propagate outwards and result in acoustic waves in the water. By varying the eccentricity of the ellipse and the transmission rate in the drive assembly it may be adapted to different situations and frequency contents. In other embodiments, different sound emitting surfaces may be chosen. As an example, the fastening devices 4 may be fastened directly to pistons, in which a relatively large movement of the drives will provide a small movement of the pistons.

[0066] Spring elements 8 can be attached to the transmission element 6. This spring element 8 can be a be of any kind, but preferably a spring that will have a frequency dependence. The preferred material in the spring elements 6 and 8 can be carbon fiber, glass fiber, or spring steel. With the same understanding as in FIGS. 4A-4C the spring element 8 can have different resonance modes. The third resonance mode can be between the 25 Hz and the 80 Hz resonance modes, for example 50 Hz.

[0067] For the third resonance mode (FIG. 3C) we have:

[00014]$K_3={\left(2\right)}^2{\left(5.40f_1\right)}^2$

[0068] Comparing 50 Hz with 90 Hs would result such that the spring constant for the 50 Hz resonance is 13 times lower than for the 90 Hz.

[0069] The resonance for the first resonance can be described as:

[00015]$f_1=\frac{1}{2}\sqrt{\frac{K_1+\frac{K_1}{13}}{M_{\mathrm{plunger}}}}$

[0070] The first resonance will only change about 4% with the added spring element (8). This makes it possible to add another resonance without affecting other resonances very much. The resonance can be selected rather freely in the frequency band 1-300 Hz. This added spring will increase the broadband performance of the source and improve the efficiency.

[0071] In FIG. 7 is a 3D view of the source shown in FIG. 6B.

[0072] In FIGS. 8A and 8B an embodiment of the technology is shown in which the transmission elements 6 have a slightly arched shape and the linear motors 7 are centrally mounted on the frame 5 and the transmission elements 6 respectively. FIG. 8A shows a cross-sectional view of the embodiment along plane A-A of FIG. 8B. Linear motors 7 can also be centrally mounted on the frame 5 and the sound emitting surfaces 3. The transmission elements 6 may be shaped as flexible plates or rods and are preferably rotatably fastened to the fastening devices 4. The distance from the central part of the transmission elements 6 to the axis between the fastening devices 4 is substantially less than the distance from the central part to the fastening devices 4. This way a transmission is provided in which a large movement of the drive part 8 on the transmission element 6, but with a relatively small force, leads to a small movement of the fastening devices 4, but with a correspondingly larger force.

[0073] The transmission will depend on the curvature of the transmission elements 6. If the transmission elements are essentially straight, a force doubling can be obtained compared to the movements of the drive. The fastening devices 4 are shown in FIG. 8B as beams, but the fastening of the transmission elements 6 to the sound emitting surfaces may also be done directly to the sound emitting surfaces. The sound emitting surfaces 3 in FIG. 8B are elliptic. When the fastening devices 4 are pulled inwards by the transmission elements the ellipse can widen, creating a pressure wave in the environment. This way the movements of the linear motor drives can propagate outwards and result in acoustic waves in the water. By varying the eccentricity of the ellipse and the transmission rate in the drive assembly it may be adapted to different situations and frequency contents.

[0074] In other embodiments, different sound emitting surface solutions may be chosen. As an example, the fastening devices may be fastened directly to pistons, in which a relatively large movement of the drives can provide a small movement of the pistons. Having linear motors 7 on both the spring elements 6 and the sound emitting surfaces 3 can improve the acoustic output. The linear motors 7 attached to the sound emitting surfaces can be used for the first resonance determined by the sound emitting surfaces 3 interaction with the equivalent water mass. The linear motors 7 acting on the spring elements 6 can be used for the second resonance. All linear motors 7 can be used between the resonances.

[0075] In FIG. 9 is a 3D view of the source shown in FIG. 8B.

[0076] In FIGS. 10A and 10B an embodiment of the invention is shown in which the transmission elements 6 have a slightly arched shape and the linear motors 7 are centrally mounted on the frame 5 and the transmission elements 6 respectively. FIG. 10A shows a cross-sectional view of the embodiment along plane A-A of FIG. 10B. Linear motors 7 can also be centrally mounted on the frame 5 and the sound emitting surfaces 3. The transmission elements may be shaped as flexible plates or rods and can preferably be rotatably fastened to the fastening devices 4. The distance from the central part of the transmission elements 6 to the axis between the fastening devices 4 can be substantially less than the distance from the central part to the fastening devices 4. This way a transmission can be provided in which a large movement of the drive part 8 on the transmission element 6, but with a relatively small force, leads to a small movement of the fastening devices 4, but with a correspondingly larger force.

[0077] The transmission can depend on the curvature of the transmission elements 6. If the transmission elements 6 are essentially straight a force doubling can be obtained compared to the movements of the drive. The fastening devices 4 are shown in FIG. 10B as beams, but the fastening of the transmission elements 6 to the sound emitting surfaces may also be done directly to the sound emitting surfaces. The sound emitting surfaces 3 in FIG. 10B can be elliptic. When the fastening devices 4 are pulled inwards by the transmission elements the ellipse can widen, creating a pressure wave in the environment. This way the movements of the linear motor drives will propagate outwards and result in acoustic waves in the water. By varying the eccentricity of the ellipse and the transmission rate in the drive assembly it may be adapted to different situations and frequency contents.

[0078] In other embodiments of the sound emitting surfaces other solutions may be chosen. Having linear motors 7 on both the spring elements 6 and the sound emitting surfaces 3 can improve the acoustic output. The linear motors 7 attached to the sound emitting surfaces can be used for the first resonance determined by the sound emitting surfaces 3 interaction with the equivalent water mass. The linear motors 7 acting on the spring elements 6 can be used for the second resonance example, the fastening devices 4 may be fastened directly to pistons, in which a relatively large movement of the drives can provide a small movement of the pistons. Spring elements 8 can be attached to the transmission element 6. This spring element 8 can be a be of any kind, but preferably a spring that will have a frequency dependence. The preferred material in the spring elements 6 and 8 is carbon fiber, glass fiber or spring steel. With the same understanding as in FIG. 6 the spring element 8 will have different resonance modes (FIG. 3). Another frequency can be selected which will improve the broadband performance of the source as explained in FIG. 6.

[0079] In FIG. 11 is a 3D view of the source shown in FIG. 10.

[0080] FIG. 12 shows a towing vessel 13 towing flextensional sources 9 and 10. The towing vessel 13 can be a catamaran which can be manually operated or autonomous. The sources 9 and 10 can be towed by umbilicals 11 and 12. The seismic sources 9 and 10 can be activated to produce a down-going seismic wave that is at least partially reflected by a subsea interface or boundary. The up-going reflected seismic wave then travels towards sensors in a stream or on the seabed.

[0081] FIG. 13 shows a conventional type of marine seismic surveying with towing vessel 13 towing a seismic source 9 and a plurality of marine streamers 14. A marine cable or streamer 15 incorporating pressure sensing hydrophones 16 is designed for continuous towing through the water. A marine streamer 15 might be made up of a plurality of active or live hydrophone arrays 16 separated by spacer or dead sections. Usually, the streamers are nearly neutrally buoyant and depth controllers or depressors are attached to depress the streamer to the proper towing depth. A tail buoy with a radar reflector is typically attached to the end of the streamer. The entire streamer may be 3-15 Km in length and may be towed by a towing vessel 13.

[0082] Systems and methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the systems and methods of this disclosure have been described in terms of various embodiments, it will be apparent to those of skill in the art that variations may be applied to the systems and methods and in the steps or in the sequence of steps of the methods described herein without departing from the concept, spirit, and scope of the disclosure.