Abstract:
An acoustic vector sensor mounted in a housing is provided for measuring ultra low frequency acoustic wave particle velocities in a liquid, comprising a horn for amplifying the acoustic wave particle velocities, and a neutrally buoyant object supported in a liquid contained in the horn. The neutrally buoyant object, in reacting to the amplified acoustic wave particle velocities, produces displacements that are sensitively measured.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Section 111(a) application relating to commonly owned, co-pending U.S. Provisional Patent Application Ser. No. 61/072,481, filed Mar. 31, 2008, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to underwater acoustic measurement devices, and, more particularly, to underwater acoustic measurement devices related to seismic exploration, underwater surveillance (detection of divers, submarines, torpedoes, etc.), tsunami and seismic activity monitoring, and other similar activities. 
     BACKGROUND OF THE INVENTION 
     Traditional underwater acoustic sensors are pressure sensors (e.g., hydrophones) responsive to oscillating pressure in the field of the acoustic wave. Pressure is a scalar quantity, so a single hydrophone (smaller than the acoustic wavelength) has no directionality. Unlike hydrophones, vector sensors are responsive to water oscillatory velocity (or pressure gradients) associated with the same acoustic wave. Because velocity is a vector, the vector sensor has directionality even if its size is much smaller compared to the acoustic wavelength. This may not be a big advantage at relatively high acoustic frequencies (tens of kHz), but it has an unsurpassed advantage at low frequencies (tens-thousands of Hz) and, especially, at ultra low frequencies in the range of a fraction of a Hz to tens of Hz. 
     To measure particle velocity in the water, conventional vector sensors (or particle velocity sensors) are designed to be neutrally buoyant in the water column. Developing a highly sensitive, low-noise vector sensor in a small, neutrally buoyant package is extremely challenging. It becomes even more challenging for the ultra low frequency range. One reason accelerometers used in conventional sensors are not suitable for sensitive measurements at ultra low frequencies is that acceleration is proportional to f*v, where v is the particle velocity and f is the frequency. As the frequency decreases, the accelerometer&#39;s sensitivity decreases accordingly. It will be more advantageous to measure displacement, x, rather than acceleration, as the displacement is proportional to v/f. That is, for the given particle velocity, the displacement is increased as the frequency goes down. 
     Another challenging problem is the suspension of the sensor in a water column. The conventional sensor is configured as a neutrally buoyant body containing an inertial measuring element such as an accelerometer. Yet the sensor should be fixed at a particular location in the water column using some sort of suspension element. However, the suspension element restricts the free motion of the neutrally buoyant body which interferes with measurements. The neutrally buoyant body is also directly exposed to water currents which force the body to float away from its designated location, while the suspension element restricts this forced motion. All of these factors causes additional noise and interference with the sensor operation, especially at low frequencies. 
     SUMMARY OF THE INVENTION 
     The problems and disadvantages associated with the prior art are overcome by the present invention, which includes an low frequency acoustic vector sensor for measuring acoustic wave particle velocities in a liquid, such as water. The acoustic vector sensor includes a housing, a horn positioned within the housing for amplifying the acoustic wave particle velocities, and a buoyant object positioned within the horn. The buoyant object is moveable between a stationary position and a displaced position in response to the amplified acoustic wave particle velocities. A sensor senses the displacement of said buoyant object. The buoyant object, which can consist of any desired shape and size (e.g., spherical, cylindrical, etc.) may be mounted to horn either mechanically or magnetically. and is neutrally or near-neutrally buoyant. 
     Further features and advantages of the invention will appear more clearly on a reading of the detailed description of the exemplary embodiments of the invention, which are given below by way of example only with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is made to the following detailed description of exemplary embodiments considered in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of an ultra low frequency vector sensor having a neutrally buoyant sphere suspended in a horn, according to a first embodiment of the present invention; 
         FIG. 2 . is a graphical plot of acoustic wave particle velocity versus position along the longitudinal axis of the horn of the vector sensor shown in  FIG. 1 ; 
         FIG. 3 . is a perspective breakaway view of an ultra low frequency vector sensor with a neutrally buoyant cylinder magnetically suspended in a horn, according to a second embodiment of the present invention; and 
         FIG. 4 . is a perspective view of conventional eddy current sensing apparatus that may be utilized for measuring movements of the sphere shown in  FIG. 1  and the cylinder shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates an ultra low frequency acoustic vector sensor (herein “the acoustic sensor”)  10  constructed in accordance with an exemplary embodiment of the present invention. The acoustic sensor  10  is adapted to measure ultra low frequency liquid particle oscillations when positioned in a body of water S (e.g., a sea or lake). More particularly, the acoustic sensor  10  includes a spherically-shaped housing  12  which has a liquid-tight compartment or horn  14  positioned centrally therein. The horn  14  is shaped in the form of an axis-symmetrical body of revolution having its longitudinal axis coincident with a diameter of the housing  12 . A tube  16  having openings  18 ,  20  is centrally positioned in the horn  14 . Sound transparent membranes or screens  22 ,  24  are located at opposite ends of the horn  14  for allowing acoustic wave oscillations (not shown) to enter and leave the horn  14 , while preventing water S from entering the horn  14 . 
     A frustoconically shaped section  26  of the horn  14  is located between the screen  22  and the opening  18  of the tube  16 , and is tapered with its largest cross-sectional area A 1  located at the screen  22  and its smallest cross-sectional area A 2  located at the opening  18  of the tube  16 . An identically shaped section  28  of the horn  14  is located between the screen  24  and the opening  20  of the tube  16  and is tapered with its largest cross-sectional area A 1  located at the screen  24  and its smallest cross-sectional area A 2  located at the opening  20  of the tube  16 . The interior of the horn  14  is filled with water W or other liquid that has similar acoustic impedance as water. 
     A small neutrally buoyant object in the form of a sphere (herein “the sphere”)  30  is supported in the tube  16  of the horn  14  by a rod  32 . The rod  32  is rigidly attached to a pivot  34  that is positioned in a liquid-tight manner centrally in the circumference of the tube  16 . Another rod  36  is rigidly attached to the pivot  34  and extends outside the horn  14 . The axis of the rod  32  is coincident with the axis of the rod  36  (i.e., the rods  32 ,  36  are oriented in a straight-line). The pivot  34  permits the rods  32 ,  36  to freely rotate in the directions indicated by the arrow R. A pad  38  attached to the end of the rod  36  is positioned between conventional displacement sensors  40 ,  42 , to facilitate measurement of the oscillations of the pad  38 . The length of the rod  36  is denoted as L 1  and extends from the end of the pad  38  to the center of the pivot  34  and the length of the rod  32  is denoted as L 2  and extends from the circumference of the sphere  30  to the center of the pivot  34 . 
     The direction of the oscillations of the acoustic wave particles is substantially parallel with the longitudinal axis of the horn  14 . As the sphere  30  oscillates in the field of the acoustic wave, the pivoted arrangement of the rods  32 ,  34  mechanically amplifies the displacements of the sphere  30 . The amplification factor K P  is equal to the ratio of the lengths L 1  and L 2  of the rods  32 ,  36 , respectively. More particularly, K P =L 1 /L 2 . The amplified displacement of the pad  38  at the end of rod  36  is measured with displacement sensors  40 ,  42 . Alternately,  FIG. 4  depicts examples eddy current sensing elements (e.g., manufactured by Micro-Epsilon, Model Number EddyNCDT 3700) that may be used in place of the displacement sensors  40 ,  42 . Eddy current sensors and other motion sensing alternatives are discussed hereinafter. Signals generated by the displacement sensors  40 ,  42  are routed to sensor controllers  54 ,  56  and to a data acquisition system  58  for signaling processing. 
       FIG. 2  depicts a graph of acoustic wave particle velocity (i.e., digitally simulated) in the horn  14  (i.e., plotted on the Y-scale) versus positions along the longitudinal axis of the horn  14  (i.e., plotted on the X scale). The 0 position on the X scale denotes the center of tube  16  of the horn  14  in which vicinity the particle velocities are at the highest amplitudes. As the acoustic wave enters and progresses through the horn  14 , the wave particle velocity increases in the vicinity of the sphere  30  by a factor of K H =A 1 /A 2 . 
     Referring again to  FIG. 1 , the combination of the amplification of the particle velocities (i.e., amplified by sections  26 ,  28  of horn  14 ) and the mechanical amplification of the displacements of the sphere  30  (i.e., amplified by the pivotal arrangement of the rods  32 ,  36 ) yields a total measured amplification of the particle velocity of the wave as follows:
 
 v   measured   =K   P   *K   H   *v   wave ,
 
where v measured  is the particle velocity measured by the sensors  40 ,  42  and v wave  is the particle velocity in the incident acoustic wave.
 
     Velocity sensitivity (S V ) of the acoustic sensor  10  can be estimated using the following formula:
 
 S   V =2 πf*S   D /( K   H   *K   P ),
 
where f is the frequency and S D  is the sensitivity of the displacement sensor. For example, for amplifications K H =100 and K P =10; S D =0.4 nm, f=1 Hz, the sensitivity of the acoustic sensor  10  will be S V =0.0025 nm/s.
 
     Referring to  FIG. 3 , another exemplary embodiment of the present invention is illustrated. The elements illustrated in  FIG. 3  which correspond to the elements described above with reference to  FIG. 1  have been designated by corresponding reference numerals increased by one hundred, while new elements are designated by odd reference numerals in the one hundreds. The embodiment of the present invention shown in  FIG. 3  operates and is constructed in a manner consistent with the embodiment shown in  FIG. 1 , unless otherwise indicated.  FIG. 3  illustrates a horn  114  which has frustoconically shaped sections  126 ,  128  connected to a centrally positioned tube  116 . A neutrally buoyant object in the form of a cylinder (herein “the cylinder”)  159  is magnetically suspended in the tube  116  by a plurality of permanent magnets  161 . An aluminum disc  163  and a bismuth disc  165  are positioned at opposite ends of the cylinder  159  to facilitate the magnetic suspension of the cylinder  159  in the horn  114 , although other magnet-responsive metal elements may be utilized for this purpose. Movements of the cylinder  159  are measured by displacement sensors  140 , 142 , and signals emanating from the sensors  140 ,  142  are routed to sensor controllers  154 ,  156  and to a data acquisition system  158  for signaling processing. It is noted that this embodiment of the present invention differs from the embodiment described hereinabove, in that a) no mechanical amplification of the displacement of the cylinder  159  is provided, and b) the displacement sensors  140 ,  142  are positioned inside the horn  114 . 
     Unlike conventional vector sensors, the present invention provides a number of features leading to unsurpassed sensitivity and utility at the ultra low and the low frequency ranges spanning from a fraction of Hz to thousands of Hz. The placement of the sphere  30  inside the horn  14  facilitates measurements of the motion of a neutrally buoyant object, and this factor in conjunction with the combined total amplification enables a very high sensitivity of the acoustic sensor  10  at ultra low and low frequency ranges. The sphere  30  of the horn  14  is supported within the housing  12  such that it reacts to the forces exerted on it by the particle velocities of the acoustic wave oscillations without being disturbed by current flows or other disturbances that occur outside the horn  14  of the acoustic sensor  10  (i.e., no exposure to interfering currents and environmental degradation, such as bio-fouling, mineral deposits, etc). As stated above, the direct measurement of the displacements of the pad  38  is advantageous as compared with the inertia-based measurement approach utilized in conventional sensors. 
     It will be understood that the embodiment described herein is merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. For instance, the frustoconically shaped sections  26 ,  28  of the horn  14  may be in form of any appropriate shape that amplifies the acoustic wave particle velocity oscillations. Different suspension arrangements of the neutral or near-neutral buoyancy object could be implemented, for example, mechanical suspension using strings, etc. Multiple (e.g., differential) motion sensors could be used for displacement measurements of the sphere  30 . The housing  12  may be shaped in any form and fabricated from any material that is appropriate for immersion in sea water for extended periods of time. Likewise, since the sphere  30  of the horn  14  does not contain any internal elements, it may be formed in any size or shape that is appropriate (e.g., symmetrical) to minimize any distorting effect of the incident acoustic wave field impinging thereon. There are a number of displacement or velocity sensing techniques (including optical/laser based, electro-magnetic, and others) that could be utilized to measure the motion of the sphere  30 . For example, eddy current displacement sensors, such as those shown in  FIG. 4 , could be used. These sensors, used for precision displacement measurements in semiconductor manufacturing, optical telescopy and microscopy, and many other applications requiring precision displacement control, offer extremely high resolution of 0.4 nm in the frequency range from 0 to 1 kHz and approximately 1 nm in the range above 1 kHz to 10 kHz. All such variations and modifications, including those discussed above, are intended to be included within the scope of the invention as defined in the appended claims.