US 3832762 A
A seismic cable having minimum response to speed changes is provided. The cable includes a plurality of miniature seismic pressure sensitive detectors coupled one to another in such a manner so as to minimize through cancellation undesired electrical disturbance resulting from speed changes. Each detector includes transducers of the ceramic piezoelectric crystal type having matched geometric and physical property parameters. Each ceramic piezoelectric crystal is provided with metal electrodes such as, for example, gold, nickel, platinum, or rhodium which, in addition to being corrosion resistant and inactive as to the constituents in the ceramic material, are insoluble in liquids encountered by the detector in its use environment to avoid dendrite type growth of the metal through minute cracks developing in the ceramic which causes short circuits in the piezoelectric element.
Beschreibung (OCR-Text kann Fehler enthalten)
United States Patent 1 Johnston et a1.
[4 1 Sept. 3, 1974 METHOD OF PRODUCING A MATCHED PARAMETER ACCELERATION CANCELLING HYDROPHONE  Inventors: Roy C. Johnston, Richardson;
Lawrence B. Sullivan, Plano, both of Tex.
 Assignee: Texas Instruments Incorporated,
 Filed: Dec. 26, 1973 ['21 Appl. No.: 427,881
Related US. Application Data  Continuation-in-part of,Ser. No. 255,503, May 22,
 US. Cl 29/2535, 310/87, 310/91,
340/8 R, 340/10  Int. Cl B0lj 17/00  Field of Search 29/2535; 340/10, 8 R,
340/8 PC, 8 FT; 310/83, 8.7, 9.1
3,555,503 1/1971 Morris 340/10 X Primary ExaminerCharles W. Lanham Assistant ExaminerCarl E. Hall Attorney, Agent, or Firm-l-larold Levine; Rene E. Grossman; Alva H. Bandy 5 7] ABSTRACT A seismic cable having minimum response to speed changes is provided. The cable includes a plurality of miniature seismic pressure sensitive detectors coupled one to another in such a manner so as to minimize through cancellation undesired electrical disturbance resulting from speed changes. Each detector includes transducers of the ceramic piezoelectric crystal type having matched geometric and physical property parameters. Each ceramic piezoelectric crystal is provided with metal electrodes such as, for example, gold, nickel, platinum, or rhodium which, in addition to being corrosion resistant and inactive as to the constituents in the ceramic material, areinsoluble in liquids encountered by the detector in its use environment to avoid dendrite type growth of the metal through minute cracks developing in the ceramic which causes short circuits in the piezoelectric element.
5 Claims, 15 Drawing Figures  References Cited UNITED STATES PATENTS 2,448,365 8/1948 Gillespie 340/10 3,187,300 6/1965 Brate 340/10 3,458,857 7/1969 Hancks et a1. 340/10 PATENTED 3EP3 74 SHEET 1 BF 4 PATENTED 393 7 snmeord ACCELERATION FORCE PRESSURE FORCE METHOD OF PRODUCING A MATCI-IEI) PARAMETER ACCELERATION CANCELLING HYDROPHONE This is a continuation-in-part of application Ser. No. 255,503 filed May 22, 1972 now abandoned.
BACKGROUND OF THE INVENTION I. Field of the Invention This invention relates to an improved hydrophone and more particularly to an improved pressure sensitive detector element therefor.
2. Description of the Prior Art Seismic cables utilized in water work are commonly referred to as streamers. In a streamer the cable and detectors are built into a single piece of equipment, sections of which are joined before they are placed in the water. The streamer is handled by a large powerful winch mounted on the stern of a survey ship. Each section includes a tube and up to 30 detectors or more. The tube, which is generally constructed of a heavy duty plastic, is made neutrally buoyant by being filled with a liquid, e.g., kerosene or a plastic foam. Kerosene also provides very effective acoustic coupling from marine pressure changes to the detectors. The tension load is carried by steel wire rope strain members running the length of the streamer. Conductors are provided for feeding seismic information generated by the detectors to data recording equipment and depth information from depth transducers to a depth meter console. The signal-to-noise ratio is the ratio of the amplitude of a desired seismic signal at any time on the seismic trace to the amplitude of undesired electric disturbance (noise) signals at the same time on the trace.
The problem of enhancing the signal-to-noise ratio has long confronted the art; efforts to enhance their ratio have been made. These efforts resulted in a method of supporting the detectors within the plastic tube to make them susceptible to random transverse accelerations and decelerations induced by towing, and to include, in detectors using piezoelectric transducers, an impedance matching step-down transformer for each detector group. This latter improvement is necessary because the output impedance of a group of such detectors is usually unacceptably high as an input to ordinary seismic amplifiers and is undesirable because of noise pickup problems.
Pressure-sensitive detectors utilize piezoelectric elements which generate an electromagnetic force (emf) when stressed by an external force applied to their surface. In the past, pressure-sensitive detectors or hydrophones employed as their piezoelectric elements anistropic crystals such as quartz or rochelle salt. More recently, however, these anisotropic crystals have been replaced by polycrystalline and isoptropic ceramic materials. An anisotropic substance exhibits different properties in different directions; while isotropic substances exhibit the same properties (e.g. electrical or optical) in all directions.
The piezoelectric elements or transducers used in hydrophones are desired to operate over a relatively large frequency range which extends up to several octaves below the devices resonant frequency. Thus they are often referred to as nonresonant transducers. The transducers comprise a ceramic dielectric having metal electrodes formed on each major surface. The electrical equivalent circuit for a nonresonant transducer can be approximated by an ideal voltage generator in series with a capacitor. The capacitor represents the electrical capacitance between the electrodes, and its value depends on the physical dimensions of the piezoelectric element and the dielectric constant of the particular type of ceramic used. The capacitance of the typical piezoelectric element used today ranges from 10 to I00 nanofarads. It is sufficient for piezoelectric elements used in hydrophones or seismic detectors to have a capacitance ranging from 9 to 11 nanofarads (nf) since using 30 such detectors in parallel results in a minus 3 dB point of approximately 6 Hz for the section. To obtain this value the thickness of the ceramic or dielectric material of the piezoelectric element must be substantially reduced. Efforts to reduce the thickness of the dielectric of prior art ceramic type piezoelectric elements of transducers which utilize as electrodes metals capable of supporting dendrite type growth such as, for ex- I ample, silver failed in use because electrical short circuits developed between the electrode plates. It has been determined that these electrical shorts were the result of silver of the silver electrodes migrating through cracks in the ceramic dielectric through what is believed to be a dendrite growth type action.
SUMMARY OF THE INVENTION It is an object of the invention to provide an improved seismic cable or streamer.
It is another object of the invention to provide an improved piezoelectric element for improving the lifetim of a pressure type seismic detector.
It is a further object of the invention to provide a detector and a seismic cable or streamer having respectively first and second orders of acceleration and deceleration noise cancellation.
Still another object of the invention is to provide a detector having high sensitivity.
Yet another object of the invention is to provide an improved method of manufacture of a detector having a high sensitivity.
The above objects and other objects of the invention are accomplished by fabricating piezoelectric crystals or elements for detector elements, selecting two detector elements having matched geometric and physicalproperty parameters for each pressure-sensitive detector to reduce the detectors sensitivity to acceleration and deceleration noise (hereinafter referred to as acceleration noise), and by constructing a seismic cable or streamer having its pressure-sensitive detectors connected one to the other with the polarity of one reversed as to the polarity of the other for the same acceleration noise. Thus the acceleration noise of one detector, being out of phase relative to the other, subtracts or cancels the acceleration noise of the other detector to enhance the signal-to-noise ratio of the seismic signals generated by the cable.
The seismic cable may be constructed in different ways; however, the preferred construction will be described hereinafter in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS:
FIG. 1 is a side view of a seismic streamer being towed through water;
FIG. 2 is a perspective view of a miniature detector constituting an embodiment of this invention;
FIG. 2A is an isometric view of a miniature detector constituting another embodiment of this invention;
FIG. 3 is a perspective view of the miniature detector subassembly;
FIG. 4 is a sectional view of the miniature detector taken along lines 4-4 of FIG. 2;
FIG. 5 is a schematic view of the miniature detector and showing in dotted lines its response (greatly exaggerated) to pressure forces;
FIG. 6 is a view similar to FIG. 5 showing the miniature detectors response (greatly exaggerated) to acceleration forces;
FIG. 7 is a perspective view of the piezoelectric element;
FIG. 8 is a diagram of the equivalent electromechanical circuit of the detector of FIG. 4;
FIG. 9 is a schematic diagram showing a pair of hydrophones of FIG. 4 arranged in a seismic streamer with the polarities as shown;
FIGS. 9A and B are waveforms of the voltage outputs of the hydrophones as arranged in FIG. 9 due to the acceleration force;
FIG. 10 is a schematic diagram showing the same pair of hydrophones of FIG. 9 with one of the hydrophones physically oriented 180 from its position of FIG. 9;
FIGS. 10A and B are waveforms of the voltage outputs of the hydrophones as arranged in FIG. 10 due to the same acceleration force (FIG. 9).
Referring to the drawings, a seismic streamer 10, having a plurality of sections 12, is towed through the water at a constant depth by a ship 14 (FIG. 1). The ship 14 has a winch 16 for laying and retrieving the seismic streamer 10. The streamer 10 is made up of one or more tubular sections joined together by connectors, not shown, called integrated couplers. These couplers provide electrical continuity for all the conductor pairs from the detectors, hereinafter described, and the depth transducers, and water-break transducers (not a part of the present invention). In addition the couplers provide mechanical coupling for the stress members running from one end to the other of each section. Each section contains the required number of conductor pairs necessary to carry all the hydrophone-group circuits into the instruments. Normally up to 70 pairs of conductors or more are provided, 54 of which are for seismic data, 4 are for spares, 6 are for the depth transducers and 6 are for the water-break transducers.
Each section 12 contains a plurality of hydrophones arranged in the streamer 10 in accordance with the second order acceleration noise cancellation embodiment of the invention. Each hydrophone includes detector elements 58 and 58' (FIGS. 5 and 6) constructed and combined to form a hydrophone subassembly 22 having a first order acceleration noise cancellation mode hereinafter described.
It will be understood that ideally two detector elements 58 and 58' (FIG. 4) are used in each hydrophone and they are to have the same sensitivity to acceleration so that the acceleration noise will be cancelled completely and the response of each detector element to pressure forces will be substantially equal. This desired result can be closely approximated by matching geometric and physical property parameters and electrical properties of elements of the hydrophone prior to and during the construction. Parameters which can be changed are determined by equating electrical units to mechanical units in accordance with the mathematical relationship derived from an equivalent electrome- Ill chanical circuit. In deriving the mathematical formula the electrical units and analogous mechanical units set forth in TABLE I are utilized.
TABLE I Electrical Unit Analogous Mechanical Unit Voltage Force Current Velocity Charge Displacement Capacitan ce Compliance Inductance Mass Impedance Mechanical Impedance Each ceramic piezoelectric element is subjected during use to both acoustic wave (pressure) forces and acceleration noise forces; and equivalent electrical circuit for these conditions is shown in FIG. 8. The pressure forces are presented in the mechanical side by F (t), and the acceleration noise forces are represented by velocity perturbations V,,(t). The mass of each piezoelectric element is M, the compliance of its mounting is C,,,. The electromechanical transformer has a turns ratio of N, and the capacitance C, and output voltage E, are shown on the electrical side. The impedances of the detector elements 58 and 58' are represented respectively by Z and Z The equivalent circuit is divided into a mechanical side and an electrical side by the electromechanical transformers 70 and 72 for the first detector element 58 and the electromechanical transformers 74 and 76 for the second detector element 58. The electromechanical transformers represent the electrical response to the force applied to the piezoelectric elements 44, 46, 48 and 50, (FIG. 4) respectively. The transformers 70, 72, 74 and 76 (FIG. 8) have turn ratios respectively N 1, N 1, N 1, and N 1. The mechanical side of the equivalent circuit for the first detector element 58 includes a force generator 78 and a velocity generator 80 which are equivalent respectively to the pressure forces (F )t and the velocity disturbance perturbations V (t). The positive terminals of these generators 78 and 80 are coupled to an inductor 82 which represents the mass (M of the piezoelectric electric element 44. The inductor 82 is coupled to one end of a parallel circuit comprising a capacitor 84 which represents the compliance of the piezoelectric element 44, coupled across the primary of transformer 70. The other end of the parallel circuit is coupled through a capacitor 86 to 5 an inductor 88 representing respectively the compliance (C and mass (M,,) of an electrically conductive diaphragm 40 (FIG. 4). Inductor 88 is coupled to an inductor 90 (FIG. 8) which represents the mass (M of piezoelectric element 46. The inductor 90 is in turn coupled to one end of a parallel circuit comprising a capacitor 92 which represents the compliance of the piezoelectric element 46 coupled across the primary winding of transformer 72. The other end of the parallel circuit is attached to the other end of the generators 78 and to complete the mechanical side of the first detector element. The electrical side of the first detector element includes the following elements coupled end to end or in series: the positive terminal, capacitor 94 which represents the electrical capacitance (C of piezoelectric element 44, the secondary of electromechanical transformer 70, capacitor 96 which represents the electrical capacitance (C of the piezoelectric element 46, the secondary of electromechanical transformer 72, and negative terminal 28. The output of the first detector element is designated E The mechanical and electrical sides of the equivalent circuit of the second detector element 58 are substantially identical to those of the first detector element 58. The principal differences are that piezoelectric elements 48 and 50 and diaphragm 42 replace the piezoelectric elements 44 and 46 and diaphragm 40. Thus the mechanical side includes a force generator 98 and velocity perturbation generator 100 which represent respectively the pressure forces F(t) and acceleration forces V,,(t) exerted on the second detector element 58. Positive terminals of the generators 98 and 104 are connected through an inductor 102 which represents the mass (M of the piezoelectric element 50 to one end of a parallel circuit having a capacitor 164 which represents the compliance of piezoelectric element 50 coupled across the primary of the electromechanical transformer 74. The other end of the parallel circuit is attached to an inductor 106 which in turn is connected to a capacitor 108 which represent respectively the mass (M and compliance (C of the diaphragm 42. The capacitor 108 is coupled through an inductor H0 representing the mass (M of piezoelectric element 48 to one end of a parallel circuit comprising a capacitor 112 which represents the compliance of piezoelectric element 48 coupled across the primary of electromechanical transformer 76. The other end of the parallel circuit is connected to the other end of the force and velocity gener- B. The voltage output owing to an acceleration disturbance on the second detector comprising piezoelectric elements 48 and 50 is C. The combined open-circuit, output of the detector is given by ators 98 and 100 to complete the mechanical side of Wherei n and 12 are respectively the electromechanthe second detector element. The electrical side of the second detector element comprises the following elements connected in series. From the positive terminal 26 the secondary of electromechanical transformer 76, capacitor 114, which represents the electrical capacitance (C of piezoelectric element 50, secondary of electromechanical transformer 74, capacitor 116, which represents the electrical capacitance (C of piezoelectric element 48, and the negative terminal 28. The electrical output of the second detector element is designated E The following mathematical formulas are derived from the equivalent circuit.
A. The voltage output owing to the same acceleration force on the first detector comprising piezoelectric elements 44 and 46 is f w g omar ill A nmaa:
(91112 ml m2)/( l M2 M11): al
angular frequency for detector 58 mounting ical impedance of the first and second detector elements.
M M M M Referring to FIG. 2 for a description of the first embodiment of the invention, the detector or hydrophone construction comprises a housing 20 for a hydrophone subassembly 22 (FIGS. 2 & 3). The housing 20 may be constructed of any suitable shock resistant material such as, for example, stainless steel and has its major exterior surface coated or otherwise covered with a shock absorbent and electrically resistant sleeve 21 or coating made of rubber or a suitable plastic. One end of the housing 20 supports a pair of output terminals 26 and 28 fixed thereto in insulators 29. The inner surface of the housing 241) is provided with a hydrophone subassembly mounting means 30 (FIG. 4) which may be either a groove or a series of spaced holes.
The hydrophone subassembly 22 (FIG. 3) includes a retaining means 32 (FIG. 4) adapted to mate when cov- 5 ered by an encapsulating plastic with the hydrophone subassembly mounting means 30 to position the hydrophone subassembly within the housing 20. The retaining means 32 may be formed as an integral part of a de tector element frame 34 (FIG. 4). The frame 34 has opposing ends 36 and 38 having recesses which open inwardly to receive respectively electrically conductive diaphragms 40 and 42 of detector elements 58 and 58' and to retain them in spaced axial alignment. The frame 34 may be formed by molding any suitable plastic such as, for example, a polycarbonate plastic which after setting has the rigidity to hold the diaphragms with a reproducible compliance to pressure forces. The electrically conductive diaphragms 40 and 42 may be constructed of any suitable material such as, for example, brass, beryllium, copper, phosphor bronze or other copper alloy metal. The diaphragms are of a thickness of about 0.016 inches, the thickness criterion being that the diaphragms have sufficient strength to withstand expected hydrostatic pressures yet deform sufficiently in an acoustic pressure field to generate adequate electrical signals. The diaphragms 40 and 42 have major opposing surfaces adapted to receive respectively piezoelectric elements 44 and 46, and 48 and 50 to form the detector elements 58 and 58.
The piezoelectric elements 44-50 are identical in construction and therefore only one need be described in detail. Each piezoelectric element (FIG. 7) comprises a dielectric member 52 constructed of suitable ceramic materials such as, for example, barium titanate, lead zirconate, and lead titanate or a mixture of lead zirconate and lead titanate. Although the dielectric member 52 may be formed in any desired shape, as shown in FIG. 7, it is extruded in the form of a cylinder and then sliced into flat circular disks having a substantially uniform thickness of less than about 0.015 inches. Next, dendrite growth inhibitor metal electrodes 54 are formed on opposing sides of the ceramic disk 52. The metal must be corrosion resistant, inactive as to the constituents of the ceramic material, and insoluble or very stable as to liquids encountered by the hydrophone in its use environment to inhibit dendrite type growth of the metal through minute cracks developing in the ceramic and subsequent short circuiting of the piezoelectric element. A suitable metal is, for example, gold, nickel, platinum, or rhodium with gold or nickel being preferred. Gold electrodes are formed by depositing a thin layer of gold or gold frit onto the major surfaces of the ceramic disk by a thick film process to be described. If the piezoelectric elements are to be soldered to a supporting diaphragm 58, a border 56 (FIG. 7) is left between the peripheries of the gold electrodes and ceramic disk to prevent short circuiting by arcing or otherwise around the edge of the ceramic disk. However, if the piezoelectric elements are glued to the diaphragm 58 the border is eliminated. The gluing technique, hereinafter described, is preferred because the border 56 is not required, thus for a given size ceramic disk the electrode size is increased.
The above mentioned thick film process is well known to those skilled in the art; therefore, only a brief description need be given. The process consists of spreading a gold paste to a thickness of 0.0007 inches over a 200-325 mesh nylon screen and drying at room temperature for to 15 minutes, and then firing, i.e., heating to temperatures below 750C through a conveyor type furnace with a 45 minute cycle to a peak temperature of 750C for 5-10 minutes to obtain good electrode adherence. Temperatures above 750C are to be avoided to prevent damage to a lead compound ceramic disk through evaporation of the lead. A suitable gold paste is DuPonts Gold Conductor Composition 8115.
If nickel electrodes are desired they may be formed on the ceramic disks by vapor deposition techniques well known to those skilled in the art.
After forming the electrodes, the ceramic disk is polarized by a very powerful d.c. electric field, e.g., 40,000 to 5 X 10 V/m. A dot or other marking is used to designate the side the positive electrode was attached for polarizing the ceramic disk. The polarized piezoelectric elements are then labeled, weighed and placed in a capacitance measuring instrument and the weight and capacitance of each piezoelectric element recorded. The piezoelectric elements are then placed in bins according to their weight and capacitance.
Piezoelectric elements 44 and 46 are selected from the bins with substantially matching weights and capacitances for attachment to a weighed diaphragm 40 to form detector element 58. These measurements are fed into an automatic machine programmed to solve the above mentioned mathematical formulas to determine the mass and capacitance for piezoelectric elements 48 and 50, and diaphragm 42, for the second detector element 58' required to match it to the first detector element for a noise cancellation hydrophone. Piezoelectric elements 48 and 50 and diaphragm 42 having the required parameters or combination thereof to substantially match the response of the first detector are then selected from the bins. Piezoelectric elements 44 and 46, and piezoelectric elements 48 and 50, respectively, are then attached selectively, either by soldering with an indium alloy or gluing with glue, GAl l 1, sold by Gulton Industries, to the diaphragms 40 and 42 with the positive marked electrodes facing outwardly as indicated by the arrows in FIGS. 4, 5, and 6 to complete the detector elements 58 and 58 for the hydrophone subassembly 22. Gluing is preferred over soldering in that; the glue may be spread evenly at a desired thickness over a plate, portions of the glue, identical to the shape (circular) of the electrode, cut by a cutter and applied to the negative electrode to attach the piezoelectric element to the diaphragm. Thereafter the detector elements are stacked in a pressure exerting type holder and the glue cured for about two hours at room temperature. By gluing the mass of the detector elements can be controlled and the possibility of shorting between the piezoelectric elements and the diaphragm is alleviated. The detector elements are then weighed and their capacitances measured. The weight and capacitance measured is recorded for each detector element. If these parameters match substantially a hydrophone is fabricated as follows:
Electrical leads 60 and 62 are attached as shown in FIG. 5 to the electrodes that are to form the innermost plates of the hydrophone, and the detector elements 58 and 58 mounted in the recesses (FIG. 4) formed in the ends 36 and 38 of the detector element support member or frame 34 with the electrical leads 60 and 62 brought out through the detector element support member 34 (FIG. 3). Electrical leads 64 and 66 are; then attached to the outermost electrodes as shown ini FIG. 5 to complete the hydrophone subassembly 22.
The hydrophone subassembly 22 (FIGS. 2, 3, and 4) is then mounted in the housing 20 by positioning the retaining means 32 of the subassembly in line with the mounting means 30 of the housing in order that they will mate when the subassembly 22 is encapsulated with a suitable plastic such as, for example, polyurethane. The subassembly is then encapsulated with thicknesses of encapsulating material covering the major surfaces equal or selectively varied one to another to adjust the effective mass of the piezoelectric elements. After encapsulation the electrical leads 60 and 66 are attached to output terminals 26 and 28 (FIG. 2).
In another embodiment of the invention (FIG. 2A) the housing is eliminated. F IG. 2A depicts this embodiment with identical parts bearing the same numbers with a prime. Terminals 26 and 28 are terminals 26 and 28 modified as shown and attached to leads 60 and 66'. The hydrophone subassembly 22 is positioned in an encapsulating mold and selectively encapsulated; that is, the thickness of the encapsulating material on the major surfaces of the detector elements is controlled to change the piezoelectric elements response to acceleration noise for cancellation. The encapsulation step completes the fabrication of the hydrophone.
With the detector elements 58 and 58' electrically connected as above described for either embodiment of the invention, a series-parallel relationship is established through which acceleration noise (FIG. 6) may be detected and cancelled one from the other while the pressure wave (FIG. 5) may be detected by both with- 2 5 out cancellation. The above-described acceleration noise cancellation is referred to as first order cancellation.
The hydrophone is then given a shake test to determine its response to acceleration noise and an acoustic lating material from the faces of the hydrophone by sanding.
Although hydrophones constructed in accordance with the first order noise cancellation embodiments of the invention under ideal conditions will reduce acceleration noise output to zero, it will be apparent that in actual use it will be difficult to achieve zero noise cancellation. Thus further acceleration noise cancellation is desirable and is provided by what is referred to as second order acceleration cancelling.
Second order acceleration noise cancellation is acquired by subjecting each hydrophone prior to its incorporation into the streamer or streamer section, to tests to determine capacitance, acceleration sensitivity, acoustic sensitivity, acoustic polarity, terminal to terminal resistance and terminal to mount resistance. The
tests include subjecting the hydrophone to a known acceleration force to determine its electrical response, i.e., its amplitude and phase response as to the acceleration force. Thereafter, using these parameters, the hydrophones are matched one with another or others to provide second order noise cancellation. Here again the final thickness of the encapsulating material of the hydrophones to be matched may be varied by sanding to match the hydrophones one to another. It will be appreciated that the second embodiment of the invention 1 (FIG. 2A) lends itself more readily to adjustment of the encapsulating material. FIG. 9 shows two hydrophones 118 and coupled in parallel with their polarities such that their outputs (FIGS. 9A and B) are in phase and thus no cancellation occurs. FIG. 10 shows the same two hydrophones 1118 and 120 with the polarity of hydrophone 120 changed by turning it about in the same streamer. Although the parallel connection is the same, the response to acceleration noise as shown in FIGS. 10A and B are out of phase and subtract or cancel. In this arrangement the detectors response to seismic waves is not affected. The above described arrangement is only one of many that can be applied to any number of detectors whose acceleration noise response may vary over a range of values with improved results. Nevertheless, the ideal situation exists when the sum of the negative and positive voltage producing detectors approach zero and the wavelength of the acceleration noise is large compared with the length of the section. When the noise wavelength is equal to or less than the streamer section length the spacing of the hydrophones becomes a factor in positioning the hydrophones. Further, it will be understood by those skilled in the art that the hydrophones can be rotated 90 and transverse acceleration noise cancelled in a manner similar to the cancelling of the longitudinal noise described above. Thus, various combinations of transverse and longitudinal orientations and series and parallel wiring techniques or combinations thereof can be employed, as is well known in the art, to provide any desired noise cancellation depending on knowledge of the magnitude and direction of the acceleration noise.
TABLE II lists the acceleration sensitivity measured in milliamps per gram (ma/g) for 30 hydrophones used in test cables. Case I represents an arrangement of the hydrophones without regard to acceleration polarities or the worst case; Case II represents the acceleration polarity matching of hydrophones in accordance with an embodiment of the invention. Other methods of evaluating the hydrophones include, for example, the product of the capacitance times the acceleration sensitivity.
TABLE II HYDRO- CASE I CASE II PHONE Acceleration Sensitivity Acceleration Sensitivity (Unmatched) (Matched) l 3.1 3.1 2 3.5 -3.5 3 10.5 l0.5 4 12.0 12.0 5 9.0 9.0 6 8.5 8.5 7 8.5 8,5 8 I 1.0 -1 1.0 9 8.5 *8.5 10 12.0 12.0 I l 9.0 9.0 12 9.0 9.0 Iii 7.5 7.5
TABLE II Continued HYDRO- CASE 1 CASE ll PHONE Acceleration Sensitivity Acceleration Sensitivity (Unmatched) (Matched) 14 12.5 12.5 15 10.0 10.0 16 12.5 l2.5 17 27.5 27.5 18 30.0 30.0 19 25.0 25.0 20 20.0 20.0 21 13.5 13.5 22 15.5 15.5 23 21.0 21.0 24 31.0 3l.0 25 12.5 12.5 26 14.0 14.0 27 l 1.0 1 1.0 28 12.5 12.5 29 15.0 15.0 30 21.0 21.0
NOTE: indicates acceleration polarity is reversed.
In the streamers constructed utilizing the hydrophones of TABLE ll, the first hydrophone was positioned 13.66 feet from one end coupling of the streamer, the next 14 hydrophones were equally spaced throughout the next 54.67 feet of the streamer, nol ydrophones were positioned within the center 27.33 feet of the streamer where the 16th hydrophone was positioned, the remaining l4 hydrophones were equally spaced within the next 54.67 feet of the streamer; and the other end coupling was spaced 13.66 feet from the last hydrophone. The results of the trial runs are shown in the following chart in which the relative response of the streamers to acceleration forces are plotted on the ordinate axis and the wave number X in cycles per foot and the corresponding wavelength A in feet per cycle. The chart clearly shows the improved performance where the WORST CASE POLARITY MATCHED Relative Response in Decibels A,
Frequency in Cycles Per Second -60 l l l l 1 1 I l 1 Wave Number K in Cycles Per Foot llllnlinlilnulilimlilllil l l 10 5 3 2 1.5 1.0.9 ,B .7 .6 .5
Wave Length A in 100 Feet Per Cycle ing substantially equal response to pressure waves the improvement comprising:
a. determining the geometric and physical property parameters and electrical parameters of a first detector element and its response to a selected acceleration noise wave and a pressure wave;
b. selecting a second detector element having geometric and physical property parameters and electrical parameters which when combined electrically with the first detector element to form a hydrophone having substantially matching responses to acceleration noise waves and pressure waves;
c. mounting said first and second detector elements in a frame such that they are coupled in an acceleration noise cancelling mode; and
d. selectively varying geometric and physical property parameters of at least one of the detector elements after the mounting step and during further construction of the hydrophone to vary the response of the hydrophone to acceleration noise and pressure waves to produce an hydrophone having substantially acceleration noise cancelling response and duplicate pressure wave response.
2. A method for producing a pressure sensitive hydrophone comprising:
a. determining the geometric and physical property parameters and electrical parameters for a plurality of piezoelectric elements;
b. selecting a first pair of piezoelectric elements having substantially matching geometric and physical property parameters and electrical parameters;
0. selectively mounting the pair of piezoelectric elements on opposing sides of a first support member of known weight and compliance to form a first detector element;
d. determining the geometric and physical property parameters and electrical parameters of the first detector element;
e. selecting a second pair of piezoelectric elements having geometric and physical property parameters and electrical parameters substantially matching those of said first pair;
f. selectively mounting the second pair of piezoelectric elements on opposing sides of a second support member having properties substantially matching those of the first support member, to form a second detector element having geometric and physical property parameters and electrical parameters substantially matching those of the first detector element;
g. determining the geometric and physical property parameters and electrical parameters of the second detector element;
h. selectively mounting the first and second detector elements in opposing ends of a hydrophone frame;
i. selectively electrically coupling the detector elements in an acceleration noise cancelling mode; and
j. selectively varying geometric and physical property parameters of at least one of the detector elements by selectively encapsulating the frame and detector elements thereby producing a hydrophone having substantially acceleration noise cancelling response and duplicate pressure wave response.
3. A method according to claim 2, wherein the step of selectively encapsulating the frame and detector elements includes selective removal of encapsulating material covering the detector elements of the hydrophone.
4. A method for producing a pressure sensitive hydrophone according to claim 2 wherein the second detector element is off set to the first detector element through geometric and physical property parameters and electrical parameters including for the electrical side the voltage output (E of each detector element, the capacitance (C of each piezoelectric element; and for the mechanical side the pressure force (F) owing to acoustic forces and a velocity force (V) owing to acceleration and deceleration forces, compliance (C of each detector element mounting, mass (M of each piezoelectric element and diaphragm (M turns ratio (N representing electromechanical transformer action of each piezoelectric element and the mechanical impedance (Z) of each detector element where x equals 1, 2, 3 or 4 indicating the particular piezoelectric elements and p equals 1 or 2 indicating the particular diaphragm.
5. A method for producing a pressure sensitive hydrophone according to claim 4 wherein the first and second detector elements are off set by feeding the known geometric and physical property parameters and electrical parameters of a first piezoelectric element of the first detector element into an automatic machine programmed to determine matching parameters for the remaining piezoelectric element and the second detector element in accordance with the following relationships:
2 omar 111 1 l CIIIICIIIL'. ,3 m! um! where,
"111 m1 m2)/( 1+ 2 n) for the second detector element "112 m3 m4)/( 3 4 n) and for the combined open-circuit output of the hydrophone oc iz 01 n o2)/( t2 n) where