AU747855B2 - Method of making a hydrophone - Google Patents

Description

S F Ref: 413945D1

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name and Address of Applicant: Actual Inventor(s): Address for Service: Invention Title: Input/Output Inc.

12300 Parc Crest Drive Stafford Texas 77477-2416 UNITED STATES OF AMERICA Win H. Chang, Algernon S. Badger and Bernard Simensky Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia Method of Making a Hydrophone The following statement is a full description of this invention, including the best method of performing it known to me/us:- 5845

I

Method of Making a Hydrophone Field of the Invention This invention relates to a hydrophone for use in a seismic streamer cable at depths down to 300 metres and, more particularly, the invention relates to a method of making a hydrophone.

Background of the Invention US Patent Nos. 3,187,300 to Brate and 3,832,762 to Johnson et al disclose hydrophones having component-to-component seals of epoxy.

US Patent No. 3,970,878 discloses an acoustic transducer which has no plastic or epoxy parts 1o exposed to the outside of the transducer unit, to avoid electrical leakage caused by the epoxy or plastic absorbing salt from exposure to salt water. However, the disclosed transducer appears, from the patent, and from the technical specifications of the commercial embodiment, to be limited to depths of less than one hundred and fifty feet.

US Patent No. 4,999,819 discloses an acoustic transducer for use in "deep submergence 5 applications under high hydrostatic pressures." Col. 2, lines 31-32. However, because it is designed for high hydrostatic pressures, it is not as sensitive as desired for shallower water, where streamer cables are usually used. What is needed is a transducer with greater sensitivity, for more accurate readings.

The construction of this transducer also requires, after assembly, curing for ten minutes at 600 S 20 degrees Centigrade. As a result of such heat, if the piezoelectric cell had been poled before assembly, it then has to be repoled. Piezoelectric cells may be purchased already poled, so it is a .i waste of time and money to repole them. What is needed is a transducer that does not require a curing which destroys the original poling.

Furthermore, after the repoling, the transducer then has to be stored for at least ten days, to let the piezoelectric cell age, before calibrating the transducer. Because the ageing is not linear, most of the ageing occurs within the first ten days. The required storage time increases manufacturing time, and increases storage costs. What is needed is a transducer that does not require an ageing period after assembly.

Summary of the Invention According to one aspect of the present invention there is provided a method of making a hydrophone, comprising the steps of: a. assembling a previously polarised piezoelectric crystal with a pair of solid circular metal plates positioned to sandwich the piezoelectric ceramic crystal between them, wherein an epoxy adhesive is interposed between the metal plates and the piezoelectric ceramic crystal to form an acoustic transducer assembly; [R:\LIBFFI02105.doc:NJC 2 b. curing the acoustic transducer assembly at temperatures less than 150 degrees Centigrade; c. encapsulating the acoustic transducer assembly in a flexible case with a polyurethane potting sealant to form a potted assembly; and d. curing the potted assembly at temperatures less than 150 degrees Centigrade to form a hydrophone.

The epoxy adhesive is preferably a conductive epoxy adhesive.

Brief Description of the Drawings Other aspects of the present invention will become more apparent from a description of the preferred embodiment when read in conjunction with the accompanying drawings. In the drawings the same members have the same reference numerals.

FIG. 1 is a general overall view of an illustrative seismic streamer cable towed behind a boat, the cable containing many hydrophones.

15 FIG. 2 is a perspective view of a hydrophone, containing an acoustic transducer.

o c* *o oco o*° *oco o*o [R:\LIBFF]02105.doc:NJC -3- FIG. 3a is a perspective view of the two metal plates of an acoustic transducer; and FIG. 3b is an exploded view of one metal plate and a piezoelectric ceramic crystal.

FIG. 4 is a plan view of a solid circular metal plate, which comprises part of an acoustic transducer.

FIG. 5 is a side view of the metal plate of FIG. 4, taken along the line 5-5 of FIG. 4.

FIG. 6 is an enlarged view of a portion of FIG. DETAILED DESCRIPTION OF THE PREFERRED

EMBODIMENT

FIG. 1 depicts a seismic streamer cable 10 towed behind a boat 12. The cable contains hydrophones 14.

FIG. 2 is a perspective view of a hydrophone 14, containing an acoustic transducer 20. The transducer 20 has a leader wire 22 attached to a metal plate 24, and a leader wire 26 attached to a metal plate 28. The transducer 20 is surrounded by polyurethane 30, filling a boot 32. The polyurethane 30 must be, as much as i possible, acoustically transparent. This is done by selecting a polyurethane which has, as much as possible, the same acoustic characteristics as sea water, or as the Isopar H, manufactured by Exxon, in which the hydrophone is immersed inside the i marine seismic streamer cable 10. Similarly, the boot 32 must be, as much as possible, acoustically transparent. The boot 32 is made of a thin, soft, flexible vinyl.

The polyurethane 30 is model no. HMP-85, manufactured by Fluid Polymers, in Las Vegas, NV, which is a division of Chase Corp. in Pittsburgh, PA. The boot 32 is a boot manufactured by Mocap Inc. in St. Louis, MO.

FIG. 3a is a perspective view of the two metal plates 24, 28 of the acoustic transducer 20. The two plates are identical. Each has a diameter less than one inch, and is slightly concave. FIG. 3b is an exploded view of the metal plate 24 and a piezoelectric ceramic crystal 36.

FIG. 4 is a plan view of a concave side 38 of the solid circular metal plate 24.

The plate 24 includes an outer rim 40 and a recessed inner rim 42. The diameter of the piezoelectric ceramic crystal 36 is less than the diameter of the plate 24, but is greater than the diameter of the recessed inner rim 42. The two plates 24, 28 are put together with their concave sides 38 facing each other, the piezoelectric ceramic crystal 36 between them, and an epoxy adhesive 54 is interposed between the metal -4plates 24, 28 and the piezoelectric ceramic crystal 36. The outer rim 40 of each plate is bonded to the piezoelectric ceramic crystal 36 by the epoxy adhesive 54, the pair of plates thus forming a cavity between them. The epoxy adhesive 54, with a conductive filler, is Eco-bond, made by Emerson Cuming, in Woburn, Massachusetts.

The recessed inner rim 42 acts as a wicking barrier, so that when the epoxy adhesive is placed on the plates 24, 28, it does not "wick" further along from the outer rim 40 towards the center of the piezoelectric ceramic crystal 36 and the plates 24, 28. Another less effective form of a wicking barrier would be to put masking tape along the inner perimeter of the outer rim FIG. 5 is a side sectional view of the metal plate 24, taken along the line of FIG. 4. Proceeding from the recessed inner rim 42 towards the center of the plate, the plate thickens and then thins to be thinner at the center than at either of the rims 40, 42. Also depicted in FIG. 5 is the crystal 36, glued to the outer rim 40. The crystal 36 is a little more than twice as thick as the depth of the recessed inner rim 42. A convex side 44 of the plate 24 includes a small radius curve 46 and a large i radius curve 48. In operation, as pressure increases on the outside of the hydrophone :14, a center 50 of the convex side 44 is pushed inwardly. The crystal 36 acts as a flexing stop to stop the inward movement of the center 50. To achieve less flexing, a flexing stop could also be achieved by a pedestal rim attached to each of the concave sides 38, midway between the center 50 and the outer rim 40. Still another flexing stop could be achieved by a small pedestal attached at the center Referring to both FIG. 4 and FIG. 5, the inner dotted circle 52 shown in FIG.

4 is where the small radius curve 46 meets the outer rim 40 on the convex side 44.

FIG. 6 is an enlarged view of a portion of FIG. 5, at the point of the juncture of the crystal 36 and the rims 40, 42. The width of the recessed inner rim 42 is greater than the width of the outer rim The hydrophone 14 is made in the following steps. First, the previously polarized piezoelectric crystal 36 is positioned to be sandwiched between the pair of solid circular metal plates 24, 28. The epoxy adhesive 54 is then applied to the outer rims 40 of the plates. The plates 24, 28 and the crystal 36 are then put together, with the crystal sandwiched between them, and the plates are held together by a clamp while the epoxy adhesive 54 cures, thus forming the acoustic transducer assembly The acoustic transducer assembly 20 is cured at temperatures less than 150 degrees centigrade. In the preferred method, the curing temperature is 65 degrees centigrade, about one hundred forty-nine degrees fahrenheit. This low temperature curing, as opposed to the high temperatures required to cure an assembly made with solder, avoids the prior art problem of destroying the polarization of the piezoelectric ceramic crystal 36. The curing temperature can be varied according to the manufacturer's specifications. For example, it could be cured at 95 degrees centigrade for one hour, or at room temperature for twenty-four hours. Different manufacturers of the epoxy adhesive 54 would have different curing temperatures and times.

~After the acoustic transducer assembly 20 has been cured, it is encapsulated in the flexible case, or boot 32, with the polyurethane potting sealant 30 to form a potted assembly 20. Finally, the potted assembly 20 is cured at temperatures less 00 0 than 150 degrees centigrade to form the hydrophone 14.

Although an illustrative embodiment of the invention has been shown and described, other modifications, changes and substitutions are intended in the foregoing disclosure. Accordingly, it is appropriate that the appended claims be S•construed broadly and consistent with the scope of the invention.

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Claims (2)

1. A method of making a hydrophone, comprising the steps of: a. assembling a previously polarised piezoelectric crystal with a pair of solid circular metal plates positioned to sandwich the piezoelectric ceramic crystal between them, wherein an epoxy adhesive is interposed between the metal plates and the piezoelectric ceramic crystal to form an acoustic transducer assembly; b. curing the acoustic transducer assembly at temperatures less than 150 degrees Centigrade; c. encapsulating the acoustic transducer assembly in a flexible case with a to polyurethane potting sealant to form a potted assembly; and d. curing the potted assembly at temperatures less than 150 degrees Centigrade to form a hydrophone.

2. A method of making a hydrophone, substantially as hereinbefore described with S reference to Figs. 1 to 6. Dated 5 March, 2002 Input/Output Inc. O Patent Attorneys for the Applicant/Nominated Person 20 SPRUSON FERGUSON o [R:\LBFF021