WO1995024977A2

WO1995024977A2 - Hydrophone bender crystal assembly

Info

Publication number: WO1995024977A2
Application number: PCT/US1995/003144
Authority: WO
Inventors: Daniel Ming-Kwong Woo
Original assignee: Shaw Industries, Ltd.
Priority date: 1994-03-15
Filing date: 1995-03-14
Publication date: 1995-09-21
Also published as: AU2459795A; WO1995024977A3; CN1143918A; GB2301479A; GB2301479B; GB9618000D0

Abstract

A bender crystal assembly (30) is disclosed for mounting in a hydrophone housing to produce electrical signals in response to acoustic pressure changes in a body of water. The assembly comprises a disc-shaped mounting plate (12a) for mounting in the hydrophone housing to be bent by differential water pressure across the plate. A thin disc (14a) of piezoelectric crystal is bonded to the mounting plate by a layer (16a) of conductive adhesive to bend with the mounting plate. A reinforcing plate (32) is bonded to the side of the crystal opposite from the mounting plate by a layer (34) of conductive adhesive to increase the section moment of inertia and the depth of water in which the crystal assembly can operate.

Description

HYDROPHONE BENDER CRYSTAL ASSEMBLY

This invention relates to hydrophones generally, and to hydrophone bender crystal assemblies in particular. Specifically, the invention relates to an improved bender crystal assembly designed to increase the depths of water at which the hydrophone can operate without damage to the bender crystal.

Hydrophones are pressure sensitive devices that are used to detect acoustic energy (variations of pressure) in a body of water. Therefore, hydrophones are particularly useful in converting pressure changes traveling through the water from a seismic event to electrical signals to map subterranean formations.

A typical bender crystal assembly of a hydrophone includes a disc-shaped mounting plate and a thin disc shaped piezoelectric crystal bonded to one side of the mounting plate by a layer of conductive epoxy. Usually, another thin disc-shaped piezoelectric crystal is bonded to the other side of the mounting plate in the same manner. The mounting plate is supported in the housing on its periphery and since the load on the plate is uniform, the assembly acts like a uniformly loaded circular plate with clamped edges. Acoustic signals create pressure changes in the water that cause the assembly to bend back and forth in sync with the increase and decrease of the acoustic pressure. This causes the piezoelectric crystals to produce voltages that rise and fall with the pressure changes. Since this type of bender crystal assembly is a high-impedance device, the crystal is connected to either an impedance matching transformer or a preamplifier through which the output signal passes before reaching a recording instrument.

Typically, standard hydrophones, such as those described above, are unable to produce consistent uniform signals over the life of the crystals, because they experience capacitance changes. These capacitance changes cause a change in the resonant frequency of a transformer coupled circuit and variation in the amplitude of the output signal in a preamplifier system. The amount of capacitance in each hydrophone varies and, thus, the output signals vary from hydrophone to hydrophone, even though the hydrophones are subjected to the same stress. Therefore, even though these capacitance changes do not cause a complete failure of the hydrophones, the output signals can not be stacked (a practice commonly used in the seismic industry to reduce signal-to-noise ratio in a seismic data gathering system) , because of the variation in the output signals.

Hydrophone bender crystals have been in existence since at least the early 1970s, and the problem of non- uniform signals caused by the change in capacitance has been around as long as crystals have been used in hydrophones. There are no visible signs of the cause of the problem.

For years, unsuccessful tests have been run to determine the source of the changing capacitance problem. It was not until just recently that the inventor discovered that the capacitance changes were due to a phenomenon that the inventor calls "micro-crazing." Micro-crazing is a large number of very shallow cracks that occur when the crystals are placed under tension. These cracks can be observed only when the crystal is under tension.

If a hydrophone is lowered deep enough in a body of water, the tensile stress in the crystal will reach the point where micro-crazing will occur. This changes the capacitance of the crystal, which causes a change in the resonant frequency of a transformer coupled circuit and variations in the amplitude of the output signal in a preamplifier system. When the hydrophone is brought back to the surface, the deflection of the crystal decreases back to zero, causing the cracks to effectively close up. If subjected to a test, the hydrophone will generally perform well, and even microscopic examination will fail to reveal micro- crazing. If again sent down to significant depth, the cracks reopen and the hydrophone again fails to perform. Therefore, while the effects of the problem were well- known, the cause, and therefore the solution, were not. Because the development of micro-crazing is a consequence of tensile strain, and because the tensile strain increases with increasing depth of immersion, hydrophones which can survive moderate depths have been found to fail at the greater depths now being surveyed. Therefore, the problem generated by the micro-crazing phenomenon has become even more pronounced recently due to the need to carry out seismic surveys at greater depths than were commonly attempted in the past.

Therefore it is an object of this invention to provide a bender crystal assembly that has an increased moment of inertia to increase the depth of water at which the assembly can operate without micro-crazing occurring in the crystal.

It is another object of this invention to provide a strengthened bender crystal assembly that is micro-crazing resistant at increased depths in the water by applying a layer of adhesive to the crystal opposite the mounting plate.

It is another object of this invention to provide a strengthened bender crystal assembly that is micro-crazing resistant at increased depths in the water, so that output signal generated by hydrophones using the strengthened bender crystal assembly can be stacked.

It is also been shown that subjecting a piezoelectric crystal to thermal stress will reduce the limits of the crystal to resist micro-crazing. Therefore, the soldering of the electrical connection directly onto the piezoelectric crystal increases the likelihood of micro- crazing at pressures less than expected. It is therefore another object of this invention to provide a bender crystal assembly for use in a hydrophone that includes electrically conductive reinforcing plates bonded to the piezoelectric crystals to which the output wires can be soldered prior to bonding thereby avoiding localized heating of the crystal. In the Drawings:

FIG. 1 is a top view of the prior art bender crystal assembly.

FIG. 2 is a sectional view taken along line 2-2 of FIG. 1.

FIG. 3 is a graphical representation of a normal frequency response curve and a response from a bender crystal with a micro-crazed surface using an impedance matching transformer. FIG. 4 is a top view of a bender crystal assembly constructed in accordance with the preferred embodiment of this invention.

FIG. 5 is a sectional view taken along line 5-5 of FIG. 4. FIG. 6 is a sectional view of a dual bender crystal assembly.

FIG. 7 is a sectional view of a single bender crystal assembly utilizing an adhesive reinforcement technique. FIG. 8 is a sectional view of a single bender crystal assembly utilizing an alternate adhesive reinforcement technique.

FIG. 9 is a sectional view of a single bender crystal assembly utilizing another alternate adhesive reinforcement technique.

FIG. 10 is a schematic view of a typical seismic data gathering system utilizing hydrophones built in accordance with this invention.

FIGs. 1 and 2 show the top and sectional views of a typical bender crystal assembly, generally referred to as 10, used in a hydrophone for seismic data collecting. Assembly 10 includes mounting plate 12 and two piezoelectric crystal plates 14 bonded to opposite sides of mounting plate 12 by layers of conductive epoxy 16. Electrical contact is made to the piezoelectric crystal plates by soldering wires 18 directly to the crystals. Wires 18 are then monitored to detect voltage drops across the respective plates that are produced when the assembly is bent due to a change in pressure. As the piezoelectric crystal plates 14 bend they undergo compression on one side and tension stress on the other depending on the direction of the bend. As the assembly is submerged in the water, the static pressure increases thus, causing the assembly to bend. At some point the tensile stress on the piezoelectric crystal plates becomes so great that micro-crazing occurs. As stated earlier, the micro-crazing causes a change in capacitance, which will effect the output signals from the piezoelectric crystal plates. For example, FIG. 3 shows the output frequency response characteristics of a bender crystal assembly connected in series with a step-down transformer. The solid curve shows the output from a crystal that has not experienced micro-crazing. The dashed curve represents the frequency response of a crystal once micro-crazing occurs. The capacitance of the crystal is lowered and, as shown, the resonant frequency and damping are both higher than those associated with a crystal that is not affected by micro-crazing.

In order to reduce the amount of stress experienced by the crystals for a given pressure differential, the moment of inertia of the bender crystal assembly is increased. In the embodiment 30 shown in FIGs. 4 and 5 bender crystal assembly 30 includes mounting plate 12a with thin disc¬ shaped piezoelectric crystals 14a bonded thereto by conductive adhesive such as conductive epoxy layers 16a. With this arrangement, rather than soldering the electrical contact directly to piezoelectric crystal plates 14a, which can damage the crystals, wires 18a are soldered to reinforcement plates 32. Then reinforcement plates 32 along with wires 18a attached are bonded to piezoelectric crystal plates 14a with a second layer of conductive epoxy 34. The addition of the reinforcement plates serves two purposes. First, since the wires are soldered to the reinforcement plates, the piezoelectric crystals will not undergo the thermal stress caused by the soldering of the wires directly to the piezoelectric crystals. Second, the addition of reinforcement plates 32 has increased the moment of inertia and the stiffness of the overall assembly reducing the stresses and the amount of micro-crazing caused by bending at increased pressure, which will allow the hydrophone to operate successfully in much deeper water.

In the preferred embodiment of this invention the mounting plate thickness is approximately 0.03 inches and the diameter is approximately 1.5 inches. The crystal is approximately 0.01 inches thick, whereas the thickness of the reinforcement plate is approximately 0.003 inches. Both of the conductive epoxy layers are approximately 0.001-0.002 inches thick. The thicknesses of the various layers can be adjusted depending on the desired output voltage, maximum operating depth in water, and the Young's

Modulus of all the selected materials. Temperature is an additional factor to consider when selecting the crystal.

Also the diameter of reinforcement plate 34 is 0.80 inches which is slightly less than the 0.98 inches diameter of piezoelectric plate 14, so as to prevent the epoxy from bridging the crystal and shorting the circuit. These dimensions are not critical and again depend on the desired voltage and depths. The electrical isolation of the various layers can be achieved by many methods well known to those of ordinary skill in the art.

Both of the mounting and the reinforcement plates are made of beryllium copper, but could be made of any conductive spring material, such as stainless steel. However, it is not necessary that the reinforcement plates be conductive if electrical contact is made directly to piezoelectric crystal.

The piezoelectric crystal is a EC-65 manufactured by EDO. However, any piezoelectric material capable of producing a voltage under pressure, such as those manufactured by Channel or Motorola, could be used.

The conductive epoxy is developed from a mixture of DYMAX 847 adhesive, manufactured by DYMAX Corporation of Torrington, Connecticut and a metal dust, such as copper or silver dust. However, any conductive adhesive developed by techniques well known to those of ordinary skill in the art can be used.

Although FIGs. 4 and 5 show two piezoelectric crystal mounted to the mounting plate, i.e. a dual crystal assembly, a bender crystal assembly only requires one crystal.

In order to make a bender crystal hydrophone less sensitive to acceleration, two dual crystal assemblies A and B are mounted to brass ring 40, such as that shown in FIG. 6, to create a dual crystal, dual bender crystal assembly. The polarities of the crystals are arranged on the assemblies to effect acceleration cancellation when moved through the water. The means for connecting the crystals to the recording equipment are well known to those of ordinary skill in the art. The polarity arrangements may change depending on how these connections are configured. A single crystal, dual bender crystal assembly results from having only one crystal mounted on each mounting plate.

FIGs. 7, 8, and 9 show three alternate bender crystal assemblies, wherein the amount of micro-crazing is reduced by reinforcing the unbonded surface crystal with a layer of adhesive. All three assemblies include mounting plates 12c, 12d, and I2e with piezoelectric crystals 14c, 14d, and 14e bonded thereto by conductive epoxy layers 16c, 16d, and 16e.

In FIG. 7, wire 18c is soldered directly to crystal 14c. Then a layer of reinforcing adhesive 42 is applied over the surface of the crystal opposite from the mounting plate 12c. Any adhesive strong enough to reinforce the crystal to prevent or reduce micro-crazing can be used. For example, the DYMAX 847 adhesive can be used. Even though the crystal undergoes the thermal stress caused by soldering wire 18c directly to crystal 14c, the addition of the adhesive layer strengthens the surface of the crystal to make it less susceptible to micro-crazing. There is no requirement that the adhesive layer be conductive. This crystal assembly may not be able to withstand as much pressure as the preferred assembly. However, it is able to withstand more pressure than the prior art assemblies.

The assembly shown in FIG. 8 is capable of withstanding more pressure than the FIG. 7 assembly, because wire 18d is not soldered to crystal 18d. It is mounted to wire bonding plate 44 which is bonded to crystal 14d by adhesive layer 42a. Although electrical contact can be made by the pressure of a nonconductive adhesive pushing plate 44 against the crystal, better contact is made using a conductive adhesive. The electrical contact between the crystal and the wire of this assembly is not as effective as that shown in FIG. 7, but it is believed that this assembly is capable of performing at higher pressures than the FIG. 7 assembly, because the crystal did not undergo the thermal stress caused by soldering.

It is possible to make electrical contact with the piezoelectric crystal by bonding wire 18e directly to conductive adhesive layer 42b as shown in FIG. 9, when the wire is not insulated. In this embodiment, the crystal is somewhat insulated from the thermal stress caused by soldering directly to the crystal. However, the assemblies described above prove to be more reliable.

The thickness of adhesive layers 42, 42a, and 42b depends on the maximum operating depth in water and the physical characteristics of the adhesive. However, when using the DYMAX 847 adhesive, it is believed that the thickness of any of the layers 42, 42a, 42b should be approximately .005 inches. This thickness is approximately five times thicker than the typical thickness of any of the conductive layers 16c, 16d, and 16e, which is approximately .001-.002 inches. Thicker layers are obtained by applying several coats of the adhesive until the correct thickness is achieved. Any of the crystals described above can be used in typical seismic data gathering systems, well know to those of ordinary skill in the art, to produce output signals that can be stacked to enhance the signal-to-noise ratio. FIG. 10 shows such a typical seismic data gathering system. Hydrophones 48 are dropped in a body of water 50 that covers a subsurface area of interest 52 and anchored at specific locations by anchors 54. Buoys 56 identify the location of the hydrophones. Then seismic source 60 is towed by boat 62 though the water generating acoustic pressure changes 64 that are reflected off the subsurface and detected at hydrophones 48. The hydrophones produce output electric signals in response to the acoustic pressure changes that can subsequently be stacked by techniques well known to those of ordinary skill in the art.

From the foregoing it will be seen that this invention is one well adapted to obtain all the ends and objects herein above set forth, together with other advantages which are obvious and which are inherent to the apparatus and structure.

It will be understood that certain features and subcombinations are of utility and maybe employed without reference to other features and subcombinations. This is contemplated and is within the scope of the claims. Because many possible embodiments maybe made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not limiting in any sense.

Claims

What is claimed is:

1. A bender crystal assembly for mounting in a hydrophone housing to produce electrical signals in response to acoustic pressure changes in a body of water, said assembly comprising a disc-shaped mounting plate for mounting around its perimeter in the hydrophone housing to be bent radially from its outer edge to its center by differential water pressure across the plate, a disc-shaped piezoelectric crystal plate, a first layer of conductive adhesive between the disc¬ shaped piezoelectric crystal plate and one side of the disc-shaped mounting plate to bond one side of the disc-shaped crystal plate to the disc-shaped mounting plate so the crystal will bend radially from its outer edge to the center with the mounting plate, when the bender crystal assembly is subjected to an acoustic pressure increase, a disc-shaped reinforcing plate, a second layer of conductive adhesive to bond the reinforcing plate to the other side of the disc¬ shaped crystal plate to increase the section modulus of the bender crystal assembly to reduce the stress in the crystal plate for a given water depth and to thereby increase the depth of water in which the crystal assembly can operate without undergoing micro-crazing.

2. The bender crystal assembly of claim 1, further including a disc-shaped second piezoelectric crystal plate, a third layer of conductive adhesive between the second disc-shaped piezoelectric crystal plate and the other side of the disc-shaped mounting plate to bond one side of the second disc-shaped piezoelectric crystal plate so the crystal will bend radially from its outer edge to the center with the mounting plate, a disc-shaped second reinforcing plate, and a fourth layer of conductive adhesive to bond the second reinforcing plate to the other side of the second disc-shaped piezoelectric crystal plate to further increase the section modulus of the bender assembly to further reduce the stress in the second crystal plate for a given depth of water and to thereby increase the depth of water in which the crystal assembly can operate without undergoing micro-crazing.

3. The assembly of claims 1 or 2 in which the reinforcing plates are made of electrically conductive material.

4. The bender crystal assembly of claim 1 wherein the mounting plate and the reinforcing plate are made of copper.

5. The bender crystal assembly of claim 4 wherein the copper is beryllium copper.

6. The bender crystal assembly of claim 1 wherein the mounting plate and the reinforcing plate are stainless steel.

7. The bender crystal assembly of claim 1 wherein the reinforcement plate is smaller in outside dimensions than the piezoelectric crystal.

8. The bender crystal assembly of claim 1 wherein the mounting plate, the conductive reinforcement plate, and the piezoelectric crystal are disc-shaped.

9. The bender crystal assembly of claim 1 wherein the first metallic plate is 0.03 inches thick, the reinforcing plate is 0.003 inches thick, and the piezoelectric crystal is 0.01 inches thick.

10. A dual bender crystal assembly for mounting in a hydrophone housing to produce electrical signals in response to acoustic pressure increase in a body of water, said assembly comprising a first bender crystal assembly including a first disc-shaped mounting plate for mounting around its perimeter in the hydrophone housing to be bent radially from its outer edge to its center by differential water pressure across the plate, a first disc-shaped piezoelectric crystal plate, a layer of conductive adhesive between the disc¬ shaped piezoelectric crystal plate and one side of the first disc-shaped mounting plate to bond one side of the first piezoelectric crystal plate to the first mounting plate to bend radially from its outer edge to its center with the mounting plate, when the bender crystal assembly is subjected to an acoustic pressure increase, a first reinforcing plate, a layer of conductive adhesive to bond the first reinforcing plate to the other side of the first piezoelectric crystal plate to increase the section modulus of the first bender assembly, a second bender crystal assembly including a second disc-shaped mounting plate for mounting in the hydrophone housing to be bent radially from its outer edge from its center by differential water pressure across the second mounting plate, a second disc-shaped piezoelectric crystal plate, a layer of conductive adhesive between the second disc-shaped piezoelectric crystal plate and one side of the second disc-shaped mounting plate to bond one side of the second disc¬ shaped piezoelectric crystal plate to the disc-shaped second mounting plate to bend radially from its outer edge to its center with the second disc-shaped mounting plate, when the bender crystal assembly is subjected to an acoustic pressure increase, a second reinforcing plate, a layer of conductive adhesive to bond the second reinforcing plate to the other side of the second disc-shaped piezoelectric crystal plate to increase the section modulus of the second bender assembly, and a brass ring for mounting in the hydrophone housing having the first bender crystal assembly mounted on one side of the brass ring and the second bender crystal mounted on the other side of the brass ring, thereby creating a dual bender crystal assembly that can operate in deeper waters and is less susceptible to the effects of acceleration on the output signals of the piezoelectric crystals.

11. The bender crystal assembly of claim 10 further including a third disc-shaped piezoelectric crystal plate, a layer of conductive adhesive between the third disc- shaped piezoelectric crystal plate and the other side of the first disc-shaped mounting plate to bond one side of the third disc-shaped piezoelectric crystal plate to the first disc- shaped mounting plate to bend radially from its outer edge to its center with the first disc¬ shaped mounting plate, a third reinforcing plate, a layer of conductive adhesive to bond the third reinforcing plate to the other side of the third disc-shaped piezoelectric crystal plate to increase the section modulus of the first bender assembly, a fourth disc-shaped piezoelectric crystal plate, a layer of conductive adhesive between the fourth disc-shaped piezoelectric crystal plate and the other side of the second disc-shaped mounting plate to bond one side of the fourth disc-shaped crystal plate to the second disc-shaped mounting plate to bend radially from its outer edge to its center with the second disc-shaped mounting plate, a fourth reinforcing plate, and a layer of conductive adhesive to bond the fourth disc-shaped reinforcing plate to the other side of the fourth piezoelectric crystal plate to increase the section modulus of the second bender assembly.

12. The bender crystal assembly of claim 10 wherein the first and second piezoelectric crystal assemblies face toward the inside of the brass ring.

13. A bender crystal assembly for mounting in a hydrophone housing to produce electrical signals in response to acoustic pressure changes in a body of water, said assembly comprising a disc-shaped mounting plate for mounting in the hydrophone housing to be bent radially from its outer edge to its center by differential water pressure across the plate, a disc-shaped piezoelectric crystal plate, a first layer of conductive adhesive between the disc¬ shaped piezoelectric crystal plate and one side of the disc-shaped mounting plate to bond one side of the disc-shaped crystal plate to the disc-shaped mounting plate so the crystal will bend radially from its outer edge to its center with the disc-shaped mounting plate, a second layer of adhesive bonded to the other side of the disc-shaped piezoelectric crystal plate to strengthen the surface of the disc-shaped piezoelectric crystal plate and increase the depth of water in which the crystal assembly can operate without undergoing micro-crazing.

14. The bender crystal assembly of claim 13 wherein the second layer of adhesive is conductive and further including a wire bonded to the second layer of adhesive for making electrical contact with the piezoelectric crystal plate.

15. The bender crystal assembly of claim 13 wherein the second layer of adhesive is conductive and further include a wire bonding plate with a wire bonded thereto for bonding to the piezoelectric crystal plate by the second layer of adhesive.

16. The bender crystal assembly of claim 13 further including a wire bonded to the piezoelectric crystal plate for making electrical contact therewith.

17. In a seismic data gathering system utilizing a plurality of hydrophones in a body of water to produce a plurality of electrical signals in response to acoustic pressure changes in the water and in which system the signals from the hydrophones are stacked, and wherein each hydrophone includes a bender crystal assembly that bends in response to pressure increases in the water to thereby bend in response to acoustic energy reflected from earth formations, the improvement comprising a bender crystal assembly having a disc-shaped mounting plate, a disc-shaped piezoelectric crystal, a layer of an electrically conductive adhesive between the mounting plate and the crystal to attach the crystal to the plate to cause the crystal to bend with the plate in response to acoustic energy reflected from earth formations, a disc-shaped reinforcing plate of relatively stiff material, and a layer of adhesive between the reinforcing plate and the crystal to cause the reinforcing plate to bend with the crystal, wherein said layers of adhesive and the reinforcing plate increase the section modulus of the assembly to thereby allow a plurality of such assemblies to operate in deeper water providing output signals that can be stacked.