Acoustic sensor and array thereof

The present invention provides an acoustic sensor having one or more segments that are electrically coupled to provide a response corresponding to a hydrodynamic pressure applied to the segments. Each segment contains a substrate of a desired shape with a concavity on an outer surface that is sealingly enclosed by an active member made from a flexible, resilient piezoelectric material. PVDF material is preferably used as the piezoelectric active element. The active element may be bonded to a compliant diaphragm sealed to the substrate to provide the sealed chamber. The active member is covered with a protective layer of a suitable material, preferably a polyvinyl material. In one embodiment, the diaphragm includes a standoff ledge and is placed on the outer surface of the substrate to define the sealed chamber between the diaphragm and the outer surface of the substrate. In another embodiment, at least two substrates are used and a damping material is placed between the substrates wherein one substrate includes a concavity on an outer surface for defining the sealed chamber. In still another embodiment, the diaphragm having the standoff ledge is used with the at least two substrates.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to the field of acoustic sensors and more 
particularly to a novel hydrophone and method of making the same which may 
be used under great hydrostatic pressure and under severe hydrodynamic 
conditions. 
2. Description of the Related Art 
Piezoelectric hydrophones of various configurations have been used in a 
variety of applications. In geophysical exploration, arrays of hydrophones 
are used to detect seismic shock waves from the earth's substrata in 
response to induced shock waves at known locations on the earth. 
Hydrophones also are used in boreholes to conduct vertical seismic surveys 
and for a variety of other applications. Acoustic pressure variations 
across the hydrophone produce electrical signals representative of the 
acoustic pressure, which are processed for desired applications. 
Piezoelectric hydrophones typically contain a piezoelectric material as an 
active element which produces electrical signals when subjected to 
acoustic pressures. Ceramic materials such as barium titane or lead 
zirconate titane have been used in various configurations as one class of 
piezoelectric materials in hydrophones. U.S. Pat. No. 4,092,628 discloses 
the use of a thin ceramic wafer that operates in the bender mode. U.S. 
Pat. No. 4,525,645 shows a unit shaped as a right cylinder that operates 
in the radial mode. Ceramic materials are brittle and tend to shatter in 
the presence of a severe shock such as that produced by an explosive 
charge or air gun commonly employed for conducting seismic surveys over 
water-covered areas. 
Most hydrophones have a depth limit. An excessive overpressure can cause 
the active element to bend beyond its elastic limit, resulting in signal 
distortion and ultimate failure of the hydrophone. In the ceramic wafer 
type hydrophones, an internal stop is sometimes provided to prevent 
excessive bending of the element. The wafer, however, tends to develop a 
permanent deformation that degrades the output signal. 
Polyvinylidene fluoride ("PVDF") has been used as another class of 
piezoelectric material in hydrophones. One such material is available 
under the tradename KYNAR from AMP corporation. The PVDF material is 
useful as a hydrophone active element because its acoustic impedance is 
close to that of water and the acoustic wavefields do not produce spurious 
reflections and diffractions as they do when encountering ceramic 
piezoelectric elements. The output signals of the PVDF element are many 
times greater than the signal output of a ceramic material. Also, PVDF 
material is readily available in various sheet sizes and a wide range of 
thickness. Such PVDF material may be readily shaped and cut to fit the 
intended use. Prior to use, the PVDF material is poled or activated in the 
thickness direction by application of a high electric field at an elevated 
temperature for a requisite time period. Conductive metal electrodes are 
evaporated on the opposite sides of the PVDF film as with the ceramic 
materials. 
An external mechanical force applied to the PVDF film results in a 
compressive or tensile force strain. The PVDF film develops an open 
circuit voltage (electrical charge) proportional to the changes in the 
mechanical stress or strain. The charge developed diminishes with time, 
depending upon the dielectric constant of the film and the impedance of 
the connected circuitry. By convention, the polarization axis is the 
thickness axis. Tensile stress may take place along either the 
longitudinal axis or the width axis. 
U.S. Pat. No. 4,653,036 teaches the use of a PVDF membrane stretched over a 
hoop ring. A metallic backing is attached to the back of the ring and a 
void between the film and the backing is filled with an elastomer such as 
silicone. The device operates in the bender mode. U.S. Pat. No. 4,789,971 
shows the use of a voided slab of PVDF material sandwiched between a pair 
of electrodes. A bilaminar construction is also disclosed. A preamplifier 
is included in the assembly. The transducer operates in the 
thickness-compressive mode. 
A hydrophone array shown in U.S. Pat. No. 4,805,157 consists of multiple 
layers of PVDF material symmetrically disposed around a stiffener for 
prevention of flexural stresses. The axis of maximum sensitivity is in the 
direction transverse to the plane of the layers. This sensor is sensitive 
to compressive stress. 
U.S. Pat. No. 5,361,240, issued to the inventor of this application, 
discloses a pressure-compensated PVDF hydrophone that contains a hollow 
mandrel having a concavity at an outer surface. A flexible and resilient 
piezoelectric film, preferably made from a PVDF material, is wrapped 
several times around the mandrel over the concavity to act as the active 
element of the hydrophone. The volume between the surface of the inner 
layer of the film and the concavity provides a pressure compression 
chamber. This hydrophone has been found to be responsive to varying 
hydrodynamic pressure fields but is substantially insensitive to 
acceleration forces, localized impacts and variations in hydrostatic 
pressures. 
To perform seismic surveys in water-covered areas, one or more arrays of 
hydrophones, each array having a plurality of serially coupled 
hydrophones, are deployed on the bottom of a water-covered area or are 
towed behind a vessel. In bottom cable applications, hydrophones are 
commonly built as an integral part of the cable. Each hydrophone is 
hermetically sealed with a suitable material, such as polyurethane. Such 
cable constructions are not conducive to easy repairs in the field. 
Defective hydrophone sections are removed and a cable section containing a 
working hydrophone is spliced in the place of the defective hydrophone. 
Such repairs are usually less reliable than unitary constructions and 
require excessive repair time, which can significantly increase the cost 
of the surveying operations, especially when performing three-dimensional 
seismic surveys as the down time can cost several thousand dollars per 
hour. Thus, there has been an unfilled need to provide a hydrophone which 
is easy to assemble into a hydrophone cable and easy to repair in the 
field. 
The present invention addresses the above-noted problems and provides a 
segmented hydrophone that preferably utilizes a flexible and resilient 
piezoelectric material and a method of making same. The hydrophone 
segments may be combined to form a single hydrophone. The hydrophone 
segments removably attach to the cable which is suitably configured to 
accommodate the hydrophone segments. Any hydrophone segment can readily be 
replaced without requiring any splicing of the cable. The hydrophone is 
responsive to varying hydrodynamic pressure fields, but is substantially 
inert to acceleration forces, localized impacts and variations in 
hydrostatic pressure. 
SUMMARY OF THE INVENTION 
The present invention provides an acoustic sensor having one or more 
segments that are electrically coupled to provide a response corresponding 
to an acoustic pressure applied to the segments. Each segment contains at 
least one substrate of a desired shape with a chamber on an outer surface 
that is sealingly enclosed by an active member made from a flexible, 
resilient piezoelectric material. Polyvinylidene fluoride material is 
preferably used as the piezoelectric active element. The active element is 
preferably bonded to a compliant diaphragm sealed to the substrate to 
provide the sealed chamber. The active element is covered with a 
protective layer of a suitable material, preferably a polymer material. A 
single segment may be used as a sensor or a plurality of segments may be 
coupled to form the sensor. The sensor of the present invention may be 
used in hydrophone cables for performing seismic surveys. In such 
applications, a plurality of spaced sensors made according to the present 
invention are attached along the length of a suitably configured cable. 
Each such sensor preferably contains two electrically coupled hydrophone 
segments are placed on a rigid member placed on the cable. One or both 
hydrophone segments are coupled to a conductor in the cable, preferably 
via an under water plug-in connector. The cable is configured to 
accommodate the hydrophone segments between a nose section and a rear or 
tail section. One or more of such hydrophone cables are usually arranged 
in a matrix or an array for performing seismic surveys. The sensor of the 
present invention, however, may also be used in other applications 
requiring the use of a hydrophone. 
In one embodiment, the diaphragm has a standoff ledge and is placed on the 
outer surface of the substrate to define a sealed chamber between the 
diaphragm and the outer surface of the substrate. The piezoelectric 
material is placed over the diaphragm. In a second embodiment, at least 
two substrates are provided and a damping material is placed between the 
outer surface of a first substrate and the inner surface of a second 
substrate. The second substrate includes a concavity on an outer surface 
wherein a diaphragm is placed over the concavity of the second substrate 
to define a sealed chamber between the diaphragm and the concavity. The 
piezoelectric material is placed over the diaphragm. In a third 
embodiment, at least two substrates are provided. The diaphragm includes 
the standoff ledge and is placed on the outer surface of a second 
substrate to define a sealed chamber between the diaphragm having the 
standoff ledge and the outer surface of the second substrate. A damping 
material is placed between an outer surface of a first substrate and an 
inner surface of the second substrate. 
Examples of the more important features of the invention thus have been 
summarized rather broadly in order that the detailed description thereof 
that follows may be better understood and in order that the contributions 
to the art may be appreciated. There are, of course, additional features 
of the invention that will be described hereinafter and which will form 
the subject of the claims appended hereto.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
For convenience and simplicity and not as a limitation, the sensor of the 
present invention is described as a two-segment hydrophone placed on a 
type of cable typically used for conducting seismic surveys for 
geophysical prospecting. Accordingly, FIG. 1 shows a partial 
cross-sectional view the preferred embodiment of a two-segment hydrophone 
coupled to a cable according to the present invention. FIG. 1A, FIG. 1B, 
and FIG. 1C are cross-sectional views of the hydrophone assembly of FIG. 1 
taken along A--A shown therein each showing a different embodiment of the 
hydrophone assembly. FIG. 2 shows an exploded perspective view of the 
hydrophone assembly shown in FIG. 1. 
Now referring to FIGS. 1, 1A-1C, and 2, the hydrophone assembly includes a 
cable 10 having a plurality of twisted pairs of conductors 8 placed around 
a core member 9 along the cable length. The conductors 8 are encased in 
one or more layers 12 of protective coatings. A molded section 14 is 
formed over the cable 10 to suitably accommodate hydrophone segments 
between a nose (front end) 34 having a smoothly increasing cross-section 
from a point of attachment 33 on the cable 10 to a point 38 and a smoothly 
decreasing cross-section tail (rear end) 14a that extends from a point 20 
of maximal diameter d.sub.1 to a point 18 on the cable exterior. The front 
end 34 preferably has a higher slope than the tail end 14a to reduce the 
noise effect due to hydrodynamic turbulence produced when the cable 10 is 
pulled under water. Longitudinal flanges 22a and 22b, each having a 
desired width d.sub.2 extends axially along the cable 10 from the maximal 
diameter point 20 up to the point 38 on opposite sides of the cable 10. 
Each flange contains a plurality of holes 24 for accommodating therein 
suitable tying elements such as bolts. Conductors 16 taken out from the 
cable are coupled to a suitable underwater connector 17 for providing 
electrical connection between the cable 10 and the hydrophone segments. 
The connector 17 and the conductors 16 are preferably molded in the 
section 14a exposing only the mating end of the connector 17 to the end 
20. 
Hydrophone segments 30 and 32, each having an inner surface that 
substantially conforms to the outer surface of suitable rigid members 
placed on the cable, are placed over the rigid member juxtaposed with the 
edge 20. The rigid members provide support for the hydrophone segments and 
prevent bending of the hydrophones when the cable 10 bends during handling 
and use. The hydrophone segments 30 and 32 may be conveniently secured on 
the cable by attaching the segments to the flanges 22 by bolts 26 placed 
through the holes 24 and corresponding access holes 28 made in the 
segments 30 and 32. The hydrophone segment 30 contains a connector 40 that 
may be removably connected to the cable connector 17. The connectors 40 
and 17 sealingly mate with each other to prevent fluid leakage into the 
conductors connected to such connectors for carrying electrical signals. 
The hydrophone segments 30 and/or 32 may contain a preamplifier 45 suitably 
coupled between the connector 17 and an active element of the hydrophone 
(described later) to amplify signals received from the hydrophone segments 
30 and 32 prior to transmitting such signals through the cable 10. 
Hydrophone segments 30 and 32 are electrically coupled to each other by 
suitable means known in the art, such as connectors. 
The nose 34 preferably has two halves 34a and 34b, each such half having an 
inner surface that substantially conforms to the cable outer surface, are 
placed against the hydrophone segments 30 and 32 and securely attached to 
the cable 10 by a suitable means such as bolts. The outer surface of the 
nose 34 preferably has a smooth surface 39 extending from the cable 
diameter to the diameter of the hydrophone segments at the end 38. The 
hydrophone segments 30 and 32 may be easily removed from the cable 10 by 
removing the nose section 34 and the bolts 26 to repair or replace a 
particular hydrophone segment. Alternatively, the nose 33 (shown in dashed 
lines) is comprised of a single section having an inner surface that 
substantially conforms to the cable outer surface, and is placed against 
the hydrophone segments 30 and 32. The nose 33 is securely attached to the 
cable 10 and hydrophone segments 30 and 32 using the screw threads 35 on 
the nose 33 which attach to suitable matching screw threads 35 (shown in 
dashed lines) on the hydrophone segments 30 and 32. A suitable attachment 
means, such as bolts, may additionally be used. 
The above-described cable hydrophone utilizes two hydrophone segments 
attached around a cable. One segment contains a preamplifier and a 
connector electrically coupling the hydrophone to a conductor take-out 
from the cable. Although the cable hydrophone described herein has two 
segments, the hydrophone according to the present invention, however, may 
contain one or more segments that are electrically coupled to each other. 
In certain applications, it may be desirable to utilize more than two 
hydrophone segments placed around the cable. In such cases, provision is 
made for placing the desired number of segments around the cable in a 
manner similar to that described above. The elements and construction of 
the hydrophone segments will now be described while referring to FIGS. 
1-6. 
FIG. 4 shows a plan view of the hydrophone segment 30 shown in FIG. 1 and 
FIGS. 5A-5C show cross-sectional view taken along B--B of the hydrophone 
segment 30 shown in FIG. 4. 
FIGS. 1A and 5A show a first embodiment of the hydrophone of the present 
invention including a compliant diaphragm 68 having a standoff ledge 69 
and made from a suitable polymer material, such as polycarbonate. The 
diaphragm 68 having the standoff ledge 69 is more clearly shown in FIG. 3. 
The hydrophone segment contains a mandrel or a substrate 50 having an 
inner surface 52 that preferably conforms to the surface on which such 
segment is intended to be mounted. In the case of the cable hydrophone 
shown in FIG. 1, the substrate 50 has a concave inner surface that 
conforms to the outer surface of a rigid member mounted on the outer 
surface of the cable 10. 
The diaphragm 68 is place on the outer surface 54 of the substrate 50 to 
define an enclosed chamber 66 between the diaphragm 68 and standoff ledge 
69 and the outer surface 54 of the substrate 50. The size of the diaphragm 
68 is sufficient to cover the entire outer surface 54 of the substrate 50. 
A rectangular chamber 66 is preferred for use in the cable hydrophones. 
The depth of the chamber 66 depends upon the desired application. The 
diaphragm 68 is hermetically sealed at the standoff ledge 69 by a suitable 
means, such as an adhesive or a clamp, placed around the entire periphery 
of the standoff ledge 69 to form a sealed space or chamber 66 between the 
diaphragm 68 and the outer surface 54 of the substrate 50. The use of the 
standoff ledge 69 on the diaphragm 68 allows for the use of un-machined 
tubing for manufacturing the substrate 50. Forming the standoff ledge 69 
into the molded diaphragm 68 provides a cost efficient method of 
manufacturing and assembling of the hydrophone. 
FIGS. 1B and 5B show a second embodiment of the hydrophone of the present 
invention including a first substrate 49 and a second substrate 50 having 
a concavity 56 on an outer surface 54. The first substrate 49 has a 
concave inner surface that conforms to the outer surface of the rigid 
member mounted on the outer surface of the cable 10 and the second 
substrate 50 has a concave inner surface that conforms to the outer 
surface of the first substrate 49. The outer surface 54 of the second 
substrate 50 has a concavity 56 of a desired shape and depth bounded by 
shoulders 58. A rectangular concavity 56 is preferred for use in the cable 
hydrophones. The depth of the cavity 56 depends upon the desired 
application. 
A damping compound or damping material 51, such as a polyurethane, or other 
suitable elastomeric compound, is placed between the outer surface of the 
first substrate 49 and the inner surface of the second substrate 50 to 
isolate energy present in the cable 10 and provide for vibration damping. 
Preferably, the first substrate 49 is made of an aluminum material and the 
second substrate 50 of a carbon fiber material. However, both substrates 
can be made of aluminum material, carbon fiber material, or other suitable 
stiff material. Preferably, the first substrate 49 is thicker in size than 
the second substrate 50. Alternatively, one or more additional substrates 
are used including a damping material placed between each additional 
substrate to provide further vibration damping. 
The concavity 56 is preferably covered by a compliant diaphragm 68 made 
from a suitable polymer material such as polycarbonate. The size of the 
diaphragm 68 is sufficient to cover the entire concavity 56. The diaphragm 
68 is hermetically sealed at the shoulders 58 by a suitable means 64, such 
as an adhesive or a clamp, placed around the entire periphery of the 
concavity 56 to form a sealed chamber or space 66 between the diaphragm 68 
and the inner surface of the concavity 56. The diaphragm 68 shown in FIGS. 
1B and 5B is shown without a standoff ledge, however, alternatively, a 
diaphragm having a standoff ledge may be used with the substrate having a 
concavity to form a sealed chamber. 
FIGS. 1C and 5C show a third and preferred embodiment of the hydrophone of 
the present invention wherein the diaphragm 68 having the standoff ledge 
69 (shown in greater detail in FIG. 3) is used in a hydrophone having at 
least two substrates, shown as first substrate 49 and second substrate 50. 
The diaphragm 68 is hermetically sealed at the standoff ledge 69 by a 
suitable means, such as an adhesive or a clamp, placed around the entire 
periphery of the standoff ledge 69 to form a sealed space or chamber 66 
between the diaphragm 68 and an outer surface 54 of the second substrate 
50. 
The first substrate 49 has a concave inner surface that conforms to the 
outer surface of the rigid member mounted on the outer surface of the 
cable 10 and the second substrate 50 has a concave inner surface that 
conforms to the outer surface of the first substrate 49. The damping 
material 51 is placed between the outer surface of the first substrate 49 
and the inner surface of the second substrate 50 to isolate energy present 
in the cable 10 and provide for vibration damping. Preferably, the first 
substrate 49 is made of an aluminum material and the second substrate 50 
of a carbon fiber material. However, both substrates can be made of 
aluminum material, carbon fiber material, or other suitable stiff 
material. Preferably, the first substrate 49 is also thicker in size than 
the second substrate 50. 
The sealed chamber 66 in each embodiment of the present invention usually 
is filled with air or an inert gas such as nitrogen either at the ambient 
pressure or at higher pressure. Also in each embodiment of the present 
invention, an active element 60 in the form of a thin sheet or film of a 
flexible resilient piezoelectric material, preferably a polyvinylidene 
fluoride ("PVDF") material, is placed over the diaphragm 68. The active 
element 60 is preferably bonded to the diaphragm. The active member and 
the diaphragm flex in response to acoustic signals in the form of pressure 
waves "P" (see FIG. 1). The thickness of the PVDF film 60 is preferably in 
the order of 28 microns, although other thicknesses may be used. The 
diaphragm 68 provides spring action to the active element 60 that defines 
response of the active element 60. To make the hydrophone segment, it may 
be desirable to first bond the active member 60 to the diaphragm 68 and 
then bond the diaphragm to substrate 50. In an alternative embodiment, the 
active element 60 may be placed directly on the concavity 56 shown in 
FIGS. 1B and 5B and hermetically sealed along the shoulders 58. 
FIG. 6 shows a sectional view of the diaphragm 68 having the standoff ledge 
69 and including a plurality of convex ribs 53 longitudinally positioned 
on an inner surface of the diaphragm 68. The plurality of convex ribs 53 
is preferably included on the diaphragm 68 in each embodiment of the 
present invention. The convex ribs 53 preserve the active element 60 by 
controlling collapse of the diaphragm 68 under pressure and thereby 
preventing crumpling of the active element 60. The depth and size of the 
convex ribs 53 can be adjusted to provide varying control. 
FIG. 7 shows the electrical connection takeout from the active member 60. 
As shown in FIG. 7, a metalized electrode 70 is deposited over both sides 
of most of the active element 60, leaving an inactive or inert border or 
edge 72 around the electrode member 70. Evaporated silver or silver ink 
are most common electrode compositions although other metals, such as 
gold, may be used. Electrical leads 74 and 76 deliver electrical signals 
to signal conditioner or preamplifier 45. The signals from the signal 
conditioner are passed to the connector 40 via the conductor 47. 
Referring back to FIGS. 1-5, the active element 60 is covered by a layer 80 
of a suitable material. For cable hydrophone applications, the active 
element 60 and the substrate are preferably encapsulated by a polyurethane 
material, exposing only the necessary elements, such as the connector 40, 
to the atmosphere while retaining the overall shape that will provide a 
snug fit of the assembly shown in FIG. 4 on the cable. Holes 28 that match 
the holes 24 in the flanges 22a and 22b are provided along the edges of 
the segment 30 for attaching the segment to the flanges 22a and 22b as 
described earlier. 
In operation, uniformly distributed radial acoustic hydrodynamic transient 
pressure fields, as represented by the inwardly directed arrows "P" such 
as shown in FIG. 1. exert compressive stress along the longitudinal and 
lateral axes of the active element 60, generating a voltage output in 
response to pressure variations. Compensation for changes in hydrostatic 
pressure is provided by the volume of gas in the enclosed chamber 66. As 
the external hydrostatic pressure increases or decreases, inward 
contraction or expansion of the flexible resilient piezoelectric element 
creates corresponding changes in the pressure of the gas chamber 66, thus 
equalizing the internal and external pressures. It has been found that for 
most of the applications in the field of seismic exploration, a hydrophone 
having unpressurized air in the chamber produces adequate response. 
The hydrophone segments 30 and 32 may be configured for optimum electrical 
output at a desired range of operating depths by adjusting the volumetric 
capacity of the chamber 66: a larger volume provides for a wider operating 
range. By reason of its construction, the hydrophone is inherently 
insensitive to acceleration forces. A random localized impact, such as 
might be applied by a sharp object, will result in a small signal, but the 
resulting electrical charge will be dissipated over the entire active 
element 60 and become sufficiently attenuated so as be a virtually 
undetectable. The hydrophone, therefore, is substantially electrically 
inert to localized mechanical forces. Because of the pressure equalization 
by the gas in the chamber 66, the active element is not sensitive to 
hydrostatic pressure variations. 
Thus, the present invention provides a hydrophone having one or more 
hydrophone segments that are electrically coupled to provide a response 
corresponding to a hydrodynamic pressure applied to the segments. Each 
segment contains a chamber on an outer surface that is sealingly enclosed 
by an active member made from a flexible, resilient piezoelectric 
material. A single segment may be used as a hydrophone or a plurality of 
segments may suitably configured and electrically coupled to each other to 
form a hydrophone. 
The foregoing description is directed to particular embodiments of the 
present invention for the purpose of illustration and explanation. It will 
be apparent, however, to one skilled in the art that many modifications 
and changes to the embodiment set forth above are possible without 
departing from the scope and the spirit of the invention. It is intended 
that the following claims be interpreted to embrace all such modifications 
and changes.