Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Reexamination Certificate
1999-12-22
2003-06-17
Lobo, Ian J. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
C367S020000, C367S153000
Reexamination Certificate
active
06580661
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a hydrophone array and, in particular, to an array for use in seismic exploration and like applications.
BACKGROUND ART
Hydrophone arrays are used to perform active and passive sound imaging. One significant application of such arrays is in the field of geophysical surveying of seabeds. Hydrophone arrays generally utilise a plurality of acoustic transducers, spaced at a controlled spacing, for providing data which cannot be readily obtained from a single hydrophone.
SUMMARY OF THE INVENTION
According to a first aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of housings, each housing enclosing at least two acoustic transducers.
In a preferred embodiment, each transducer has an independent output and is able, in use, to sense the same tensor component of an elastic wave in the medium surrounding the hydrophone array.
The acoustic transducers act as acoustic receivers and acoustic transmitters as requirements dictate during use of the array. In one embodiment, the acoustic transducer can comprise a tube formed at least in part of piezoelectric material. The piezoelectric material can comprise an active polarised ceramic material, such as barium titanate or lead zirconate titanate (PZT). The piezoelectric material can, in another embodiment, be a piezoelectric polymer material, such as polyvinylidene fluoride (PVDF), or a piezo-rubber composite material.
The tube comprised at least in part of piezoelectric material can be substantially cylindrical. The tube can be fabricated from two segments or a greater number of segments. The tube can be sectioned longitudinally into longitudinal segments. The inner and outer faces of the segments can each have an electrically conductive material covering at least a part of each face and thereby provide an electrode for each face. Each electrode could comprise a coating of metal, such as silver, gold or aluminium or an intrinsically or extrinsically conductive polymer. Electrical connection to the faces can be facilitated by electrically conductive leads attached to the electrodes. The attachment of the leads to the electrodes can be through use of a conductive epoxy or adhesive metallic tape.
In the case where the tube is formed of a plurality of segments, the segments are preferably arranged such that electrical connection is provided between the respective inner faces of the segments and the respective outer faces of the segments. In one embodiment, the segments may be in an abutting arrangement with short electrical leads or conductive tape providing the electrical connection between the respective faces of the segments.
In a preferred embodiment of the invention, the piezoelectric polymer film can be bonded to a tubular support structure. In a still further embodiment, the support structure can comprise at least an inner and outer tube, with the inner tube mounted within the outer tube. The tubes preferably share a common longitudinal axis. An annular space is preferably located between the inner and outer tubes and extends at least a substantial length of the tubular structure. The inner surface of the outer tube is preferably bonded to the piezoelectric polymer film. The support structure can be fabricated from polycarbonate. The outer surface of the outer tube can also be coated with a metallic layer. This metallic layer can comprise a layer of metallic paint, such as spray paint, including nickel, gold or silver conductive paints. The outer metallic coat can have at least one electrically conductive wire connected thereto which is in turn connected to a shield or drain wire in the array.
In an alternative embodiment of the invention, the acoustic transducer can include one or more capacitors which are adapted to undergo a change in capacitance that is proportional to changes in incident pressure. In another embodiment, the acoustic transducer can comprise one or more resistors adapted to provide a varying resistance output in response to changes in incident pressure. In a still further embodiment, the acoustic transducer can comprise one or more inductors adapted to provide a varying inductance output in response to changes in incident pressure. In a still further embodiment, the acoustic transducer includes a combination of capacitors, resistors or inductors to provide an output that varies in response to changes in incident pressure. The varying outputs of the previously described acoustic transducers can be adapted to modulate the oscillation of a modulator and so provide a quantitative measurement of variations in incident pressure.
In a still further embodiment, the acoustic transducer can comprise a fiber optic transducer as is known in the art. Such hydrophones typically rely upon measuring changes in the behaviour of the light passed through a fiber optic guide due to acoustic waves being incident on the hydrophone. As an example only of one type of fiber optic hydrophone that can be used in the present invention, the fiber optic transducer can include an optical reflector that can undergo a displacement that is responsive to acoustic waves. A beam of light from a light source that is carried by a first group of fiber optic guides can be incident on the reflector. The light reflected from the reflector can be carried by a second group of fiber optic guides to a light detector. Any displacement of the reflector due to pressure waves impinging on the reflector are detected by changes in intensity of the reflected light from the light source.
In an alternative example of a fiber optic hydrophone, the hydrophone can include a first fiber optic acoustic wave detector which is subjected to incident acoustical waves. A similar fiber optic is also provided in the hydrophone in an acoustically isolated compartment where the optic fiber is not affected by the incident acoustical waves. A comparison of the light passing through each fiber optic can be used to determine the presence and magnitude of the incident acoustical wave on the hydrophone.
In a preferred embodiment, each hydrodynamic housing preferably encloses two acoustic transducers. A majority of the combined length of the two transducers is preferably positioned forward of the middle of each housing to minimise noise from turbulent boundary layer flow. The acoustic transducers are preferably arranged to be mounted around a cable. The transducers are preferably mounted symmetrically around the cable. The hydrophone array can be used as a towed, vertical or seismic downhole array. It is particularly suited to a towed sensor arrangement, where the cable is towed behind a geophysical surveying vessel. The cable could also be adapted to be laid on the sea-bed rather than towed behind a vessel in use as described above.
In another embodiment, each acoustic transducer can include:
a tube comprised at least in part of piezoelectric material and having an inner and outer surface;
a first shell member, having a first edge and a second edge, disposed outside the tube and extending longitudinally of the tube, the first and second edges being connected to or bearing against the outer surface of the tube; and
a second shell member, having a first and second edge disposed inside the tube and extending longitudinally of the tube, the first and second edges being connected to or bearing against the inside of the tube.
In this embodiment, the first and second shell members can extend the full longitudinal length of the tube and also preferably encase the outer and inner surfaces of the tube. Each transducer, in this embodiment, preferably has a shape which when viewed in perspective is substantially toroidal. The first and second shell members call be fabricated from a metal, such as an aluminium or bronze alloy to facilitate electrical connection. When each transducer, as described in this embodiment, is used as a receiver, the acoustic signal impinges as a pressure wave on the shell members causing the shell members to flex inwardly and outwardly in response to the signal.
Holt Graham E
Marschall Debra L
Marschall Richard Anton
Jordan and Hamburg LLP
Lobo Ian J.
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