(12) UK Patent Application
`
`(19) GB (11)
`
`(21) A8plication No 8628367
`
`(22) Date of filing 27 Nov 1986
`
`(71) Applicant
`The Plessey Company pie
`
`(Incorporated in United Kingdom)
`
`Vicarage Lane, llford, Essex
`
`(72) Inventors
`Michael Laurence Henning
`Christopher Lamb
`David Havard
`
`(74) Agent and/or Address for Service
`E. Pritchard
`The Plessey Company pie,
`Intellectual Property Department,
`Vicarage Lane, llford, Essex
`
`(54) Acoustic Sensor
`
`(57) An acoustic sensor comprises a sensor element 1 and a
`compensation element 4, the two elements having different
`sensitivities to mechanical loads, such that element 1 is
`sensitive to ambient pressure changes and element 4 is
`not, and being positioned together such that they will both be
`subject to the same spectrum of disturbing phenomena,
`outputs from the two elements 1, 4 being applied to suitable
`circuit means to provide an acoustic output signal in which
`noise due to unwanted vibration effects is substantially
`reduced.
`The sensor element 1 may be an optical fibre core
`surrounded by a jacket of a plastics material and the compen(cid:173)
`sation element 4 may be a similar core with a rubber jacket.
`The construction is suitable for a linear or a planar optical
`fibre sensor array, e.g. hydrophones. Alternative sensor
`elements may be piezoelectric plastics or ceramic material or
`ceramic loaded rubber. Application to an optical fibre
`magnetometer is mentioned.
`
`(43) Application published 2 Jun 1988
`
`(51) INT CL'
`G01 S 3/86 G01 H 9/00 G01V 1/36 H04R 1/44
`
`(52) Domestic classification (Edition J):
`G1G 3A 38 GA 9X ED
`H4D 265 72X 749 751 759 775 776 LOF
`H4J 30H 30N 31J AB
`U1S 1915 2141 G1G H4D H4J
`
`(56) Documents cited
`GB A 2184237
`
`GB 1088469
`
`(58) Field of search
`G1G
`H4J
`Selected US specifications from IPC sub-classes
`G01H G01S G01V H04R
`
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`1
`
`The drawings(s) originally filed was (were) informal and the print here reproduced is taken from a later filed formal copy.
`
`HALLIBURTON, Exh. 1009, p. 0001
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`1/2
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`2197953
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`HALLIBURTON, Exh. 1009, p. 0002
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`2197953
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`2/2
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`HALLIBURTON, Exh. 1009, p. 0003
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`

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`2197953
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`-1-
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`ACOUSTIC SENSOR
`
`This invention relates to an acoustic sensor.
`
`It
`
`relates particularly to a sensor construction that can be
`
`used to form a linear or a planar arrangement of sensitive
`
`pattern and in which the effect of one source of
`
`interference with the possible output of the sensor can be
`
`reduced.
`
`In many applications of an acoustic sensor, the
`
`device is required.to detect acoustic signals in an
`
`environment where there is a high ambient level of
`
`vibration, for example in the presence of machinery noise.
`
`One example of this is in the construction of a hydrophone
`
`which is intended to be towed behind a marine vessel or to
`
`be mounted in a planar arrangement as a flank array on a
`
`hull surface of the vessel. The output from such a
`
`hydrophone will be heavily influenced by vibration from
`
`the vessel's own machinery and this will make it difficult
`
`for any weaker signals to be detected. Sometimes it has
`
`been proposed that the sensing system which receives
`
`signals from the hydrophone will have a built-in
`
`capability for cancelling out the unwanted noise. The
`
`present invention provides an alternative approach in
`
`which the sensor itself has an inherent ability to reject
`
`the unwanted noise.
`
`HALLIBURTON, Exh. 1009, p. 0004
`
`

`

`-2-
`
`According to the invention, there is provided an
`
`acoustic sensor comprising a sensor element and a
`
`compensation element, the two elements having different
`
`sensitivities and being positioned together such that they
`
`will both be subject to the same spectrum of disturbing
`
`phenomena, outputs from the two elements being applied to
`
`suitable circuit means to provide an acoustic output
`
`signal in which noise due to unwanted vibration effects is
`
`substantially reduced.
`
`Preferably, the elements comprise similar core
`
`constructions which are encapsulated in jacket materials
`
`having differing sensitivities to mechanical loads, such
`
`that one element is sensitive and the other element is
`
`insensitive to ambient pressure changes. The two elements
`
`may be located together in an interleaved arrangement
`
`whereby they will be subjected to similar acoustic and
`
`mechanical stresses when in operation.
`
`The elements may be constructed in linear form such
`
`as an optical fibre, a length of piezoelectric plastics
`
`material or a piezoelectric rubber strip. Where the final
`
`sensor shape is required to be in a planar rather than a
`
`linear form, the shape may be formed from a spirally ~ound
`
`or folded arrangement of the linear form.
`
`By way of example, some particular embodiments of the
`
`invention will now be described with reference to the
`
`~
`
`HALLIBURTON, Exh. 1009, p. 0005
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`

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`-3-
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`accompanying drawing, in which:
`
`Figure 1 shows an acoustic sensor element comprising
`
`an optical fibre in a jacket of a plastics material;
`
`Figure 2 is a cross-sectional view on an enlarged
`
`scale of the element of Figure l;
`
`Figure 3 shows the element interleaved with a similar
`
`element and formed into a spiral transducer;
`
`Figure 4 shows a different arrangement where the
`
`elements are interleaved into a curvilinear transducer;
`
`Figure 5 shows a further arrangement where the
`
`elements have been positioned one above the other as a
`
`stacked transducer;
`
`Figure 6 shows sensor elements arranged in a line to
`
`forn a towable array, and;
`
`Figure 7 is a circuit diagram of the optical and
`
`electronic circuits of the acoustic sensor.
`
`As shown in Figures 1 and 2, an acoustic sensor
`
`element 1 comprises an optical fibre core 2 which is
`
`surrounded by a jacket 3 of a plastics material.
`
`In this
`
`example, the jacket 3 is of a thermoplastic plastics
`
`material and this has resulted in the element 1 becoming a
`
`hydrostatically pressure-sensitive sensor.
`
`A compensation element 4 may be constructed by taking
`
`an identical core 2 and forming the jacket 3 of a rubber
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`' ·•
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`composition. This results ~n the element becoming a
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`HALLIBURTON, Exh. 1009, p. 0006
`
`

`

`-4-
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`hydrostatically pressure-insensitive sensor.
`
`The sensitivity of an optical fibre to mechanical
`
`strain depends upon the nature of the strain and the
`
`encapsulant in which the fibre is embedded. As a
`
`generalisation, an encapsulant with high Young's Modulus
`
`will always produce low sensitivity. However, an
`
`encapsulant with low Young's Modulus and low Poisson's
`
`ratio produces high sensitivity to hydrostatic stress
`
`whilst a high Poisson's ratio produces virtually zero
`
`sensitivity to hydrostatic pressure. Thus two coils of
`
`fibre identical in every respect save the nature of their
`
`secondary jackets can be made to have very different
`
`sensitivity to various mechanical loads.
`
`The two forms of element may be fastened alongside
`
`one another so that in operation they will both be subject
`
`to the same spectrum of disturbing phenomena.
`
`It is
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`convenient if the two forms of element are joined together
`
`by a semi-reflecting splice 6 at one end so that the
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`elements are effectively connected in series. This
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`construction is then capable of being towed behind a
`
`marine vessel for use as a hydrophone.
`
`In order to obtain an output signal from the
`
`resulting hydrophone, a coherent light pulse or a light
`
`pulse pair is launched into the fibre which in the marine
`
`environment is being subjected to deforming forces such as
`
`HALLIBURTON, Exh. 1009, p. 0007
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`

`

`-5-
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`acoustic waves. A suitable sensing system for sensing
`
`acoustic waves is disclosed in our published United
`
`Kingdom Patent Application No. 2126820A. This system thus
`
`acts to control the production of the light pulses and it
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`receives an output light signal at an output end of the
`
`fibre as well as any small proportion of the original
`
`pulse that may be transmitted back to the input end by
`
`reflection from the splice locaten between the two element
`
`bodies. The sensing system then is capable oE providi~
`
`an output signal and, by use of the sensing and
`
`compensating elements of the present invention, any noise
`
`in this signal due to unwanted vibration effects has been
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`substantially reduced.
`
`Figure 3 shows a different construction of acoustic
`
`sensor where the twin pair of elements comprising a
`
`sensor element 1 and compensation element 4, joined at one
`
`end by a semi-reflecting splice 6, have been coiled into a
`
`flat spiral. After a suitable encapsulation process to
`
`give mechanical protection to the sensors, this
`
`construction forms a planar pattern of sensor which could
`
`for example be mounted as a flank array on the hull
`
`surface of a marine vessel.
`
`Figure 4 shows a different arrangement where the
`
`interleaved elements have been folded to forin a
`
`curvilinear transducer. This arrangement wouln be also
`
`suitable for mounting on a flat surface such as the hull
`
`of a vessel.
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`HALLIBURTON, Exh. 1009, p. 0008
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`

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`-6-
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`Figure 5 is a side view of a stacked transducer where
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`the semi-reflecting splice 6 has been located within the
`
`thickness of the two layers of the device and the
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`construction is supported in a body 7 of an encapsulation
`
`material.
`
`Figure 6 shows a number of linear optical fibre
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`sensor elements arranged in a line to form a towable
`
`array. The sensor elements 1 have references 81, 82, 83,
`
`etc. whilst the insensitive vibration compensation
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`elements 4 with references 81', 82 1
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`, S3' etc. are
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`arranged to be interleaved with the sensor elements 1. A
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`reference sensor R is providea to cancel out the effects
`
`of system phase noise as disclosed in our copending Uniter~
`
`Kingdom Patent Application No. 8525924.
`
`Se~i-reflecting
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`splices rl, r2, r3, etc. are located at the junctions
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`between the sensor and compensation elements.
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`When the towed array is being used under water, it
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`receives, in operation, acoustic signals al, a2, a3, etc.
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`which impinge on the sensor elements 81, 82, 83, of the
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`array. Of course, the acoustic signals al, a2, a3, etc.
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`similarly impinge on the compensation elements 81',
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`82', 83' etc. but these elements have a built-in lack
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`of sensitivity to hydrostatic pressures and they are not
`
`affected.
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`HALLIBURTON, Exh. 1009, p. 0009
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`

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`-7-
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`In this construction, the light· pulses enter the
`
`array from the left hand end and pass through the first
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`semi-reflecting splice rl, then the reference sensor R and
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`enter the second semi-reflecting splice r2. At each
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`semi-reflecting splice, a small proportion of the signal
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`passing along the fibre is reflected back to the beginning
`
`of the fibre whilst the remainder of the signal passes
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`through the splice and enters the next length of the
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`optical fibre in the array.
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`After the second splice r2, the entering light p~lses
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`pass into the first sensor element Sl which is capable of
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`being disturbed by the acoustic signal al that may be
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`present in the aquatic environment where the towable array
`
`is being used. At the end of the sensor element Sl, the
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`entering light pulses pass through the third splice r3 and
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`enter a further sensor element Sl' which is arranged to be
`
`in a vibration compensation situation with the sensor
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`element Sl as has been already described.
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`The light pulses entering the array at the left hanrl
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`end as shown in Figure 6 are produced by an optical
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`circuit as depicted at the left hand side of Figure 7.
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`Similarly, the pulses reflected back from the splices in
`
`the array are returned to the optical circuit. The
`
`-~'
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`optical circuit depicted is similar to that nisclosea in
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`our aforementioned copending patent application, where a
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`HALLIBURTON, Exh. 1009, p. 0010
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`-8-
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`laser 8 produces light pulses that are directed through a
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`Bragg cell 9 and into a downlead 11 leading to the sensor
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`array.
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`Light pulses returned from the array pass back along
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`the downlead 11 and into the Bragg cell 9 where they are
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`deflected via a mirror 12 onto a photo detector 13. The
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`photo detector 13 forms part of the electronic circuit
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`depicted at the right hand side of Figure 7.
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`The electronic circuit shown is similar to that
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`disclosed in the aforementioned copending patent
`
`application where the photo detector 13 is connected
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`through an amplifier 14 to demultiplexing means 16. The
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`demultiplexed signals together with a reference signal are
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`fed via band-pass filters 17 to demodulators 18. Phase
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`noise and microphony compensation is provided by
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`difference amplifiers 19 and further difference amplifiers
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`21 give vibration and acceleration compensation. Finally,
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`electrical output signals al and a2 are produced and these
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`are proportional to the acoustic signals which impinged on
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`the sensor elements of the array as described in
`
`connection with Figure 6.
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`It will be apparent that the arrangement in the
`
`towable array of Figure 6 of a sensor element together
`
`with a compensation element, arranged such that they will
`
`both be subject to the same spectrum of disturbing
`
`HALLIBURTON, Exh. 1009, p. 0011
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`

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`-9-
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`phenomena, has given the resulting acoustic sensor a
`
`built-in ability to reject noise due to unwanted vibration
`
`effects since the two elements react differently to
`
`hydrostatic pressures. The electronic circuit can of
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`course be used to provide compensation against further
`
`disturbances such as microphony and sensitivity to inputs
`
`other than acoustic.
`
`The foregoing description of embodiments of the
`
`invention has been given by way of example only and a
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`number of modifications may be made without departing from
`
`the scope of the invention as defined in the appended
`
`claims. For instance, although the principle of the
`
`invention has been described in connection with an optical
`
`fibre hydrophone, the invention will also be suitable for
`
`other types of linear array such as a cable made from a
`
`polyvinylidene fluoride plastics material, a piezoelectric
`
`ceramic material or a ceramic loaded rubber. Other
`
`configurations of sensor such as a planar shape which may
`
`be attached to a surface are also possible. Since the
`
`principle of the invention relies on the use of different
`
`coatings to provide different sensitivities in the two
`
`parts of the sensor, the invention may also be applied to
`
`other devices such as an optical fibre magnetometer.
`
`HALLIBURTON, Exh. 1009, p. 0012
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`

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`-10-
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`CLAIMS
`
`1.
`
`An acoustic sensor comprising a sensor element and a
`
`compensation element, the two elements having different
`
`sensitivities and being positioned together such that they
`
`will both be subject to the same spectrum of disturbing
`
`phenomena, outputs from the two elements being applied to
`
`suitable circuit means to provide an acoustk output
`
`signal in which noise due to unwanted vibration effects is
`
`substantially reduced.
`
`2.
`
`A sensor as claimed in Claim 1, in which the elements
`
`comprise similar core constructions which are encapsulated
`
`in jacket materials having differing sensitivities to
`
`mechanical loads, such that one element is sensitive and
`
`the other element is insensitive to ambient pressure
`
`changes.
`
`3.
`
`A sensor as claimed in Claim 2, in which the two
`
`elements are located together in an interleaved
`
`arrangement whereby they will be subjected to similar
`
`acoustic and mechanical stresses when in operation.
`
`4.
`
`A sensor as claimed in any one of Claims 1 to 3, in
`
`which the said elements are fibre optic devices.
`
`HALLIBURTON, Exh. 1009, p. 0013
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`

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`-11-
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`5.
`
`An optical fibre hydrophone or magnetometer
`
`comprising an acoustic sensor as claimed in Claim 4.
`
`6.
`
`An acoustic sensor substantially as hereinbefore
`
`described with reference to the accompanying drawing.
`
`HALLIBURTON, Exh. 1009, p. 0014
`
`

`

`12
`
`The preceding claims have been superseded by
`the following claims:-
`
`CLAIMS
`
`1.
`
`An acoustic sensor comprising a sensor element and a
`
`compensation element. the two elements having different
`
`sensitivities and being positioned together such that they
`
`will both be subject to the same spectrum of disturbing
`
`phenomena, outputs from the two elements being applied to
`
`suitable circuit means to provide an acoustic output
`
`signal in which noise due to unwanted vibration effects is
`
`substantially reduced. in which the elements comprise
`
`similar core constructions which are encapsulated in
`
`jacket materials having differing sensitivities to
`
`mechanical loads, such that one element is sensitive and
`
`the other element is insensitive to ambient pressure
`
`changes.
`
`2.
`
`A sensor as claimed in Claim 1, in which the two
`
`elements are located together in an interleaved
`
`arrangement whereby they will be subjected to similar
`
`acoustic and mechanical stresses when in operation.
`
`3.
`
`A sensor as claimed in Claim 1 or 2 in which the said
`
`elements are fibre optic devices.
`
`HALLIBURTON, Exh. 1009, p. 0015
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`

`

`I ?i'
`
`-
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`4.
`
`A sensor as claimed in any one of Claims l to 3. in
`
`whicp the two elements are shaped so as to form a spirally
`
`wound transducer.
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`5.
`
`A sensor as claimed in any one of Claims 1 to 3, in
`
`which the elements are shaped in a spirally wound or
`
`folded arrangement to give a planar form of transducer.
`
`6.
`
`An optical fibre hydrophone or magnetometer
`
`comprising an acoustic sensor as claimed in any one of
`
`Claims l to S.
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`7.
`
`An acoustic sensor substantially as hereinbefore
`
`described with reference to the accompanying drawing.
`
`HALLIBURTON, Exh. 1009, p. 0016
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`