`
`(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
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`(58) Field of search
`G1G
`H4J
`Selected US specifications from IPC sub-classes
`G01H G01S G01V H04R
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`1
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`1
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`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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`3
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`FIG.4
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`6
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`FJG.5
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`HALLIBURTON, Exh. 1009, p. 0002
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`2197953
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`2/2
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`r5 521 r6 53
`r3 51 1 r4 52
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`HALLIBURTON, Exh. 1009, p. 0003
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`2197953
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`-1-
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`ACOUSTIC SENSOR
`
`This invention relates to an acoustic sensor.
`
`It
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`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
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`interference with the possible output of the sensor can be
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`reduced.
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`In many applications of an acoustic sensor, the
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`device is required.to detect acoustic signals in an
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`environment where there is a high ambient level of
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`vibration, for example in the presence of machinery noise.
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`One example of this is in the construction of a hydrophone
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`which is intended to be towed behind a marine vessel or to
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`be mounted in a planar arrangement as a flank array on a
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`hull surface of the vessel. The output from such a
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`hydrophone will be heavily influenced by vibration from
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`the vessel's own machinery and this will make it difficult
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`for any weaker signals to be detected. Sometimes it has
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`been proposed that the sensing system which receives
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`signals from the hydrophone will have a built-in
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`capability for cancelling out the unwanted noise. The
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`present invention provides an alternative approach in
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`which the sensor itself has an inherent ability to reject
`
`the unwanted noise.
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`HALLIBURTON, Exh. 1009, p. 0004
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`
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`-2-
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`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
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`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
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`mechanical stresses when in operation.
`
`The elements may be constructed in linear form such
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`as an optical fibre, a length of piezoelectric plastics
`
`material or a piezoelectric rubber strip. Where the final
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`sensor shape is required to be in a planar rather than a
`
`linear form, the shape may be formed from a spirally ~ound
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`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
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`~
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`HALLIBURTON, Exh. 1009, p. 0005
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`
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`-3-
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`accompanying drawing, in which:
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`Figure 1 shows an acoustic sensor element comprising
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`an optical fibre in a jacket of a plastics material;
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`Figure 2 is a cross-sectional view on an enlarged
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`scale of the element of Figure l;
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`Figure 3 shows the element interleaved with a similar
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`element and formed into a spiral transducer;
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`Figure 4 shows a different arrangement where the
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`elements are interleaved into a curvilinear transducer;
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`Figure 5 shows a further arrangement where the
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`elements have been positioned one above the other as a
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`stacked transducer;
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`Figure 6 shows sensor elements arranged in a line to
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`forn a towable array, and;
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`Figure 7 is a circuit diagram of the optical and
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`electronic circuits of the acoustic sensor.
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`As shown in Figures 1 and 2, an acoustic sensor
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`element 1 comprises an optical fibre core 2 which is
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`surrounded by a jacket 3 of a plastics material.
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`In this
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`example, the jacket 3 is of a thermoplastic plastics
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`material and this has resulted in the element 1 becoming a
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`hydrostatically pressure-sensitive sensor.
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`A compensation element 4 may be constructed by taking
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`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
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`
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`-4-
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`hydrostatically pressure-insensitive sensor.
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`The sensitivity of an optical fibre to mechanical
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`strain depends upon the nature of the strain and the
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`encapsulant in which the fibre is embedded. As a
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`generalisation, an encapsulant with high Young's Modulus
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`will always produce low sensitivity. However, an
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`encapsulant with low Young's Modulus and low Poisson's
`
`ratio produces high sensitivity to hydrostatic stress
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`whilst a high Poisson's ratio produces virtually zero
`
`sensitivity to hydrostatic pressure. Thus two coils of
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`fibre identical in every respect save the nature of their
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`secondary jackets can be made to have very different
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`sensitivity to various mechanical loads.
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`The two forms of element may be fastened alongside
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`one another so that in operation they will both be subject
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`to the same spectrum of disturbing phenomena.
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`It is
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`convenient if the two forms of element are joined together
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`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
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`marine vessel for use as a hydrophone.
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`In order to obtain an output signal from the
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`resulting hydrophone, a coherent light pulse or a light
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`pulse pair is launched into the fibre which in the marine
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`environment is being subjected to deforming forces such as
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`HALLIBURTON, Exh. 1009, p. 0007
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`
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`-5-
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`acoustic waves. A suitable sensing system for sensing
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`acoustic waves is disclosed in our published United
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`Kingdom Patent Application No. 2126820A. This system thus
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`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
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`fibre as well as any small proportion of the original
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`pulse that may be transmitted back to the input end by
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`reflection from the splice locaten between the two element
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`bodies. The sensing system then is capable oE providi~
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`an output signal and, by use of the sensing and
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`compensating elements of the present invention, any noise
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`in this signal due to unwanted vibration effects has been
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`substantially reduced.
`
`Figure 3 shows a different construction of acoustic
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`sensor where the twin pair of elements comprising a
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`sensor element 1 and compensation element 4, joined at one
`
`end by a semi-reflecting splice 6, have been coiled into a
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`flat spiral. After a suitable encapsulation process to
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`give mechanical protection to the sensors, this
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`construction forms a planar pattern of sensor which could
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`for example be mounted as a flank array on the hull
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`surface of a marine vessel.
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`Figure 4 shows a different arrangement where the
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`interleaved elements have been folded to forin a
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`curvilinear transducer. This arrangement wouln be also
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`suitable for mounting on a flat surface such as the hull
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`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
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`thickness of the two layers of the device and the
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`construction is supported in a body 7 of an encapsulation
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`material.
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`Figure 6 shows a number of linear optical fibre
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`sensor elements arranged in a line to form a towable
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`array. The sensor elements 1 have references 81, 82, 83,
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`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
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`of system phase noise as disclosed in our copending Uniter~
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`Kingdom Patent Application No. 8525924.
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`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
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`affected.
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`HALLIBURTON, Exh. 1009, p. 0009
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`-7-
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`In this construction, the light· pulses enter the
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`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
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`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
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`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
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`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
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`the array are returned to the optical circuit. The
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`-~'
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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
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`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
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`connection with Figure 6.
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`It will be apparent that the arrangement in the
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`towable array of Figure 6 of a sensor element together
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`with a compensation element, arranged such that they will
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`both be subject to the same spectrum of disturbing
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`HALLIBURTON, Exh. 1009, p. 0011
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`-9-
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`phenomena, has given the resulting acoustic sensor a
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`built-in ability to reject noise due to unwanted vibration
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`effects since the two elements react differently to
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`hydrostatic pressures. The electronic circuit can of
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`course be used to provide compensation against further
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`disturbances such as microphony and sensitivity to inputs
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`other than acoustic.
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`The foregoing description of embodiments of the
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`invention has been given by way of example only and a
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`number of modifications may be made without departing from
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`the scope of the invention as defined in the appended
`
`claims. For instance, although the principle of the
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`invention has been described in connection with an optical
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`fibre hydrophone, the invention will also be suitable for
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`other types of linear array such as a cable made from a
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`polyvinylidene fluoride plastics material, a piezoelectric
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`ceramic material or a ceramic loaded rubber. Other
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`configurations of sensor such as a planar shape which may
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`be attached to a surface are also possible. Since the
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`principle of the invention relies on the use of different
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`coatings to provide different sensitivities in the two
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`parts of the sensor, the invention may also be applied to
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`other devices such as an optical fibre magnetometer.
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`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
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`which the said elements are fibre optic devices.
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`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
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`comprising an acoustic sensor as claimed in Claim 4.
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`6.
`
`An acoustic sensor substantially as hereinbefore
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`described with reference to the accompanying drawing.
`
`HALLIBURTON, Exh. 1009, p. 0014
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`
`
`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.
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`HALLIBURTON, Exh. 1009, p. 0015
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`
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`I ?i'
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`-
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`4.
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`A sensor as claimed in any one of Claims l to 3. in
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`whicp the two elements are shaped so as to form a spirally
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`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.
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`6.
`
`An optical fibre hydrophone or magnetometer
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`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.
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`HALLIBURTON, Exh. 1009, p. 0016
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