GB2518216A

GB2518216A - Non-isotropic acoustic cable

Info

Publication number: GB2518216A
Application number: GB1316362.1A
Authority: GB
Inventors: Daniel Finfer; Dimitry Strusevich; Sergey Shatalin
Original assignee: Silixa Ltd
Current assignee: Silixa Ltd
Priority date: 2013-09-13
Filing date: 2013-09-13
Publication date: 2015-03-18
Anticipated expiration: 2033-09-13
Also published as: GB2518216B; GB201316362D0

Abstract

Directional acoustic sensing is achieved by adapting the acoustic response of a fibre optic cable so as focus the acoustic sensitivity of the cable directionally along the entire length of the fibre optic cable, such that the acoustic sensitivity is non isotropic in the plane normal to the length of the fibre optic cable. The adapting of the acoustic response may be performed by providing regions of filler 104 which bridge the gap between the optical fibre arrangement 100 and an outer layer 101, by selectively providing regions of acoustic insulation within an acoustically reactive mass (300,301 fig.10), by off-centre positioning of the fibre in acoustic insulation (fig. 12), by proving the fibre in square or star-shaped acoustic insulation (fig. 14, fig. 16), or by arranging the fibre in a casing that has regions of high and low acoustic coupling (fig. 18).

Description

Nonisotropic Acoustic Cable

Lethnical Field

The present invention relates to a I ibre optic cable, and in some embodiments provides a fibre optic cable which has an acoustic sensitivity that is non-isotropic. In another aspect there is provided a fibre optic cable which comprises an array of discrete acoustic coupling regions.

Background to the Invention and Prior A**

To detect an acoustic signal, distributed acoustic sensing is commonly and effectively used.. This method employs fibre optic' cables to provide distributed acoustic sensing 1 0 whereby the fibre optic cable acts as a string of discrete acoustic sensors,, and an optoeieetronic device measures and processes the returning signal. The operations of such a device is described next.

A pulse of light is sent into the optical fibre, and a small amount of light is naturally back seattered along the length of the fibre by Rayleigh,. BrilIiouin and Raman scattering mechanisms. The scattered light is captured by the fibre and carded back towards the source where the returning signal is measured against tine, allowing measurements in the amplitude, frequency and phase of the scattered light to be determined, .11 an acoustic wave is incident upon the cable, the glass strncture of the optical fibre is catised to contract and expand within the vibro-acoustip fie1d consequently varying the optical path lengths between the back scattered light scattered from different locations airing the fibre, This variation in path length is measured as a relative phase change. allowing the optical phase angle data to be used to measure the position of the acoustic. signal at the point at which light is reflected. The returning signal can also be processed in order to determine the frequency of oscillation of vihraiion in the structure, in known distributed acoustic sensing systems (D.AS), standard fibre optic cables are utilised to obtain a measurement profile from along the entire length of the fibre at intervals ranging from 1-10 metres. Further details regarding the operation of a suitable DAS system, such as the iDASIM, available from Silixa Limited, of Elstree. UK are given in W020 10/0 i 36809. Systems such as these are able to digitally record acousti.c i fields at every interval location along an optical. fibre at frequencies up to I 00kHz, Since the position of the acoustic sensors is known (the fibre depicyment being known). the source of any acoustic signal can he thus identified by means of tirneott arrival calculations. In a typical deploym.çnt, the sensing points are usually created by S clamps which are used to secure the fibre optic cable to the structure or area ii is monitoring.

By way of example, Figure 1 shows a common arrangement of a known fibre optic cable I, comprisin.g at least one optical fibre, contained in a series of concentric tubular structures. The cable generally comprises firstly an inner tubular structure, typically called a fibre-in-metal-tube (FIIMT) 2, which provides a way of encapsulating very long lengths of optical fibres 5 within a hermetically sealed tube 4A gene ml construction of a FIMT 2 includes at least one optical fibre 5 encapsulated in a metal tube 4.

Additionally, it is common to fill this'metal tube 4 with a thixotropic gel 6 it. order to protect the optical fibres S from environmental disturbances, prevent damage from 1 5 micro-bending conditions and to help imnimise the forces applied during spooling and deployment of the cable. Most importantly for distributed acoustic sensing, the thixotropic gel 6 supports the optical fibre 5, preventing excessive movement within the metal tube 4 which reduces the amount of resonant frequencies. The FIMT 2 is typically then encapsulated by a further outer tube 3 usually containing a filler material.

The optical fibres 5 are typically made of flexible., transparent fibres of glass. The filler material 3 surrounding the FIMT 2 has a lower refractive index than the optical fibres 5 such that light which has been tbeused into the optical fibres 5 is confined due to total internal reflection, hence enablIng the light to propagate down the length of the optical fibres 5 without any light being lost.

There are many applications for which distrbuted acoustic sensing may be used, for example, monitoring hydraulic fracturing of oil or gas structures and surveillance methods of assets such as oil or gas pipelines and airport runways. in order to monitor such assets, the fibre optic cables are usually secured to the structure or area by clamps distributed along the length of the fibre optic cable. By way of example, Figure 2 illustrates how l5hre optic cables I may be used to monitor structures or areas using distributed acoustic sensing..

Figure 2 shows a fibre c ptic cable 1 being used to* *ionitc)r a pipeline 7 that has been deployed underground 9. The fibre optic cable 1 is positioned to run parallel alongside the pipeline 7 and is secuied by a senec of clamps 8, which are distributed along the length of' the pipeline 7. These clamps 8 allow the fibre optic cabl.e I to monitor the S pipeline 7 through distributed acoustic sensing since the clamps 8 themselves act as an array of acoustic coupling regions. The clamps 8 transmit any vibrations in the pipeline 7, such that the acoustic energy is transmitted to the optical fibres. 5.

The clamps are spaced along the fibre at a distance at least equai to or greater than the sensing resolution of the distributed acoustic sensing, typically 1-5 metres. This provides discrete sensing points along the fibre matched to the sensing resolution and prevents any ancialiasing effects in the detected acoustic signal.

in some deployments, however, it is not possible to secure the cable with clamps, and instead the cable may be inserted in a concrete trench or the like running parailei to a pipe, well, or any other structure being monitored. in this case there are no discrete IS sensing points as.is provided by the clamps., and hence the fibre can sense at all points along its length.

As a consequence, due to the sensing resolution of the fibre being less than the actual resolution of the points at which acoustic energy is being sensed, aliasing effects can occur in the signal, due to undersampling.

Another problem faced when using fibre optic cables in distributed acoustic sensing is that acoustic signals incoming from one direction may overcome acoustic signals incoming ft om another direction, making it difficult for the fibre: optic cabl.e to detect the latter. This may prove problematic for certain applications of distributed acoustic sensing. Consider, by way of example, fibre optic cables used for surveillance of an asset. Acoustic signals emitted by the asset itself may obscure any acoustic signals incoming from the surroundings towards the asset. 1-lowever, in security surveillance, it is the incoming acoustic signals caused by disturbances in regions surroundIng the asset that are of interest. Therefore, it would be advantageous if the fibre optic cable was more acoustically sensitive in the directions, corresponding to the surrounding area such that. the ability to detect acoustic signals in these direction. is greater.

Summary of the Invention

Embodiments of the prescnt. invention address the above noted abasing problem by providing a plurality of discrete acoustic sensors along a cable whereby acoustic signals may be measured in situations where th:e fibre optic cable has. not been secured to a structure or area by a series of clamps, as described in the prior at, Acoustic sampling points are achieved by selectively enhancing the acoustic coupling between an outer layer and at least one optical fibre arrangement, such that acoustic eneigy may be transmitted selcetively from the outer layer to the at least one optical fibre arrangement.

The resulting regions of acoustic coupling along the cable allow the optical fibre to 10: c:eteet acoustic signals..

Further embodiments of this invention address the above noted problems associated with interfering acoustic signals by providing directional acoustic*sensing This is achieved by adapting the acoustic response of a fibre optic cable so as fbcus the acoustic sensitivity of the fibre optic cable directionally along the entire length of the 1 5 fibre optic cable, such that the acoustic sensitivity is non isotropic in the plane normal to the length of the cable. In sonic embodiments, the adapting of the acoustic response is performed by the provision of' an acoustically reactive mass, such as a sleeve or coating, being placed around the fibre optic cable, the mass having spatial acoustic properties required to give the directional acoustic response required.

2.0 Acording to one aspect of the present invention, there is provided a fibre optic cable, comprising at least one optical fibre arrangement, and at feast one outer layer. The fibre optic cable further comprises an acoustic insulating layer between thc at least one optical fibre arrangement and the outer layer, the insulating layer being interspersed along the length of the fibre opti.c cable.. ith discrete acoustic coupling regions for transmitting acoustic energy front the outer layer to the at least one optical fibre arrangement.

Preferably, the at least one optical fibre arrangement comprises a fibre-in-metal-tube (FiMT), as described in the above prior art. Ibis is a standard and widely used arrangement fbr the. cores of fibre optic cables, therefore existing cables which have already been deployed may be conveniently adapted to incorporate the features of the present invention, In some embodiments. the acoustic insulating layer includes a layer of air. Air is a matenal with low acc.ustic coupling such that it effectively absorbs acoustic energy and reduces its transmission. Acoustic coupling relates io the resistance of the materials particles to the mechanical vibrations of an acoustic signal. That is to say, materials that do not resist the mechanical vibrations easily couple with the mechanical vibrations and have high acoustic coupling properties. Since air particles provide a large amount of resistance to the vibrations, air exhibits low ac.ustic coupling and is considered to be a god acoustic insulating material, which i.s also convenient and cost-effective to use.

In a preferred embodiment of the invention, a filler is inserted between the at least one optical fibre arrangement and the outer layer. The filler comprises of built up regions intorspersed along the of the fibre optic cable, wherein the built up regions of tiller provide the discrete acoustic coupling regions.

The built up regions of filler connect the at least one optical fibre arrangement and the outer layer such that acoustic energy can be transmitted between them, therefore 1 5 enhancing the acoustic coupling between the at least one optical fibre arrangement and the outer layer. Therefore, fibre optic cables with built in discrete acoustic coupling regiors may be deployed and used to detect acoustic signals without the use of clamps securing them to the monitored structure or area.

In another preferred embodiment of the invention at least one layer concentrically outside the acoustic insulating layer is narrowed at points interspersed along the length of the fibre optic cable so as to divide the acoustic insulating layer and provid.e discrete acoustic coupling regions.

The narrowed points bridge the gap between the at least one optical fibre arrangement and the. outer layer such that acoustic e:n.ergy can be transmitted between them, therefore enhancing the acoustic coupling between the at least one optical fibre arrangement and the outer layer, Therefore, fibre optic cables with built in discrete acoustic coupling regions may be dcployed and used to detect acouslic signals without the use of clamps securin.g them to the monitored structure or area, Preferably, the narrowed points are achieved by crimping the fibre optic cable at points interspersed along its length such that tie. inner face of the outer layer immediately next to the insulating layer comes into contact with the layer inwards of the insulating layer towards the. at least, one optical fibre arrangement. In doing this, the crimped portions effectively shortcircuit the insulating layer to provide the discrete acoustic coupling legions.

Preferably, the distance between acoustic coupling regions is at least 1 metre. This ensures that the sensing resolution of the fibre matches the actual, resolution of the points at which acoustic energy is being sensed so as to avoid aliasing effects as a result of undersamp'l kg.

In a further embodiment, the size of the discrete acoustic coupling regions along the length of the fibre optic cable i.s at most' 50cm. This i.s a suitable size value that ensures that the acoustic coupling regions are sufficiently small that they provide discrete points to detect acoustic sign2le, but large enough tha.t''they are able to couple with acoustic vibrations.

In another preferred embodiment, the. size of the discrete acoustic coupling regions along the length of the fibre optic cable is at least 10cm, This is a preferred size value that 1 5 ensures that the acoustic coupling regions are sufficiently small that they provide discrete points to detect acoustic signale, but large enough that they are able to couple with acoustic vibrations, According to.a further embodiment of the present invention, wherein the discrete acoustic coupling regions comprise a periodi.c structure. Preferably, the periodic structure is achieved by dividing. the discrete acoustic coupling regions into equal portions. This periodic, structure provides discrete acoustic coupling points within the discrete coupling region.

Preferably, the size of the equal portions along the length of the fibre optic cable is at most 5cm, and the size of the equal portions along the length of the fibre optic cable is at least 2.5 1cm.

In view of the above, from another a spect, the present invention provides a distributed acoustic sensing system comprising a fibre optic cable wherein discrete acoustic coupling regions are interspersed along the length of the fibre optic cableS As shown in the prior art, known distributed acoustic sensing systems utilise clamps, which secure the cable to the structure or area that is being monitored. The clamps act as acoustic coupling points such that they detect acoustic ignais by transmitting the acoustic energy of the sigrals to the at least one optical fibre arrangement. In some skuations, the use of clamps is not possible and the fibre no longer consists of an array of acoustic sensing points, resulting in aliasing effects. To resolve this deficiency., the present invention provides.a fibre: optic cable that includes the feature of discrete acoustic coupling points, wherein the acoustic coupling between the at least one optical fibre arrangement and the outer layer has been enhanced, Consequently, the cable may iO he deployed alongside a structure or area, without the use of clamps, and be used to detect acoustic signals.

Preferably, the locations of the discrete acoustic. coupling regions are known and match the resolution of a distributed acoustic sensor system.

Distributed acoustic sensor systems are able to resolve acoustic signals with a spatial 1.5 resolution of' up to I in, thus it is preferable that the plurality of discrete acoustic sensors match this resolution. In doing this, the sensing points ill be phase matched, thus enhancing the dcteetion sensitivity.

A farther aspect of the present invention provides a fibre optic cable which has an acoustic sensitivity that is non isotropic. ThIs enables incoming acoustic signals to be preferentially detected from particular directions. Preferably, the a:coustic sensitivity is adapted in at least one or more directions extending in the plane nonnal. to the length of the. fibre optic cable.

In one embodiment of this aspect of the invention, a fibre optic cable is piouded wherein a filler is inserted in a gap between at least one optical fibre. arrangement and an outer layer, wherein the filler comprises built up regions that bridge the gap between the at least one optical fibre arrangement and the outer layer so as to enable acoustic energy to be transmitted from thc outer layer to the at least one optical fibre arrangement, Furthermore, the built up regions of filler extend in at least one direction in the plane normal to thc length of the fibre optic cable so as to directionally adapt the regions of acoustic coupling.

A preferred embodiment of this aspect of the invention, provides a fibre optic cable wherein an acoustically teactive mass surrounds the fibre optIc cable. The acoustically reactive mas.s is preferably a material with high acoustic coupling such that it effectively transmits acoustic energy. This allows the acoustic sensitivity to he directionally adapted in the plane normal to the length of the fibre optic cable, whereby the direction of detection of incoming acoustic signals in this plane may be chosen.

Preferably at least one segment of acoustic insulation is placed in the acoustically reactive mass so as to reduce the acoustic coupling of the fibre optic cable in at least one direction in the plane normaL to the length of the fibre optic cable. in doing this, acoustic signals will be detected with greater sensitivity in the directions corresponding to regions of the mass where acoustic insulation has not been incorporated, thus adapting the acoustic sensitivity in these regions. For example, the acoustic insulation attenuates incoming acoustic waves from directions which are incident on the insulation, thus preventing or reducing the detection.

iS In another embodiment, at least one segment of ac.oustkall.y reactive material is placed in the. acoustically reactive mass surrounding the fibre optic cable so as to further adapt the acoustic sensitivity in at least one direction in the plane normal to the fibre optic cable. This wilt result in greater acoustic sensitivity in directions correscponding to regions where the segments of acoustically reactive material have been places such that incoming acoustic signals will be easily detected.

In one embodiment, the at least one segment of acoustically reactive material i.s ananged to be interspersed at intervals along the length of the fibre optic cable to produce a plurality of discrete acoustic coupling, regions, which also have directional acoustic sensitivity. Alternatively, the at least one segment of acoustically reactive mass may extend sibtantiaHy continuously along thc length of the fibre optic cable.

Another embodiment of this aspect of the invention provides a fibre optic cable wherein an acoustically insulating mass surrounds the fibre optic cable., whereby the acoustic sensitivity is directionally adapted in the plane normal to the length of the fibre optic cable by the acoustically insulating mass. In doing this, acoustic signals vil1 be detected with lower sensitivity in directions corresponding to regions o.f greater acoustic insulation.

Preferably the fibre optic cable is placed in the acoustically insulting mass in.a non-isotropic configuration so as to vary the acoustic coupling in the plane normal to the length of the fibre optic cable That is to say acoustic signals will he detected with greater sensitivity in regions corresponding to a higher acoustic coupling.

In anothei preferred embodiment of this 4spect of the invention the acoustically insulating mass has a non-isotropic configuration so as to vary the acoustic coupling in the plane normal to length of the fibre optic. cable. In doing this, the acoustic sensitivity is greater in regions wherein incoming acoustic waves are attenuated to a lesser degree.

In one embodiment, at least one segment of acoustic insulation is placed in the to acoustically insiJating mass so as to further adapt the acoustic. sensitivity of the fibre optic cable in at least one direction rn the plane normal to the length of the fibre optic cable, By doing this, acoustic signals will he detected with less sensitivity in the directions corresponding to regions of the mass where acoustic insulation has been incorporated, thus adapting the acoustic sensitivity in these regions 1.5 In another embodiment at least one segment of acoustically reactive material is placed in the acoustically insulating, mass so as to Ilirther adapt the acoustic sensitivity in at least one direction h.i the plane normal to the fibre optic cable. This will result in greater acoustic sensitivity in directions corresepondin.g to regions where the segments of' acoustically reactive material have been places such that incoming acoustic signals will be easily detec ed.

In a ftirther embodiment of the present invention, the at least one segment of acoustically reactive material is arranged to be interspersed at intervals along the length of the fibre optic cable to produce a plurality of discrete acoustic coupling regions, which also have directional acoustic sensitivity. Alternatively, the at least one segment of acoustically reactive material may extend subtantially continuously along the length.

of the fibre. optic cable.

Brief description of Drawings

The present invention will now be described by way of example only, and with reference to the accompanying drawings in which: Figure 1 illustrates an example fibre optic cable of the prior art; Figure 2 illustrates a. typical fibre optic cable deployment for distributed acoustic sensing of the priorart; Figure 3 shows in schematic form a preliminary step in an enibodirnent of the present invention, whereby a layer of acoustic insulating material is placed between the outer layer and the at least one optical fibre.; Figure 4 shows in schematic form a first embodiment of the: present invention, wherein acoustic sensing points are achieved by the insertion of a filler so as to produce regions of acoustic coupling; 1 0 Fi:gure 5 shows in schematic fdnn a second embodiment of the present invention.

whereby acoustic sensing points are achieved by crimping, the fibte optic cable, thereby creating regions of acousti.c coupling.; Figure 6 shows in schematic form a further embodiment of the: present invention, wherein a plurality of acoustic sensing points with different periodidties are provided within the same fibre optic cable; Figure 7 shows in schematic form the application of a fibre optic cable according to a ombodirents. of> the present invention in systems of distributed acoustic sensing; Figure 8 shows a cross sectional View of another embodiment of the present invention whereby acoustic sensing points arc produced so as to he directional in the plane' normal to the length of the fibre optic cable; Figure 9 shows a graphical representation of the non isotropic acoustic sensitivity in relation to the embodiment shown. in Figure 6; Figure 10 illustrates a preferred embodiment of the present invention, whereby regions of acoustic insulation are adapted so as to focus acoustic signals directionally iii the plane norm& to the length of the fibre optic cable; Figure 1.1 shows.a graphical representation of the. non isotropic acoustic sensitivity in relation to the embodiment. shown in Figure 10; 1.0 Figure 1 2 shows in schematic form another embodiment of the present invention, wherein a fibte optic cable is surrounded by a layer o' acoustic insulation material so as to provide non isotropic acoustic sensitivity; Figure 13 shows a graphical represenl.atioit of the non isotropic acoustic sensitivity in S relation to the embodiment shown in Figure 12; Figure 14 shows in schematic form another embodiment of the present invention, wherein a fibre optic cable is surrounded by a layer of acoustic insulatien material so as to provide non isotropic acoustic sensitivity; Figure 15 shows a graphica representation of the non isotropic acoustic sensitivity in 1 (1 relation to the embodiment shown in Figure 14; Figure 16 shows in schematic form another embodiment of the present invention, wherein a fibre optic cable is surrounded. by a layer of acoustic insulation material so as to provide non isotropic acoustic sensitivity; and Figire 17 shows a graphical representation of the non isotropic acoustic sensitivity in relation to. the embodiment shown in Figure 16; Figure 1 8. shows in schematic form another embodiment of the present invention, wherein a fibre optic cable is surrounded by a mass comprising portions of acoustic insulation and portions of material with high acoustic coupling; Figure 19 shows a graphical representation of the non isotropic acoustic sensitivity in relation to the embodiment shown in Figure 18,

Detailed description of p,efened embodiments

In a particular embodiment of the invention, described, here in order to provide an example of a preferred implementation of the present invention, a distributed acoustic sensor is provided along a fibre optic cable, which emulates having a plurality of discrete sens'ng points In order to emulate the d!cerctc points of acoustic coupling the acoustic coupling between the outer layer and the at least one optical fibre arrangement is adapted as will be described.

With reference to Figure 3, there is provided a length of fibre opiic cable 10 comprising at least one optical fibre arrangement 100 surrounded concentrically by an outer layer 101, wherein a gap 200 is provided between the tt least one optical fibre arrangement and the outer layer 101. The gap 200 eomçri.ses at least one acoustic insulating material, typically air, which exhibits low acoustic coupling The air layer 200 acts, as a sound insulating layer between the outer laycr 101 and the at least one optical fibre arrangement lOQat least one optical fibre arrangement 100.

i-\ preferred embodiment of the present invention is illustrated by Figure 4, wherein a filler 102 is inserted into the gap 200 between the at least one optical fibre arrangement 100 and the outer layer 101. The filler 102 is configured so as to provide built up regions. at points interspersed along the length of the fibre optic cable 20. thus creating regions of acoustic insulation 201 between the built up regions of filler 102. The built up regions of filler 102 bridge the gap between the outer layer 101 and the at least one optical fibre arrangement 1 00 so as to produce regions of relative acoustic coupling 202. This couples the outer layer 101 and the at least one optical fibre arrangement 100 such that the acoustic energy, as a result of acoustic signals incident on the fibre optic cable 20, may be transmitted to the at least one optical fibre arrangement 100 at the acoustically coupling regions 202, hence enabling incident acoustic signals to be detected by the fibre optic cable 20 at these points along its length. The regIons of acoustic insu:lation 201 adapt the acoustic eouplin.g between the at least one optical fibre arrangement 100 and the outer Layer 101 such that these pcints along the fibre optic cable 20 have lower acoustic coupling and the transmission of acoustic energy is impeded, hence enhancing the effect of the regions of acoustic coupling 202.

Preferably, the acoustic insulating regiot s 201 are typically I *5 metres in length, so that the sensing resolution of the fibre optic cable 20 matches the actual: resolution of the points at which acoustic energy i. being sensed, The built up: regions of filler 102, that is to say the coupling regions, are sufficiently small that they provide discrete points at which the acoustic signal may be detected. For example. the acoustic coupling regions may be approximately 10 to 50 cm in length. The built up region.s of filler 1 02 should not, however, be so small, for example, smaller than 1cm. that thcy do not provide a region large enough to transmit the acoustic energy.

An alternative embodiment of the present inventirn is illustrated by Figure. 5, wherein a.

fibre optic cable 10 described by Figure 3 is crImped, fbr example by a manual means., at intervals along its length to produce fibre optic cable 30. The crithped portions 103 of the flbre optic cable 30 are such that the inner face of the outer layer 101 comes into S contact with the at least one optical fibre arrangement 1 00, therefOre bridging the insulating gap 200 between the outer layer 101 and the at least one optical fibre arrangement 100 at that point. The fibre optic cable 30 is not crimped insofar that it squashes the at least one optical fi:brc arrangement 100 in any way. The crimped portions 103 thus provide regions of acoustic coupling such that the outer it yet 101 is able to transmit acoustic energy to the at least one optical fibre arrangement 100 at discrete points corresponding to the crimped portions. This results in regions of acoustic insulation 201 along the length of the fibre optic cable 30 between each. poilit of acoustic coupling 103.

The crimped portions 103 couple the outer layer 101 and the at least one optical fibre arrangement 10:0 such that the acoustic energy, as a result of acoustic signals incident on the fibre optic cable 30, may be transmitted to the at least one optical fibre arrangement 100, hence enabling acoustic signals to be detected by the fibre optic cable 30: at these points along its length. The regions of acoustic insulation 201 help to adapt the acoustic coupling between the at least one optical fibre arrangement 100 and the outer layer 101 such that these points alon.g the fibre optic cable 30 have lower acoustic coupling and the transmission of acoustic energy is impeded, hence enhancing the effect of the regions of acoustic coupling.

Preferably, the acoustic insulation regions 201 are typically I-S metres in 1ength so that the sensing resolution of the fibre opti.c cable 30 matches the actual resolution of the points at which acoustic energy is being sensed, when, for example, the fibre optic cable 1 is held by clani ps such as shown in Figure 2, The crimped portions 103, that is to say the coupling regions, are sufficiently small that they provide discrete points at which the acoustic signal may be detected. Prefrrably, the acoustic coupling regions are 10 to 50cm. in length. The crimped portions 103 should not, however, be so small, for example, smalle.r than 1cm, that they do not provide a region large enough to transmit the acoustic energy.

A futher embodiment of the present invention is illustrated by Figure 6, wherein a fillet 203 is inserted into a gap 201 betweenat. least one optical fibre arrangement 100 at least one optical fibe arrangement 100 (for example, a FIMI) and an outer layer 101.

Similar to th at illustrated by Figure 4, the tiller 203 is configured to produce a. plurality S of built up regions 204, 205 at points interspersed along the fibre optic cable 40 so as to provide discrte coupling points. The built up regions may then. be periodically divided into smaller sections, thus producing smaller sensing points within each. built up region that are evenly spaced apart. For example. a first built up region 204 and a second built up region 205 both. represent discrete coupling points of' equal length along the fibre optic cable 40. The first built up region 204 has been equally divided into two smaller coupling points 204a-b, whereas the. second built up region 205 has been equally divided into three smaller coupling points 205ac. Preferably, the built up regions are to 50cm in length along the fibre optic cable 40, arid are pcriodically divided such that the smaller coupling points axe approximately I Ia 5cm in length along the fibre optic cable 40.

By periodically dividing the discrete coupling regions 204, 205, the resolution at which acoustic energy is sensed is increased since the periodic structure of the discrete coupling regions 204, 205 increases the spatial resolution of the fibre optic cable 40.

Additionally, the periodic structure of the discrete coupling regions 204, 20.5 can be used to track the eddy flow of a fluid. contained within a pipeline or vessel bein.g monitored by the fibi e optic cable 40, An eddy is a current of fluid that results when a fluid flows past an object in its path, causing the current cd'the fluid to change direction with respect to the general motion of the whole fluid. The individual eddies are capable of producing acoustic vibrations, and so by tracking the eddie.s within the discrete regions of acoustic coupling, 204. 205, an object or defect in the vessel containing the fluid can be detected To track the eddies, the penodic structure of the thsLretc coupling regions 204, 205 can be configured such that the sp ing between the periodic sending points 204a-b, 205a-c matches the life of the eddies within the monitored Pipeline or vessel.

Another embodiment of the present invention is illustrated by Figure 7, wherein a fibre optic cable 20 according to the present invention is used in conjunction with a system for performing distributed acoustic sensing (DAS), for example, the iDASTM.

available from Silixa Limited. of Elstree. UK. in Figure 7, a fibre optic cable 20 as described, by Figure 4 is shown, but it should be appreciated that any fibre optic cable according to the present invention may be used in DAS systems. The DAS system 50 is capable of obt2.ining a measurement profik. along the length of the fibre optic cable 20, digitally recording acoustic fields at intervals along at least one optical fibre 51 contained within the fibre optic cable 20.

A DAS system 50 injects pulsed light into the at least one optical fibre 51 which propagates down the entire length of the at least one optical fibre 51. Light that is then reflected or back scattered by the at least one optical fibre 51 is returned to the DAS system 50, wherein the optical phase data of the returned signal i.s measti red, such that variations in the optical path of the returned signal due to acoustic vibrations arc detected. Preferably, the optical phase data measurements are made at discrete sampling points along the length of the at least one optical fibre 51 so that the position of any acoustic vibrations may be determined.

1 5 In Figure 7, the DAS system 50 is controlled such that it is possible to position where the DAS system 50 takes its measurements along the length of the at least one optical fibre 51, by time synehronising the pulsed light with the locations of the discrete coupling regions 202. For example the DAS system 50 can control its internal processing such that the positions of its effective acoustic measurement points can he controlled with respect to the positions of the discrete coupling regions.. In this respect, the DAS system 50 measures the optica.l phase data of any light reflecte.d or back scattered 52a-h from along the fibre, with changes in the back scatter as a. result of incident acoustic vibrations being detected and used to recreate the inc.ideiit acoustic signal. The processing perkrmed in. the DAS can be controlled such that the effective acoustic measurement points along the fibre can be set with respect to the positions of the discrete acoustic coupling regions. For example, as described, above in many embodiments it will be beneficial to control the positions of the acoustic measurement poInts along the fibre so as to coincide with the positions of the discrete acoustic coupling regions. However, in other embodiments there may be modes of operation, such as test modes or calibration modes., or even some operational modes, where it is desirable to synthetically shift (as a result of the signal processing applied in the DAS) the acoustic measurement points with respect to the acoustic coupling regions. is

For example. in a test or calibration mode it may be desirable to move" the. acoustic sampling points to be between *th.e acoustic cc.upiin:g regions, such that acoustic coupling to. the sensing points is minimised, so as to reduce background noise for testing or calibration purposes. Additionally or alternatively, in some operational scenarios it may he desirable to synthetically move° the acoustic sampling points away &cn the acoustic coupling regions, if for example the acoustic ccupling regions are enhancing or highlighting one signal (tbr example via resonant effects) to: the detriment of the detection of others, it will thetefbrehe understood that the DAS can control the relative positions of acoustic sampling, points along the fibre with respect to the positions of the acoustic coupling i egions, so as to make then) coincide, or to be displaced from each oilier by a varying controllable amount, For example, the acoustic sampling poirts can be controlled so as to positionaily coincide with the acoustic coupling regions (e.g. he in phase with each other), which is the envisaged preferred mode of operation for most applications, or controlled so as to be in any position between the acoustic samphn regions inc ludrng, to gis c a minima signal, positioned substantially half-way between the acoustic coupling regions i.e. such that the acoustic coupling regions and the acoustic sampling points along the fibre are essentially located in anti-phase positions with respect to each other.

in a second aspect ci the present invention, a fibre optic cable is provided wherein the acoustic sensitivity of the cable is non isotropic. The fibre optic cable is. adapted to pros ide regions of directional acoustic coupling such that incident acoustic signals are only detected from particular directions, Examples of how this non isotropic sensitivity may be achieved is described below.

A t\irther embodiment, with reference to Figure 8, provides a fibre optic cable 60 comprising a at least one optical fibre arrangement 100 surrounded concentrically by an outer layer lOl, such that a gap 200 is provided between the at. least one optical tibre arrangement 100 and the miter layer 101. A filler 104 is inserted between the at least one opUeai fibre arrangement 100 and the outer layer 101, and is configured to provide budt up regions which bridge the gap be'.ween the outer layer 101 and the at least one optical fibre arrangement 100 in order to facilitate the transfer of acoustic vibrations to the at least one optical fibre arrangement. 100. The built up regions are located in at least one direction in the plane normal to the length of the fibre optic cable 60, so as to provide acoustic coupling that is directional in the plane normal to the fibre optic cable 60.

This results in reions of acoustic insulation 201,. with low acoustk coupling, in all other directions in the plane normal to the length of tie fibre optic cable 60 other than the built up regions.. That is to say that incoming acoustic signals will be detected with greater sensitivity in the direction corresponding to the acoustic coupling paints produced by the built up regions of filler 104, Additionally, in some embodiments the built up regions may also be interspersed along thefl leigth of the fibre optic cable 60, as illustrated by the embodiment shown in Figure 4, so as to provide discrete coupling points along the length of the fibre optic cable 60 that are also directional in the plane nomni to the fibre optic cable 60.. In other embodiments.., however, the filler regions 104 extend substantially continuously along the length. of the fibre optic cable 60.

Figure 9., by way of example, iLlustrates 4 possible distiibution ofacoustic sensitivity 61 and 62 that results from a fibre optic cable 60., as stown in Figure 8. Figure. 9 shows a response plot relating to the fibre optic cable 60 in the plane normal, to its length aid the resulting acoustic sensitivity distribution 6! and 62 corresponding to the configuration of fibre optic cable 60 given in Figure 8. The acoustic sensitivity 61 is focused and enhanced by the regions of acoustic coupling produced by the built up regions of filler 104, since these are the most acoustically reactive regions of the fibre optic cable 60. In comparison,, the acoustic sensitivity 62 corresponding to the regions of acoustic insulation 201 is reduced As a result, an incident acoustic signal is detected by these coupling regions more readily, resulting in an acoustic sensitivity profile 60 and 62, as shown, which is not only directional but also dependent on the size of the regions of acoustic coupling.

A further alternative embodiment is shown in Figure 10, whereby a fibre optic cable 1 is surrounded by an acoustically reactive mass 300. Segments of acoustic insulation 301 are inserted into the mass 300 so as to provide regions of low acoustic eoupling in specific directions in the plane normal to the length of the fibre optic cable 1. The insulation results in regions of higher acoustic coupling between the segments of acoustic insulation. 301, such that acoustic signals are more readily detected in the directions of the plane normal to the length of the fibre optic cable 1 corresponding to these regions of higher acoustic coupling.

Figure 11, by way of example, illustrates a possible distributioii of acoustic sensitivity 302 and 303 that results from a fibre optic cable 1 surrounded by an acousticalTy reactive mass 300, such as that shown in Figute 1 0. Figure 11 shows the fibre optic.

cable I in the plane normal to its length and: the acoustic sensitivity distribution 302 and 303 corresponding to the configuration provided by the embodiment of Figure [0. The acoustic sensitivity 302 is focused and enhanced by the regions f higher acoustic coupling that result from the segments of acoustic insulation 301 inserted into the acoustically reactive mass 300 into the nodai regions. The nodal regions of h.iher acoustic coupling detect incoming acoustic signals more readily, resulting in regions of acoustic sensitivity 302 which. correspond to the position and size of the regions of higher acoustic coupling In companson the acoustic sensitivity 303 corresponding to the segments of insulation acoustic 301 is reduced.

Another further embodiment is illustrated in Figure l2 wherein a fibre optic cable 1 is surrounded by a layer of acoustic insulation material 400, but such that the fibre optic cable I is not positioned centrally within the acoustic insulation 400. The acoustic insulation material 400 has low acoustic coupling properties and. impedes incoming acoustic signals. The fibre optic cab. le 1 is located ithin the. acoustic insulation 400 such that the distance from the outer edge of the fibre optic cable I to the outer edge of the acoustic insulation 400 varies around the circumference of the fibre optic cable 1..

The region where this distance is smallest has higher acoustic sensitivity since incoming, acoustic signals are inipetled to a]esser degree.

Figure. 13 illustrates a possible resulting acoustic sensitivity profile 40] and 402 for a fibre optic cable 1 surrounded by acoustic insulation material 400, such as that shown in Figure 12.. Figure 13 shows the fibre optic cable I in the plane normal to its length and the distribution of acoustic sensitivity 401 and 402 corresponding, to the embodiment of Figure 12. The acoustic sensitivity 401 is focused and enhanced by the region wherein, the distance from the outer edge of the fibre optic cable I to the outer edge of the acoustic insulation 400 is smallest since this is the region where incoming acoustic signals are least impeded and more readily detected.. in comparison, the acoustic sensitivity 402. corresponding to regions wherein the distance from the outer edge of the fibre optic cable I to the outer edge of the acoustic insulation 400 is largest, is reduced.

A further embodiment is illustrated by Figure i4 wherein a fibre optic cable I is surrounded by a layer of acoustic insulation material 500, wherein the acoustic insulation 500 is of square configuration. The acoustic insulation material 500 has low acoustic coupling properties and impedes incoming acoustic signals. The fibre optic S cable I is located centrally within the acoustic insulation 500. but due to the shape of the acoustic insulation 500 the distance from the ott ter edge of the fibre optic cable ito the outer edge of the acoustic insulation 500 varies around the circumference of the fibre optic cable 1. Regions where this distance is smaller have higher acoustic sensitivity since incoming acoustic signals are impeded to a lesser degree.

Figure 15 illustrates a possible resulting acoustic sensitivity profile 501 for a. fibre optic cable I surrounded by acoustic insulation material. 500, such as that shown in Figure 14. Figure 15 shows the fibre o pti.c. cable I in the plane normal to its length and the distribution of acoustic sensitivity 501 corresponding to the embodiment of Figure 14.

The acoustic sensitivity 501 is focused and enhanced by the regions wherein the 1 5 distance from the outer edge of the fibre optic cable 1 to the outer edge of the acoustic insulation 500 is smaller since these are the regions where incomIng acoustic signals are least impeded and more readily detected, In eO:fllparisOfl, the acoustic sensitivity 501 corresponding to regions wherein the distance from the outer edge of the fibre optic cable I to the outer edge of the acoustic insulation 500:. is Largest, is reduced.

A fttrther embodiment is illustrated by Figure 16, wherein a fib re optic cable 1 is surrounded by a layer of acoustic insulation material 600, wherein the acoustic insulation 600 is of star configuration. The acoustic insulation material 600 has low acoustic coupling jroperties and impedes incoming acoustic signals. The fibre optic cable I is located centrally within the acoustic insulation 600. but due to the shape of the acoustic insulation 600, the distance from the outer edge of the fibre optic cable ito the outer edge of the acoustic insulation 600 varies around the circumference of the fibre optic cable 1. Regions where this distance is smaller have higher acoustic sensitivity since incoming acoustic signals are impeded to a lesser degree.

Figure 17 illustrates a possible resulting acoustic sensitivity profile 601 fin a fibre optic cable 1 surrounded by acoustic insulation material 600,. such as that shown in Figure 16. Figure 1.7 shows the fibre optic cable I in the plane normal to its length and. the distribution of acoustic sensitivity 601 corresponding to the embodiment of Figure 16.

The acoustic sensitivity 601 is focused and enhanced by the regions wherein the distance fl-cm the outer edge of the fibre optic cable 1 to the outer edge.. of the acoustic insulation 600 is smaller since these are the regions where incoming acoustic signals are least impeded and more readily detected in comparison, the acoustic sensitivity 601, corresponding to regions wherein the distance from the outer edge of the fibre optic cable Ito the otter edge of the acoustic insulation 600 is largest, is reduced.

Another preferred embodiment is illustrated by Figure 18, wherein a fibre optic cable 1 is concentrically surrounded by a easing 700 which may be made from an acoustically insulating material that impedes the mechanical vibrations of any acoustic signals I 0 which are incident on it. Alternatively, the casing 700 may he made from an acoustically reactive material that easily couples with the mechanical vibrations of any acoustic signals that arc inceident on it. The casing 70Q may comprise a plurality of acoustically rcactive segments 701 a-b made of a material with high acoustic coupling properties so as to directionally enhance the acoustic sensitivity in the plane normal to the length of the fibre optic cable 1. In regions that include acoustically reactive segments 701a-b, the tThre optic cable 1. will more easily couple to acoustic vIbrations and, therefore, more readily detect incoming aecustic signals. The acoustically reactive segments 70lah may be made of a number of different materials with high acoustic coupling properties, for example, a metal, and may be positioned anywhere within the sleeve 700.

Additionally, the casing 700 may comprise a plurality of segments. of acoustic insulation 702a-b with low acoustic coupling properties.so that incident acoustic signals are impeded in these regions. That is to say, the acoustic sensitivity in directions of the plane normal to the length of the fibre optic cable I will be reduced us a result of the segments of acoustic insulation 702a-b. The segments of acoustic insulation 702a-b may be made of a number of materials with low acoustic coupling properties that act as good acoustic insulators, tbr example, air, and may he positioned anywhere within the sleeve, in some embodiments, the acoustically reactive segments 70 la-b may also be interspersed, along the length of the fibre optic cable 1, as illustrated by the embodiment shown in Figure 4, so as to provide discrete acoustic coupling points along the length of the fibre' optic cable I that are also directional in the plane normal to the fibre optic cable I. In other embodiments, however, the acoustically reactive segments 70th-b extend substantially continuously along the length of the fibre optic cable 1..

Figure 19 illustrates a possible resulting acoustic sensitnity piofile 703, 704 for a fibre optic cable 1 surrounded by a sleeve 700 that includes acoustically reactive segments 70 Ia-b and portions of acoustic instlation 702a-b, such as that shown in Figure 18.

Figure 19 si owsaresponse plot relating to the fibre optic cable I in the plane normal to its length, arid shows the acoustic sensitivity distribution 703, 704 corresponding to th:e configuration of the sleeve 700 surrounding the fibre optic cable 1 shown in Figure 18.

The acoustic sensitivity 703 is focused and enhanced by the regions corresponding to the acoustically reactive segments 7014-b since these are the most acoustically reactive regions of the arrangement. In comparison, the acoustic sensitivity 704 corresponding to the portions of acoustic insulation 702a-b are reduced. Therefore, an.y incident acoustic signals are more reacity detected by the regions of higher acoustic coupling la-h, resulting in the aCoustic sensitivity profile of Figure 19, which is not only directional hut also dependent on the size of the acoustically reactive segments lOla-b and the portions of acoustic insulation 702ab, Figures 9, ii, 13, 15, 17 and 19 are to be taken as projected distributions of acoustic sensitivity for the respective fibre optic cabte embodiments and ate merely mdicative for qualitative understanding purposes only. I'he actual resulting acoustic sensitivities may differ from these embodiments and these pro*i are only intended to provide an indication of their appearance..

Alternative embodnneriits may include fibre optic cables I ith a at least one optical fibre anangementlOl that is rot compused of a FIMT as described by the prior art Figure 1, hut of some other arrangement comprising at least one optical fibre encapsulatcd in a sealed tube.

Another example of a further embodiment may be a fibre optic cable 1, where the narrowed portions along the fibre optic cable I are achieved by some means other than crimping, such as an outer layer which is manufactured so as to include regions which are curved inwards so as to make contact with the at least one optical fibre arrangement 100 at points interspersed along the length of the fibrc optic cable 1 These points of contact produce the regions of acoustic coupling.

Another further embodiment 1:5 a fibre optic. cable I that uses an acoustic insulation material other than air, sLob as an acoustic foam. Additic.nally, a: combination of acoustic insulators may be used in order to provide regions of low acoustic coupling and hence enkiance the acoustic eouplin.g between the at least one optical fibre arrangement 100 and outer layer 101. Preferably, the acoustic insulating materials are choser. such that the acoustic coupling of the acoustic insulating region, is of an optimum value to prevent the transmission of acoustics. energy or at least sufficiently diffcrent to that of the discrete coupling regions so as to impede the acoustic signal at a different rate.

Optionally, an acoustically reactive nms.s 300, as shown in Figure 10, may be located at points intersperse d along the length of the fibre optic cable 1 so us to produced discrete coupling points along the length of the fibre optic cable 1, which are also directional in the plane normal to the length of the fibre optic cable 1.

A further modification may include a fibre optic cable, surrounded by a layer of acoustic insulation wherein the distance from the outer edge of the fibre optic cable to the outer edge of the acoustic insulation is not uniform around the circumference of the fibre optic cable, thus resulting in an acoustic sensitivity profile that varies directionally and is non isotropic.

Various, modifications, vihether by way of addition, deletion or substitution may be made to the above described embodiments to provide further embodiments, any and all of which are intended to be encompassed by the appended claims.