GB2518359A

GB2518359A - Acoustic cables

Info

Publication number: GB2518359A
Application number: GB1316364.7A
Authority: GB
Inventors: Mahmoud Farhadiroushan; Tom Parker
Original assignee: Silixa Ltd
Current assignee: Silixa Ltd
Priority date: 2013-09-13
Filing date: 2013-09-13
Publication date: 2015-03-25
Anticipated expiration: 2033-09-13
Also published as: GB2518359B; GB201316364D0

Abstract

A fibre optic cable 20 for distributed acoustic sensing has an optical fibre 100 (e.g. fibre-in-metal-tube) concentrically surrounded by an outer layer 101. Discrete acoustic sensing points are achieved by selectively enhancing the acoustic coupling between the outer layer 101 and the optical fibre 100, such that acoustic energy may be transmitted selectively from the outer layer to the optical fibre. The region between the outer layer and the optical fibre contains material 201 which is acoustically insulating (e.g. air). Regions 202 of acoustic coupling between the fibre 100 and the outer layer 101 may be provided by built up regions of filler 102 interspersed along the length of the fibre, or by crimping the outer layer to form narrowed points (103, fig. 5). By providing a plurality of discrete acoustic sensors along a cable acoustic signals may be measured in situations where the fibre optic cable has not been secured to a structure or area by a series of clamps. This may reduce aliasing.

Description

Acoustic Cables

Technical Field

The present invention relates to a fibre optic cable, and in some embodiments provides a fibre optic cable which comprises an array of discrete acoustic coupling regions.

S Background to the Invention and Prior Art

To detect an acoustic signal, distributed acoustic sensing is commonly and effectively used. This method employs fibre optic cables to provide distributed acoustic sensing wheteby the. fibre optic cable acts as. a string of discrete acoustic sensors, and an optoeleclronic device measures and processes the. returning signal. Ihe 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 scattered, along the length of the fibre by Rayleigh. Brilliouin and Raman scattering mechanisms. The scattered light is captured by the fibre and carried hack towards the source where the returning signal is measured against time, allowing measurements in the ampliwde, frequency and phase of the scattered light to he determined. If an acoustic wave is incident upon the cable, the glass structure of the optical fibre is caused to contract and expand within the vibro-acoustic field., consequently varying the optical path lengths between the hack scattered light scattered from different locations along 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 vibration in the structure.

In known distributed acoustic sensing systems (DAS), 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 iDASTM, available from Silixa Limited, of Elstree, UK are given in \O2010/01 36809 Systems such as these are able to digitall) record acoustic fields at ev.ry interval location alon.g an optical fibre at frequencies up to 100kHz..

Since the location of the acoustic sensors is known (the fibre deployment being

I

known)., the position of any acoustic signal. can be thus identified by means of time-of-arrival calculations, In a typical deployment, the sensing poins are usually created by clamps which are used to secure the fibre optic cable to the structure or area it is monitoring..

By way cf examp1e Figure i shows a common arrangement o.f a known fibre optic cable 1, comprising 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 (F1MT) 2, which provides a way of encapsulating very long lengths of optical libres.5 within a hermetically sealed tube 4. A genera.! construction 1 0 o.f a F'IMT 2 includes at least one optical fibre 5 encapsulated in a metal tube 4.

Additionally, it is common to till this metal tube 4 with a thixotropic gel 6 in order to protect the optical fibres 5 from environmental disturbances, prevent damage from micro-bending conditions and to help minimise the Threes 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 thrther outer tube 3, usually containing a filler material.

The optical fibres 5 are typically rnad.e of flexible traris.paren fibres of' glass The filler material 3 surrounding the FI.MT 2 has a lower refractive index than the optical tThres. 5 such that light whic.h has been focused. 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 fbr which distributed acoustic sensing may he used, for 2.5 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 are.a by clamps distributed, along the length of the fibre optic cable. By way of example,. Figure 2 illustrates how fibre optic cables 1 may be. used to monitor structures or areas using distributed acoustic sensing.

Figure 2 shows a fibre optic cable 1 being used to monitor a pipeflne 7 that has been depkycd underground 9. The fibre optic cable 1 is positioned to run parallel alongside the pipeline 7 and is secured by a series o.f clamps 8, which are distributed along the length of the pipeline 7. These clamps 8 allow the, fibre optic cable I to monitor the 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 ln the pipeline 7, such tht the acoustic energy s trans'mrLted to the optical fibres. 5..

The clamp's arc spaced along the fibre at a distance at least equal to or greater than the senìsing resolution of the distributed acoustic sensing, typically I 5 metres. This 1.0 provides discrete sensing points along the fibre matched to the sensing resolution and pievents any antia1'iasing 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, parallel to a.

pipe., ve1l, or other structure heinu monitored. In this ease there are no discrete 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 ftc points at which acoustic energy is being seised, ali'as'ing effects can occur in the signal, due to undersampling.

urnmary oj,the. lnverflp Embodiments of the present invention address the above noted aliasing problem by providing a plurality of discrete acoustic sampling points along a flbre optic cable whereby acoustic signals may be measured in situations where the fibre optic cable has not been secured to a structure or area by a series of clamps, as described in the prior art. Acoustic sampling points are achieved by selectively enhancing the acoustic coupling. between an. outer layer and at least one opticr'l fibre arrangement, such that acoustic energy may he transmitted selectively from the outer layer to the optical fibres, The resulting regions of acousti.e coupling along the fibre optic cable allow the optical fibre to detect acoustic signals.

According to one aspect of the present invention, there is provided a fibre optic cable, comprising at least one optical fibre arrangement, and at least one outer layer encapsulating the at least one optical fibre arrangement. The fibre optic cable further cor.prises an acousUe insulating layer between the at least one optical fibre arrangement and the outer layer, the insulating layer being interspersed aleng the length of the fibre with discrete aoousti.c coupling regions. for transmitting acoustic energy from the outer layer to the at least one optical fibre arrangement.

Preferably, the at least one optical fibre arrangement comprise.s a fibre-in-metal4ube (FIMT), as tiescribed in. the above prior art, This is a standard and wideEy used 1 0 arrangement for fibre optic etibies, therefore existing cables which have already been deployed may he conveniently adapted to incorporate the fOatures of the present In some embodiments, the acoustic insulating layer Includes a. layer of air, Air is a material with low acoustic coupling such that it effectively absorbs acoustic energy 1.5 and reduces its transmission. Acoustic coupling relates to the resistance of a material's particles to the mechanical vibrations of an acoustic signal. That is to say. ma:terials that do not resist the rnechathoal vibrations easily couple with the mechanical vibrations and have high acoustic coupling. Since air particles provide a large amount of resistance to the mechanical vibrations, air exhibits. low acoustic coupling and is considered to be a good acoustic insulating material, which is 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 ot built up regions interspersed along the length of the fibre optic cable, wherein the built u.p regions of filler provide the discrete acoustic coupling regions.

The built up regions of filler connect the at Feast (ne optical fibre arrangement and the outer layer such that acoustic energy can be transmitted betwee.n them, therefore enhancing the acoustic coupling between the at least one optical fibre arrangement and the outer layer. Therefcre fibre optic cables with built in discrete acoustic coupling 3.0 regions 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 provide discrete acoustic coupling regions.

[he narrowed points bridge the gap between the at least one optical fibre arrangeTnent and the outer layer such that acoustic energy 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 deployed and used to detect acoustic signals without the use of clamps securing them to th.e monitored structure or area.

Preferably, the narrowed points are achieved by crimping the fibre optic cable at points interspersed along Its length such that the 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 cne optical fibre arrangement. In doing this, the crimped portions effectively short-circuit the insuiatiig layer to provide the discrete acoustic coupling regions.

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 undersampling.

In a further embodiment, the size of the discrete acoustic coupling regions along the length of the fibre optic cable is at most 5Ocm..i'his is a suitable size value that 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 vi. rations.

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 ensures that the acoustic coupling regions are sufficiently small that they provide discrete points to detect acoustic signale, hut large enough that they are able to couple with acoustic vibrations.

Accordi:ng to a finiher embodiment of the present invention, wherein the discrete acoustic coupling regions comprise a penodic structure Pi ciciably, the periodic stiucture s ch.ieved by dividing the discrete aCoustic coupling regions into equal portions. This periodic structure provides discrete acoustic coupling points within the discrete coupling region.

Prerab*ly, the size of the equal portions alorg the length of the fibre optic cable is at most 5crn and the size of the equal portions along the length of the fibre optic cable is at least lent In view of the above, from another aspect, the present invention provides a distributed to acoustic sensing system comprising a fibre optic cable wherein discrete acoustic coupling regions are interspersed along the length of the. fibre optic cab1e 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 signals by transmitting the 1 5 acoustic energy of the signals to tEe at least one optical fibre arrangement, In some situations, the use of clamps is not possible. and the fibre no longer consists of an array of acoustic sensing points, resulting in ali.asing effects. To resolve this. deficiency, the present invention provides a fibre optic cable that inciude.s the feature of discrete acoustic coupling points, wherein the acoustic coupling between the at least one optical fibre arrangement artd the outer layer has been. enhanced. Consequently, the cable may be 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 4istrihuted acoustic sensor system Distributed acoustic sensor systems are able to resolve acoustic signals with a spatial resolution o.f up to im, thus it is preferable that the plurality of discrete acoustic sensors match this resolution. In doing this, the sensing points will be phase matched, thus enhancing the detection sensitivity.

Brief description of Drawings

Ihe 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 illustrate.s a typical fibre optic cable deployment for distributed. acoustic

sensIng of the prior art;

Figure 3 shows in schematic form a preliminary step in an embodiment 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 tiller so as to produce regions of acoustic coupling; Figire 5 shows in schematic form a second embodiment of the present invention, wherein acoustic sensing points are achieved by crim ping the fibre optic cable, thereby creating regions of acoustIc. coupli:ng Figure 6 shows in schematic tbrm a third embodiment of the present invention, wherein a piur&ity of acoustic sensing, points with different periodicities are provided within the same fibre optic cable;, and Figure 7 shows in schematic form the tppli.caticn of a fibre optic cable according to one or more embodiments of the present invention in systems of distributed acoustic sensing.

Deta led description of prefemd 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 sensing points. In order to emulate the discrete points of acoustic coupling, the acoustic coupling hetwee. the outer layer and the at least one optical fibre is adapted as will be described.

With reference to Figure 3, there is provided a length of fibre optic cable 10 comprising at least one optical fibre 100, typically being an optical fibre arrangement such s, for example, a Fibre-iu-MetahTube (FIMT), surrounded concentrically by an outer layer 101, wherein a gap 200 is provided between the at least o.e optical fibre 100 and the outer layeriOl, The gap 200 comprises at least one acoustic insulating material, typically air,which exhibits low acousti.c coupling. The air layer 2.00 acts as a sound insulating layer between th.e outer layer 101 and the at least one optical fibre 100.

A preferred embodiment of the present invention is illustrated by Figure 4, wherein a filler 1 02 is inserted into the gap 200 between the at least one optical fibre 100 and the outer layer 101. l'he filler 102 is configured so as to provide built up regions at points intcrspsed..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 tip regions of tiller 102 bridge the gap between the outer layer 101. and the at least one optical fibre so as to produce regions of relative acoustic coupling 202, This couples the outer layer 101 and the at least one optical fibre 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 100 at the acoustic coupling regions 202, hence enabling incident acoustic signals to he detected by the fibre optic cable 20 at these points along its length. The regions of acoustic insulation 201 adapt the acoustic coupling between the 2.0 at least one. optical fibre 100 and the outer layer 101 such that these points 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 regions 201 are typically 1-S metres in length along the fibre optic cable, so that the sensing resolution of the fibre optic cable matches the actual resolution of the points at which acoustic energy is 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. Pot example, the acoustic coupling regions may be approximately 10 to 50 cm in length. The built up regions of filler 102 should not, however, he so small, for example, smaller than 1cm, that they do not provide a region large enough to transmit the acoustic energy.

An alternative embodiment of the present invention is illustrated by Figure 5,. wherein a fibre optic cable 10 as described by Figure 3 is crimped, for example by a manual means. at intervals along its length to produce fibre optic, cable 30. The crimped portions I 0' of the fibre optic cable 30 are such that the inner face of the: Outer layer 101 comes into cortact with the at least one optical fibre 100, therefore bridging the insulating gap 200 between. the outer layer 101 and the at least one optical fibre 100 at S that point the fibre optic cable 30 is not crimped insofar that it squashes the at least cme optical. fibre 100 in any way. The crimped portions 103 thus provide regions of acoustic coupling such that the outer layer 101 is ahl.e to transmit acoustic energy to the at least one optical fibre 100 at discrete points corresponding to the crin.ed portions..

This results in regions of acoustic insulation 201 along the length of the fibre optic cablc 30 between each point of acoustic coupling 103.

The crimped portions 103 couple the outer layer 101 aix! the at least one optical fibre 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 100, hence enabling acoustic signals to be detected by the fibre optic cable 30 at these points along its 15' length. The regions of acoustic insulation 201 help to adapt the acoustic coupling between the at least one optical fibre 100 and the outer layer 101 such that these points Song 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 acous:tic insulation regions 201 are typically 1-5 metres in length, so that tIle sensing resolution of the fibre optic cable matches the actual resolution of the points at which acoustic energy is being. sensed, when, fo.r example, the fibre optic cable is held by clamps such as shown in Figure 2 The crimped portions 103 that is to say the acousticeou.pling regions, are sufficiently small that they provide discrete. points at which the acoustic signal may be detected. Preferably, the acoustic coupling regions arc to 50 cm in length.. The crimped portions 103 should not, however., be so small,, for example, smaller than 1 cm, that they do not provIde a region large enough to transmit the acoustic energy.

A father embodiment of the present inveia tion is illustrated by Figure 6,. wherein a filler 203 is inserted into a gap 201 between at least one optical fibre 100 (for example, a FIMT) and an outer layer 101. Similar to that illustrated by Figure 4, the fIler 203 i.s configured to produec a plurality of built up regions' 204, 205 at points interspersed along the fibre optic cable 40 so as to provide discrete coupling points. The built up regions are periodically divided Into smaller sections, thus producing smaller sensing points within each bit ut 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 204ah, whereas the second built up region. 205 has been equally divided into three smaller coupling points 205a-c Preferably. the built up regions are 1 0 to 50cm in length along the fibre optic cable 40, and are periodically divided such that the smaller coupling points are approximately I to 5cm in length along the tThre 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, 205 can be 1 5 used to track the eddy flow of a fluid contained within a pipeline or vessel being monitored by the fibre optic cable 40. An eddy is a current of fluid that results when a fluid flows past an obiect in its path, causing the current of 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 eddies with 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 periodic structure of the discrete coupling regions 204, 205 can be configured such that the spacing between the periodic sending points 2.04ah, 205ac 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 iDAS TM, 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 60: i.s capable of obtaining a measurement profile along the length of the fibre optic cable 20, digitally recording acoustic fields at intervals along at least one optical fibre 61 contained within the fibre optic cable 20.

A. DAS system 60 irjects pulsed Ught into the at least one optical fibre. 61 which propagates down the entire length of the at least one optical fibre.61. Light that is then reflected ci hack scattered by the at least one optical fibre 61 is returned to the DAS system 60, wherein the optical phase data of the returned signal is measured, such that variations in the optical path of the returned signal due to acoustic vibrations are detected. Preferably, the optical phase data measurements are made at discrete sampling points along the length of the at least one optical fibre 61 so that the position of any acoustic vibrations may he determined.

in Figure 7. the DAS system 0 is controlled such that it is possible to position where the DAS system 60 takes its measurements along the length of the at least one optical fibre 6i by time synehronisthg the pulsed light with the locations of the discrete coupling regions 202. For example, the DAS system 60 can control its internal 1 5 processing such that the positions of its. effective, acoustic measurement points can be controlled with respect to the positions of the discrete coupling regions. In this respect.

the DAS system 60 measures the optical phase data of any light reflected or back scattered 62a-b from along the fibre, with changes in the buck scatter as a result of incident acoustic vibrations being detected an.d used to recreate the incident acoustic signal. The processing performed in the.DAS can be controlled such that the effective acoustic measurement points along th.e fibre can be set with respect to the positions of the discrete acoustic coupling regions4 For example, as described above in many embodiments it will be beneficial to control the positions of the acoustic measurement points alon.g the fibre so as to coincide with the positions of the discrete acoustic 2.5 coupling regions, Howevex, in other embodiments there. may be modes of operation, such as test modes or calibration modes, or even some operational modes, where it i.s desirable to synthetically shift (as a result of the signal processing applied lo the DAS) the acoustic measurement points with respect to the acoustic coupling regions.

For example, in a test or calibration mode it may be desirable to "move" the acoustic sampling points to be between the acoustic coupling regions, such that acoustic coupling to the sensing points is minimised, so as to reduce background noise for testing or calibration purposes L\dditional:ly or alternatively, in some operational scenarios it may be desirable to synthetically "move" the acoustic sampling points away from the acoustic coupling regions, i1 for: example the accustic coupling regions are enhancing or highlighting cne signal (for example via resonant effects) to the detriment of the detection of others. It will thereforehe 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 regions, so as to make them coincide, or to he dIsplaced ftcm each other by a varying controllahle amount. For example,, the acoustic sampling points can be controlled so as to positionally coincide with the acoustic coupling regions (e,g. be 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 sampling regions., including, to give a minima signal, positioned substatitially half-way between the acoustic coupling regions i.e. such that the acoustic coupling regions and the acoustic sanipling points along the fibre arc essentially located in anti-phase positions with respect to each other.

Alternative embodiments may include fibre optic cables 1 with at least one optical fibre 101 that i.s not comprised of a. F1MT as' described by the prior art Figure 1, but of some other arrangement comprising, at least one optical fibre encapsulated 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 100 at points interspersed along its length. These points of contact produce the regions of acoustic coupling..

Another further embodiment is a fibre optic cable I that uses an acoustic insulation 2.5 material other than air, such as an acoustic foam. Additionally, a combination of acoustic insulators may be used in order to provide' regions of low acoustic coupling and hence enhance the acoustic coupling between the at least one optical fibre 100' and outer layer 101 Preferably, the acoustic insulating materials' are chosen 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 different to that of the discrete coupling regions so as to impede the acoustic signal. at a different rate.

Vations modifications, whether by way of addition, deletion r substitution may be made to the above descHbed entbndimeins to provide ibrthcr embodiments, any and all of wbi& are Inteaded to be thcompassed 1y the appended daunt.

Claims

Claims 1.. A fibre optic cable, comprising: at!est one optical fibre. arrangement; and at least one outer layer; the cable thither comprising an acoustic insulating layer between the at least, one optical fibre arrangement and the outer layer and discrete acoustic coupling regions interspersed along the length of the fibre for transmitting acoustic energy from the outer layer to the at least one optical libre arrangement.
2. A fibre optIc cable according to claim 1, wherein the at least one optical fibre arrangement comprises a fibre-in-metal-tube (FIMT).
3. A fibre optic cable according to any of the preceding claims., wherehi the acoustic.insulating layer includes a layer of air.
4. A fibre optic cable according to any of the preceding claims wherein a tiller is inserted between: the at least one optical fibre arrangement and the outer layer, the filler comprising built up regions interspersed alon.g the length of the fibre. optic cable; wherein the built up regions of tiller provide the: discrete accustic coupling regions.
5. A fibre optic cable according to any of claims 3 to 3, wherein at least one layer concentrically outside the acoustic insulating layer is nan owed at points interspersed along the length of the fibre optic cable so as. to divide the acoustic insulating layer and provide the.discrete acoustic coupling regions.
6. A fibre optic cable according to claim 5. wherein the fibre optic cable. is crimped at points interspersed along its length such that th.e inner face of the outer layer comes into contact with the at least one optieat fibre arrangement; wherein the crimped sections provide the discrete acoustic coupling regions.
7. A fibre optic cable according to any of the preceding claims, wherein the distance between the discrete acouslic coupling regions is at least lm.
8. A fibre optic cable according to any of the preceding claims, wherein the size of the discrete acoustic coupling regiors along the length of the fibre optic cable is at most 50cm. i4
9. A fibre optic cable according to any of the preceding claims, wherein the size of the discrete acoustic coupling regions along the length of the fibre optic cable is at least 10cm.
10. A fibre optic cable according to any of the preceding claims, wherein the discrete acoustic coupling regions comprise a periodic structure.
11. A fibre optic cable according to claim 10, wherein the periodic structure is achieved by dividing the discrete acoustic, coupling regions into equal portions.
12.. A fibre optic cable according to claim II, wherein the size of the equal portions along the length of the fibre optic cable is at most 5cm,
13. A fibre optic cable according to claim 12, wherein the slize of the equal. portions along the length of the fibre optic cable is at least 1cm.
14. A distributed acoustic sensing system, comprising a fibre: optic cable of any of the pieceding claims..
15. A distributed acoustic sensing system according to claim 14. wherein the locations of the discrete acoustic coupling regions are known and match the resolution of the distributed acoustic sensing system. is