Improved acoustic sensing method

Abstract

There is herein described a method of acoustic sensing, the method comprising performing a preliminary process, the preliminary process comprising determining a round trip duration of a preliminary optical pulse that is transmitted into a head end of an optical fibre, is scattered at a remote end of the fibre and then returns to the head end and using the determined round trip duration to determine a maximum pulse repetition frequency, performing a sensing process comprising transmitting a plurality of optical sensing pulses into the head end of the optical fibre at a frequency that does not exceed the determined maximum pulse repetition frequency and detecting the transmitted plurality of optical sensing pulses after having been scattered by the optical fibre, wherein the maximum pulse repetition frequency corresponds to a frequency at which each of the plurality of optical sensing pulses returns to the head end of the optical fibre at the same time as its immediately-subsequent one of the plurality of optical sensing pulses is transmitted into the head end of the optical fibre.

Description

[0001] Improved Acoustic Sensing Method

[0002] Distributed Acoustic Sensing (DAS) is used to determine the existence and location of events (such as leaks or impacts) that have occurred in the vicinity of an optical fibre.

[0003] In particular, optical pulses are transmitted into a fibre by a DAS unit. The optical pulses are scattered backwards at locations along the fibre that are experiencing environmental strain. As a result, the impact of the environmental strain can be measured by precision measurement of the backscattered signals. The backscattered signals return to the head end and are detected. The location of the strain event can be determined from analysis of the return signals.

[0004] Most DAS applications are in the oil and gas industries. However, DAS has also been used in telecomms networks. Telecomms applications differ from oil and gas in that they tend to include more fibres, the lengths of which can vary over time due to network extensions / splices / repairs. These differences bring implementation challenges.

[0005] It would be desirable to overcome and / or substantially mitigate some or all of the above- mentioned and / or other disadvantages of the prior art.

[0006] According to a first aspect of the invention there is provided a method of acoustic sensing, the method comprising:

[0007] Performing a preliminary process, the preliminary process comprising: determining a round trip duration of a preliminary optical pulse that is transmitted into a head end of an optical fibre, is scattered at a remote end of the fibre and then returns to the head end; and using the determined round trip duration to determine a maximum pulse repetition frequency,

[0008] Performing a sensing process comprising: transmitting a plurality of optical sensing pulses into the head end of the optical fibre at a frequency that does not exceed the determined maximum pulse repetition frequency; and detecting the transmitted plurality of optical sensing pulses after having been scattered by the optical fibre, Wherein the maximum pulse repetition frequency corresponds to a frequency at which each of the plurality of optical sensing pulses returns to the head end of the optical fibre at the same time as its immediately-subsequent one of the plurality of optical sensing pulses is transmitted into the head end of the optical fibre.

[0009] The preliminary process may comprise determining a round trip duration of a plurality of preliminary optical pulses that are transmitted into respective head ends of a plurality of optical fibres. The sensing process may comprise transmitting a plurality of optical sensing pulses into the respective head ends of the plurality of optical fibres and detecting the transmitted plurality of optical sensing pulses after having been scattered by the plurality of optical fibres. The plurality of optical fibres may be part of a telecomms network. The preliminary process and / or the sensing process may be performed the under control of a software controller. The step of using the determined round trip duration to determine a maximum pulse repetition frequency may be performed by a software controller. The maximum pulse repetition frequency may be the inverse of the determined round trip duration.

[0010] The step of transmitting a plurality of optical sensing pulses into the head end of the optical fibre and detecting the transmitted plurality of optical sensing pulses after having been scattered by the optical fibre may be performed by an Optical Time-Domain Reflectometer (OTDR). The method may further comprise the step that the software controller instructs the OTDR to perform the sensing process without exceeding the determined maximum pulse repetition frequency. The step of transmitting a plurality of optical sensing pulses into the head end of the plurality of optical fibres may be performed sequentially, with an optical switch connecting the OTDR with each of the plurality of optical fibres in turn.

[0011] The sensing process may be performed by a sensing apparatus. The sensing apparatus may comprise the optical switch and the software controller.

[0012] The step of determining a round trip duration of a preliminary optical pulse that is transmitted into a head end of an optical fibre, is scattered at a remote end of the fibre and then returns to the head end may comprise obtaining the round trip duration from network apparatus that has previously used the optical fibre. The network apparatus may write the round trip duration to a look up table. The software controller may read the round trip duration from the look up table.

[0013] The step of determining a round trip duration of a preliminary optical pulse that is transmitted into a head end of an optical fibre, is scattered at a remote end of the fibre and then returns to the head end may comprise transmitting the preliminary optical pulse from into the head end of the optical fibre and detecting the returned measurement pulse after it has scattered from the remote end. In some embodiments this step is performed by a component of the sensing apparatus. In other embodiments it is performed by a device which is external to the sensing apparatus. In either case the step may be performed by an ODTR. The ODTR may transmit the time of transmission and the time of detection of the preliminary optical pulse to the software controller.

[0014] In some embodiments the step of determining a round trip duration of a preliminary optical pulse that is transmitted into a head end of an optical fibre, is scattered at a remote end of the fibre and then returns to the head end is performed by:

[0015] (i) an internal ODTR in respect of one or more of the plurality of optical fibres, the internal ODTR being a component of the sensing apparatus; and / or

[0016] (ii) an external ODTR in respect of one or more of the plurality of optical fibres, the internal ODTR being external to the sensing apparatus; and / or

[0017] (iii) obtaining the round trip duration in respect of one or more of the plurality of optical fibres from network apparatus that has previously used the optical fibre.

[0018] According to a second aspect of the invention there is provided an apparatus for performing a method of acoustic sensing, the apparatus comprising:

[0019] A determiner adapted to determine a round trip duration of a preliminary optical pulse that is transmitted into a head end of an optical fibre, is scattered at a remote end of the fibre and then returns to the head end; and

[0020] The determiner being further adapted to use the determined round trip duration to determine a maximum pulse repetition frequency,

[0021] Sensing apparatus adapted to: transmit a plurality of optical sensing pulses into the head end of the optical fibre at a frequency that does not exceed the determined maximum pulse repetition frequency; and detect the transmitted plurality of optical sensing pulses after having been scattered by the optical fibre;

[0022] Wherein the maximum pulse repetition frequency corresponds to a frequency at which each of the plurality of optical sensing pulses returns to the head end of the optical fibre at the same time as its immediately-subsequent one of the plurality of optical sensing pulses is transmitted into the head end of the optical fibre.

[0023] An embodiment of the invention will now be described in detail, for illustration only, with reference to the appended drawings, in which:

[0024] Fig 1 is a schematic view of an arrangement according to the prior art;

[0025] Fig 2 is a schematic view of an embodiment according to the invention;

[0026] Fig 3 is a schematic view of a further embodiment according to the invention;

[0027] Fig 4 is a schematic view of a still further embodiment according to the invention;

[0028] Fig 5 is a schematic view of a still further embodiment according to the invention;

[0029] Fig 6 shows a flow chart of the steps in accordance with an embodiment of the invention.

[0030] Distributed Acoustic Sensing (DAS) is used to determine the location of leaks, breakages or other events that occur in the vicinity of an optical fibre. An optical pulse is transmitted by a DAS unit into a fibre. The time of transmission is recorded. The pulse is scattered at any location along the fibre that are experiencing strain, as well as by the remote end of the fibre. This strain may be caused by a fluid leak or some other event that has occurred in the vicinity of the fibre.

[0031] These scattered signals return to the head end, are detected, and the arrival time is recorded. The difference between the transmission time and the arrival time is determined. This is known as the round-trip delay (RTD) of the pulse. As the speed of light is known, the RTD can be used to determine the location along the fibre that the scattering event took place. In this way, any parts of the fibre that are experiencing strain can be located. This process is repeated for a series of optical pulses. The frequency at which pulses are transmitted into the fibre is known as the pulse repetition frequency (PRF). For optimal operation, a pulse is not transmitted into the fibre after the previous pulse has returned. There is therefore a maximum pulse repetition frequency (PRF). This is determined by determining the length of each fibre and / or the RTD of a pulse.

[0032] Therefore the length of the fibres needs to be known accurately.

[0033] Fig 1 is a schematic drawing of an arrangement adapted to carry out this method. In particular, there is a DAS unit 1 containing an Optical Time-Domain Reflectometer (OTDR) 2. The OTDR 2 is the part of the DAS unit 1 that provides the sensing functionality described above. The OTDR 2 is connected to industrial plant 4 by an optical fibre 3.

[0034] In some applications, such as telecomms, the length of the fibres can change over time. For example, fibres can be shortened or additional sections spliced in. As mentioned above, it is desirable to know the length of the fibres accurately. The present invention proposes three possible methods to determine the length of a given fibre. These are:

[0035] 1. Using the DAS unit to transmit a pulse and measuring the RTD of the pulse after reflection from the remote end;

[0036] 2. Using an external unit to transmit a pulse and measuring the RTD of the pulse after reflection from the remote end;

[0037] 3. Creating a look up table containing fibre length information already gathered by equipment using the fibres and reading a desired fibre length from the look up table.

[0038] These methods will now be described in turn.

[0039] Method 1

[0040] Fig 2 is a schematic drawing of an arrangement in accordance with the invention adapted to carry out method 1 above. In a typical optical sensing session, the OTDR 22 of the DAS unit 21 transmits a set of several optical pulses to an optical switch 28 over optical fibre 27. Optical switch 28 directs the set of pulses to a first fibre 201. The pulses are backscattered at points along the fibre and return to the head end, arrive at optical switch 28 and are directed via optical fibre 27, to the OTDR 22 for detection. A pulse is not transmitted until the previous pulse has returned.

[0041] The OTDR 22 of the DAS unit 21 then transmits a second set of pulses to the optical switch 28 over optical fibre 27. Optical switch 28 directs the second set of pulses to a second fibre 202 and directs the resulting backscattered pulses to the ODTR. This process continues until a set of pulses has been transmitted onto each of the fibres 200 connected to the switch 28.

[0042] Thus, a series of sets of pulses are transmitted into the set of fibres 200. The optical switch is controlled by Control Plane Software 25.

[0043] As the person skilled in the art would understand, the return signals detected the OTDR 22 are analysed to identify the location and extent of any regions of strain along the fibres. Such regions are indicative of external forces being applied to the fibres. Remedial action can then be taken.

[0044] Before the optical sensing session begins, a preliminary measuring step is performed to determine and set the maximum PRF for each fibre in the set of fibres 200. In particular, the OTDR 22 transmits a pulse over optical fibre 27 to optical switch 28 and into the first fibre 201. The pulse is scattered at the remote end of, fibre 201. The scattered pulse returns to optical switch 28 and passes to OTDR 22. The OTDR 22 transmits the transmission time and arrival time of the optical pulse to control plane software 25 over connection 24. Control plane software 25 subtracts the transmission time of the pulse and its arrival time to determine the RTD of the pulse. Control plane software 25 then uses the RTD to determine the maximum PRF for the first fibre 201, and stores it in a data store (not shown). This process is then repeated for each of the remaining fibres in the set of fibres 200 until the control plane software 25 has determined and stored a maximum PRF for each of the fibres 200.

[0045] An optical sensing session can then begin. During such a session, control plane software 25 ensures that the PRF of pulses transmitted into each of the fibres 200 does not exceed the determined maximum PRF for that fibre. In particular, control plane software 25 instructs the OTDR 22, over connection 24, to transmit pulses destined for a given fibre at or below the determined maximum PRF for fibre. Furthermore, control plane software 25 sends an instruction over connection 26 to the optical switch 28 to ensure that, when the transmitted pulse is received at the optical switch 28, it is directed to the given fibre.

[0046] In this way it is ensured that pulses are transmitted into each fibre at or below the determined maximum PRF for that fibre. The result of this is that no pulses will be transmitted into a fibre until the previous pulse transmitted into that fibre has returned.

[0047] It is important to note that the determined maximum PRF may be modified according to the user’s requirements. For example, if a user does not require the PRF to be maximised, then a lower PRF is used instead. The maximum PRF is the upper limit for the PRF.

[0048] Method 2

[0049] Fig 3 is a schematic drawing of an arrangement in accordance with the invention adapted to carry out method 2 above. There are many similarities with the arrangement of Fig 2.

[0050] In a normal optical sensing session, the OTDR 32 of the DAS unit 31 transmits a set of several optical pulses to an optical switch 38 over optical fibre 37. Optical switch 38 directs the set of pulses to a first fibre 301. The pulses are backscattered at points along the fibre and return to the head end, arrive at optical switch 38 and are directed via optical fibre 37, to the OTDR 32 for detection. A pulse is not transmitted until the previous pulse has returned.

[0051] The OTDR 32 of the DAS unit 31 then transmits a second set of pulses to the optical switch 38 over optical fibre 37. Optical switch 38 directs the second set of pulses to a second fibre 302 and directs the resulting backscattered pulses to the ODTR. This process continues until a set of pulses has been transmitted onto each of the fibres 300 connected to the switch 38. Thus, a series of sets of pulses are transmitted into the set of fibres 300. The optical switch is controlled by Control Plane Software 35.

[0052] As in method 1, a preliminary measuring step is performed to set the maximum PRF for each fibre in the set of fibres 300. The difference in method 2 is that the preliminary measuring step involves an external ODTR 132. In particular, the external OTDR 132 transmits a pulse over optical fibre 39 to optical switch 38 and into the first fibre 301. The pulse is scattered at the remote end of, fibre 301. The scattered pulse returns to optical switch 38 and passes to external OTDR 132. The external OTDR 132 transmits the transmission time of the pulse and its arrival time to control plane software 35 over connection 34. Control plane software 35 subtracts the transmission time of the pulse and its arrival time to determine the RTD of the pulse. Control plane software 35 then uses the RTD to determine the maximum PRF for the first fibre 301 and stores it in a data store (not shown). This process is then performed for each of the remaining fibres in the set of fibres 300 until the control plane software has determined and stored a maximum PRF for each of the fibres 300.

[0053] An optical sensing session can then begin. During such a session, control plane software 35 ensures that the PRF of pulses transmitted into each of the fibres 300 does not exceed the determined maximum PRF for that fibre. In particular, control plane software 35 instructs the OTDR 32, over connection 30, to transmit pulses destined for a given fibre at or below the determined maximum PRF for that fibre. Furthermore, control plane software 35 sends an instruction over connection 36 to the optical switch 38 to ensure that, when the transmitted pulse is received at the optical switch 38, it is directed to the given fibre.

[0054] In this way it is ensured that pulses are transmitted into each fibre at or below the determined maximum PRF for that fibre. The result of this is that no pulses will be transmitted into a fibre until the previous pulse transmitted into that fibre has returned.

[0055] Use of an external OTDR 132 has certain advantages. In particular, each pulse produced by the OTDR 132 typically contains three different wavelengths. The OTDR 32 in the DAS unit 31 will typically produce pulses containing only a single wavelength. The use of three wavelengths makes it easier to identify certain features of the fibre, such a bends and splices. This improves the accuracy of the maximum PRF for the fibre.

[0056] Method 3

[0057] Fig 4 is a schematic drawing of an arrangement in accordance with the invention adapted to carry out method 3 above. There are many similarities with the arrangement of Fig 2. In a normal optical sensing session, the OTDR 42 of the DAS unit 41 transmits a set of several optical pulses to an optical switch 48 over optical fibre 47. Optical switch 48 directs the set of pulses to a first fibre 401. The pulses are backscattered at points along the fibre and return to the head end, arrive at optical switch 48 and are directed via optical fibre 47, to the OTDR 42 for detection. A pulse is not transmitted until the previous pulse has returned.

[0058] The OTDR 42 of the DAS unit 41 then transmits a second set of pulses to the optical switch 48 over optical fibre 47. Optical switch 48 directs the second set of pulses to a second fibre 402 and directs the resulting backscattered pulses to the ODTR. This process continues until a set of pulses has been transmitted onto each of the fibres 400 connected to the switch 48.

[0059] Thus, a series of sets of pulses are transmitted into the set of fibres 400. The optical switch is controlled by Control Plane Software 45.

[0060] As in methods 1 and 2, a preliminary step is performed to set the maximum PRF for each fibre in the set of fibres 400. In method 3 this is achieved as follows. Existing equipment 43 is present which has operated on the fibres 400 before. As part of this prior use, the existing equipment 43 is aware of the length of each of the fibres 400 and / or the RTD of an optical pulse traversing each of the fibres 400. In the preliminary step of method 3, the existing equipment stores the determined length and / or RTD for each of the fibres 400 in a look-up table 49. The control plane software 45 reads this information from the look-up table 49 using connection 44. The control plane software 45 uses the read length / RTD information to determine the maximum PRF for that fibre.

[0061] An optical sensing session can then begin. During such a session, control plane software 45 ensures that the PRF of pulses transmitted into each of the fibres 400 does not exceed the determined maximum PRF for that fibre. In particular, control plane software 45 instructs the OTDR 42, over connection 40, to transmit pulses destined for a given fibre at or below the determined maximum PRF for that fibre. Furthermore, control plane software 45 sends an instruction over connection 47 to the optical switch 48 to ensure that, when the transmitted pulse is received at the optical switch 48, it is directed to the given fibre.

[0062] In this way it is ensured that pulses are transmitted into each fibre at or below the determined maximum PRF for that fibre. The result of this is that no pulses will be transmitted into a fibre until the previous pulse transmitted into that fibre has returned.

[0063] A benefit of method 3 is that the preliminary step does not require the sensing apparatus to measure the length / RTD for each fibre. Doing so would be timeconsuming and expensive, especially if the number of fibres is large. Instead, the maximum PRF is determined using information that is already known to the network.

[0064] Combined Method

[0065] In a telecomms network, there may be some optical fibres for which existing equipment is not aware of the fibre length / RTD. Fig 5 shows a further embodiment in accordance with the invention which addresses this. The difference between the embodiments of Fig 4 and Fig 5 is that the embodiment of Fig 5 contains an external ODTR 53, such as the one shown in Fig 3. In the embodiment of Fig 5 the RTD for a given fibre can be obtained by each of methods 1, 2 and 3.

[0066] In particular, for the fibres in optical fibre set 500 in respect of which the RTD of an optical pulse transmitted along the fibre is known from existing equipment, Control Plane Software 55 reads the RTD from the lookup table 64. For the fibres in optical fibre set 500 in respect of which the RTD of an optical pulse transmitted along the fibre is not known from existing equipment, Control Plane Software 55 determines the RTD for those fibres from either the OTDR 52 of the DAS unit 51 or the external OTDR 53, depending on the condition of the optical fibre in question. In particular, if the fibre contains significant bends and splices, the external OTDR 53 is used, as its multiple wavelength pulses make such features easier to identify, as described above. If, however, the fibre does not contain significant bends and splices, the OTDR 52 of the DAS unit 51 is used. This is preferable due to its lower cost compared with the external OTDR 53. The selection of which of the three methods to use for each fibre is made by the Control Plane Software 55.

[0067] Fig 6 shows a flow chart of the steps in accordance with an embodiment of the invention. The steps are as follows.

[0068] Step 601: for each of multiple optical fibres in a telecomms network, determine the Round Trip Duration (RTD) of an optical pulse that is transmitted into the head end of the fibre and returns to the head end following scattering at the remote end.

[0069] Step 602: use the determined RTD for each fibre to determine the maximum Pulse Repetition Frequency (max PRF) for that fibre.

[0070] Step 603: for each fibre, perform optical sensing by transmitting a series of optical sensing pulses into the head end and detecting the scattered return signals, ensuring that the transmission frequency for a given fibre does not exceed the determined max PRF for that fibre.

[0071] Step 604: analyse scattered return signals to determine location and extent of any strain experienced by each fibre.

Claims

Claims1.A method of acoustic sensing, the method comprising: performing a preliminary process, the preliminary process comprising: determining a round trip duration of a preliminary optical pulse that is transmitted into a head end of an optical fibre, is scattered at a remote end of the fibre and then returns to the head end; and using the determined round trip duration to determine a maximum pulse repetition frequency, performing a sensing process comprising: transmitting a plurality of optical sensing pulses into the head end of the optical fibre at a frequency that does not exceed the determined maximum pulse repetition frequency; and detecting the transmitted plurality of optical sensing pulses after having been scattered by the optical fibre, wherein the maximum pulse repetition frequency corresponds to a frequency at which each of the plurality of optical sensing pulses returns to the head end of the optical fibre at the same time as its immediately-subsequent one of the plurality of optical sensing pulses is transmitted into the head end of the optical fibre.

2. A method as claimed in claim 1 , wherein the step of transmitting a plurality of optical sensing pulses into the head end of the optical fibre and detecting the transmitted plurality of optical sensing pulses after having been scattered by the optical fibre is performed by an Optical Time-Domain Reflectometer (OTDR).

3. A method as claimed in claim 2, wherein the step of transmitting a plurality of optical sensing pulses into the head end of the plurality of optical fibres is performed sequentially, with an optical switch connecting the OTDR with each of the plurality of optical fibres in turn.

4. A method as claimed in any preceding claim, wherein the sensing process is performed by a sensing apparatus.

5. A method as claimed in any preceding claim, wherein the step of determining a round trip duration of a preliminary optical pulse that is transmitted into a head end of an opticalfibre, is scattered at a remote end of the fibre and then returns to the head end comprises obtaining the round trip duration from network apparatus that has previously used the optical fibre.

6. A method as claimed in claim 5, wherein the network apparatus writes the round trip duration to a look up table.

7. A method as claimed in any preceding claim, wherein the step of determining a round trip duration of a preliminary optical pulse that is transmitted into a head end of an optical fibre, is scattered at a remote end of the fibre and then returns to the head end comprises transmitting the preliminary optical pulse from into the head end of the optical fibre and detecting the returned measurement pulse after it has scattered from the remote end.

8. A method as claimed in claim 7, wherein the step of transmitting the preliminary optical pulse from into the head end of the optical fibre and detecting the returned measurement pulse after it has scattered from the remote end is performed by a component of the sensing apparatus.

9. A method as claimed in claim 7 or claim 8, wherein the step of transmitting the preliminary optical pulse from into the head end of the optical fibre and detecting the returned measurement pulse after it has scattered from the remote end is performed by a device which is external to the sensing apparatus.

10. A method as claimed in any preceding claim, wherein the preliminary process comprises determining a round trip duration of a plurality of preliminary optical pulses that are transmitted into respective head ends of a plurality of optical fibres.

11. A method as claimed in claim 10, wherein the sensing process comprises transmitting a plurality of optical sensing pulses into the respective head ends of the plurality of optical fibres and detecting the transmitted plurality of optical sensing pulses after having been scattered by the plurality of optical fibres.

12. A method as claimed in claim 10 or claim 11 , wherein the step of determining a round trip duration of a preliminary optical pulse that is transmitted into a head end of an opticalfibre, is scattered at a remote end of the fibre and then returns to the head end is performed by:(i) an internal ODTR in respect of one or more of the plurality of optical fibres, the internal ODTR being a component of the sensing apparatus; and / or(ii) an external ODTR in respect of one or more of the plurality of optical fibres, the internal ODTR being external to the sensing apparatus; and / or(iii) obtaining the round trip duration in respect of one or more of the plurality of optical fibres from network apparatus that has previously used the optical fibre.

13. An apparatus for performing a method of acoustic sensing, the apparatus comprising: a determiner adapted to determine a round trip duration of a preliminary optical pulse that is transmitted into a head end of an optical fibre, is scattered at a remote end of the fibre and then returns to the head end; and the determiner being further adapted to use the determined round trip duration to determine a maximum pulse repetition frequency,Sensing apparatus adapted to: transmit a plurality of optical sensing pulses into the head end of the optical fibre at a frequency that does not exceed the determined maximum pulse repetition frequency; and detect the transmitted plurality of optical sensing pulses after having been scattered by the optical fibre; wherein the maximum pulse repetition frequency corresponds to a frequency at which each of the plurality of optical sensing pulses returns to the head end of the optical fibre at the same time as its immediately-subsequent one of the plurality of optical sensing pulses is transmitted into the head end of the optical fibre.

14. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of any of claims 1 to 12.

15. A computer-readable carrier medium comprising the computer program of claim 14.

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