Monitoring of exposed submarine cables

Abstract

A method and system for determining exposure points along a buried submarine cable are disclosed. The method includes transmitting a query signal to a sensing optical fiber extending the length of the submarine cable, and receiving a backscattered signal from the sensing optical fiber in response to the query signal. The method further includes processing the backscattered signal to determine the temporal and / or spatial variation of strain along the sensing optical fiber, the determined spatial and / or temporal variation of strain indicating exposure points and their locations along the length of the buried submarine cable.

Description

Technical Field

[0001] Aspects of the present disclosure generally relate to methods and / or systems for monitoring subsea assets such as cables, and more particularly to methods and / or related systems for monitoring the exposure of subsea assets.

Background Art

[0002] Any reference in this specification to prior art is not an admission or suggestion that this prior art forms part of the common general knowledge in any jurisdiction, or that this prior art is understood by a person skilled in the art, considered relevant, and / or reasonably foreseeable as being combinable with other prior art.

[0003] The burial of underwater telecommunication and / or power cables is frequently required or strongly recommended by law, particularly in many areas near the coast, and the cables are laid in shallow water, for example, in water with a depth less than from 500 to 1000 meters.

[0004] To meet such requirements, optical cables are often buried to a depth of about 1 meter or more. However, in some areas where the cables need to be buried, they may not be buried. This can occur for various reasons. For example, the cable may not be buried due to high burial costs or because the cable is located far from the coast and thus inaccessible by fishing boats or other external means of attack - and thus generally has a low risk of damage - in deep water. Alternatively, the cable may be initially buried but may eventually become unburied due to, for example, the activity level of sediment near the coast around the cable, undersea currents, and / or storm activity. For example, it has been found that during storms, the exposure of cables near the coast changes dramatically due to high wave activity.

[0005] In particular, it may be desirable to monitor the buried / unburied status of submarine fiber optic cables in highly active areas where external attack factors (e.g., fishing boats or fish) could damage the cables. Such information can then be used to mitigate risks and hazards to the cables, thereby extending their lifespan. [Overview of the project]

[0006] A first aspect of the present disclosure provides a method for determining exposure points along a buried submarine cable. The method includes transmitting a query signal to a sensing optical fiber extending the length of the submarine cable; receiving a backscattered signal from the sensing optical fiber in response to the query signal; and processing the backscattered signal to determine temporal and / or spatial variations of strain along the sensing optical fiber, wherein the determined spatial and / or temporal variations of strain indicate exposure points and their locations along the length of the buried submarine cable.

[0007] Determining the spatial and / or temporal variations of strain includes determining the variations in the true amplitude and duration of the backscatter signal at locations along the length of the photosensitive fiber. Furthermore, an exposure point is detected by determining that the true amplitude of the backscatter signal at a location along the length of the photosensitive fiber exceeds a threshold and remains above the threshold for a threshold duration. In some embodiments, the threshold is determined based on baseline strain values ​​at corresponding locations along the length of the photosensitive fiber. Baseline strain values ​​may have been determined when the submarine cable was laid.

[0008] In the embodiment, determining the spatial and / or temporal variation of the strain includes determining the true amplitude and duration variation of the backscatter signal at positions along the length of the photosensitive fiber.

[0009] In an embodiment, the method further includes detecting an exposure point by determining that the true amplitude of the backscatter signal at a location along the length of the photosensitive fiber exceeds a threshold and remains above the threshold for a threshold duration.

[0010] In one embodiment, the threshold is determined based on the baseline strain value at a corresponding position along the length of the optical fiber, and the baseline strain value is determined when the submarine cable is laid.

[0011] In an embodiment, this method further includes determining the position of the exposure point based on the timing of receiving the backscatter signal.

[0012] In an embodiment, this method further includes determining a risk score associated with the detected exposure location.

[0013] In the embodiment, determining the risk score includes determining the geographical location of the exposure site and determining the risk score associated with the geographical location based on contextual data.

[0014] In one embodiment, the method further includes generating an alert if it determines that the risk score of a detected exposure location exceeds a threshold risk score.

[0015] In an embodiment, the method further includes generating a cable risk map based on detected exposure locations and communicating the cable risk map to operators of cables buried on the seabed.

[0016] A second aspect of the present disclosure provides a distributed optical fiber sensing (DFOS) system comprising: a light source configured to transmit a query signal to a sensing optical fiber extending the length of a submarine cable; a photodetector configured to receive a backscattered signal from the sensing optical fiber in response to the query signal; and a processing unit configured to process the backscattered signal to determine temporal and / or spatial variations of strain along the sensing optical fiber, wherein the determined spatial and / or temporal variations of strain indicate exposure points and their locations along the length of a buried submarine cable.

[0017] The DFOS system may be configured to detect exposure points by determining the true amplitude and duration variations of the backscatter signal at locations along the length of the optical fiber, and determining if the true amplitude of the backscatter signal at a location along the length of the optical fiber exceeds a threshold and remains above the threshold for a threshold duration. The threshold may be determined based on the baseline distortion value at the corresponding location along the length of the optical fiber. For example, the threshold may be set as the baseline distortion value. The baseline distortion value may be determined when the submarine cable is laid.

[0018] In the embodiment, in order to determine the spatial and / or temporal variations of the strain, the processing unit is further configured to determine the true amplitude and duration variations of the backscatter signal at positions along the length of the photosensing fiber.

[0019] In one embodiment, the processing unit is further configured to detect exposure locations by determining that the true amplitude of the backscatter signal at a location along the length of the photosensitive fiber exceeds a threshold and remains above the threshold for a threshold duration.

[0020] In one embodiment, the threshold is determined based on the baseline strain value at a corresponding position along the length of the optical fiber, and the baseline strain value is determined when the submarine cable is laid.

[0021] In an embodiment, the processing unit is further configured to determine the location of the exposure point based on the timing of receiving the backscattered signal.

[0022] In an embodiment, the processing unit is further configured to determine a risk score associated with the detected exposure point.

[0023] In an embodiment, determining the risk score includes determining the geographical location of the exposure point and determining a risk score associated with the geographical location based on context data.

[0024] In an embodiment, the processing unit is further configured to generate an alert when it determines that the risk score of the detected exposure point exceeds a threshold risk score.

[0025] Further aspects of the present invention and further embodiments of the aspects described in the previous paragraphs are given by way of example and will become apparent from the following description, which refers to the accompanying drawings.

Brief Description of the Drawings

[0026] [Figure 1] Schematic of an exemplary undersea cable. [Figure 2A] An example of a system for distributed acoustic sensing is shown. [Figure 2B] An example of an electrical signal generated over time by the system of FIG. 2A is shown. [Figure 3] Schematic diagram of a disclosed method of distributed acoustic sensing for detecting cable exposure according to some aspects of the present disclosure. [Figure 4A] A sequence of transmitted interrogation optical signals and a corresponding sequence of observation windows at multiple time points are schematically shown. [Figure 4B] An example of a plot of the amplitude of the returned optical signal versus distance is schematically shown. [Figure 5A]The following are exemplary power spectral density (PSD) plots generated based on the returned optical signal according to some embodiments of the present disclosure. [Figure 5B] An exemplary exposure likelihood plot, determined based on the PSD plot in Figure 5A, is shown. [Figure 6] This is an exemplary plot illustrating the determination of cable exposure locations using the DFOS and DTS methods. [Modes for carrying out the invention]

[0027] Typically, submarine telecommunications and / or power cables face four main types of hazards: defects in manufacturing, natural hazards, external attacks, and abrasion. Defects in manufacturing are usually controlled by enforcing rigorous quality protocols and standards throughout the industry. Natural hazards such as submarine landslides, tsunamis, submarine volcanoes, and earthquakes are often difficult to predict, but can be mitigated as much as possible through careful route planning, such as not laying cables in areas known to be prone to landslides, earthquakes, etc. On the other hand, hazards from external attacks and / or abrasion are both far more difficult to detect and mitigate.

[0028] External attacks refer to any type of damage to cables resulting from external activity, such as fishing nets or trawls getting caught on exposed cables, or a ship's anchor being dragged along the seabed and accidentally damaging exposed cables on the seabed. In the case of external attacks, cable exposure creates potential problems that would not exist if the cables were buried beneath the seabed. Abrasion is also associated with exposure because, if the cables are loose, they can move freely along the seabed, which can lead to excessive abrasion of the cables, such as insulation degradation due to friction with surrounding rocks / corals, cable strumming, or cable vibration (for example, as the cables are pulled by tension between two points).

[0029] The risks of both attack and abrasion can be prevented by highly insulating and / or sheathing the cables with layers of strong materials such as plastic, gel, and / or steel. While this can be effective, it significantly increases the cost of laying the cables.

[0030] Another way to reduce the risk of attack and / or abrasion is to bury the cable beneath the seabed, as previously mentioned. Figure 1 shows an exemplary cable (e.g., a telecommunications cable) 100. Cable 100 is buried about 1 meter below the seabed. However, due to currents, storms, oceanic activity, etc., cable 100 may become unburied over time. Figure 1 shows that cable 100 has become unburied at one location - 104 -. This location may be referred to in this disclosure as an exposed location. If exposed location 104 remains unburied, cable 100 in this area may become abraded over time and / or be subjected to attack from fishing boats, marine life, movement, etc. Therefore, it is desirable to monitor the exposure of submarine cables. Appropriate measures can then be taken - for example, if it is determined that one or more portions of the cable are unburied and exposed near land, those portions of the cable can be reburied, or measures can be taken to warn nearby anglers in real time about those exposed locations.

[0031] There are three common techniques currently used to assess and monitor submarine cable exposures. One technique involves periodic campaign-style surveys of cable assets. This may include scuba divers or remotely operated vehicles visually inspecting the cable path at cable exposure points. As can be understood, this method is expensive and sporadic. Cables may become exposed during the period between two surveys, which may only be discovered in the next survey.

[0032] Another technique involves the use of multibeam sonar to map seabed and submarine cable assets. Multibeam sonar surveys typically use source-receiver arrays mounted directly onto the hull of a vessel. This method of surveying cable assets is capital-intensive and time-consuming. Furthermore, these surveys are dependent on passing vessels and, logistically, cannot be conducted regularly, or even annually, to provide the data necessary to warn fishermen about exposure locations in real time.

[0033] In the third technology, cable exposure points are assessed along the submarine cable using distributed temperature sensing (DTS) systems. These systems operate on the assumption that when a portion of the optical cable is exposed or not buried, the temperature of that portion changes compared to the unexposed or buried portion. For example, the presence of a circulating ocean environment may cause the temperature to decrease in the exposed portion. Using DTS systems, temperature changes along the optical cable can be detected, and any detected temperature changes can be used to determine the location of the exposure.

[0034] In such a system, one or more optical fibers may be included in any submarine cable system. Optical signals can be transmitted to the optical fibers, and potential exposure locations can be detected by analyzing the Raman backscatter of the transmitted signals. In particular, in a DTS system, the ratio of Stokes to anti-Stokes Raman scattering is used to determine the absolute temperature along the optical fiber. If the temperature of the optical cable decreases in a particular region of the optical cable, those regions can be evaluated as exposure points.

[0035] While DTS can be used to determine exposure locations remotely, it has one or more problems. For example, DTS can typically only cover short distances (e.g., within a range of 30-40 km). Therefore, it cannot be used to identify exposure locations further from the coast. Furthermore, DTS generally requires dedicated multimode fiber for the sensing element. Multimode fiber is not very common in marine environments, and therefore, DTS cannot be implemented using existing cables laid in marine environments.

[0036] Aspects of this disclosure provide a novel method for evaluating cable exposure based on distributed fiber optic sensing (DFOS). The method and system of this disclosure can utilize single-mode optical fiber for detection and can detect cable exposure locations along longer distances, for example, beyond 100 km. Furthermore, aspects of this disclosure can provide end users with a method for determining the risk of submarine cables and then mitigating or preventing damage in near real-time.

[0037] To this end, the disclosed systems and methods use DFOS on existing submarine optical fibers to estimate cable exposure by distributing it along the length of the cable. In some embodiments, the methods and / or systems use Rayleigh scattering and / or Brillouin scattering in the optical fiber to evaluate dynamic or static strain along the optical fiber and determine whether the optical fiber is exposed (e.g., not buried). In particular, the measured strain provides information about vibrations occurring in the optical cable. Exposed or not buried cables are subject to greater strain from environmental phenomena (e.g., passing ships, storms, waves, etc.), while buried cables are presumed to be less likely to be subject to strain from such environmental phenomena because they are buried beneath the seabed. Furthermore, buried cables have specific vibration damping characteristics based on the damping characteristics of the materials forming the seabed, while not buried or exposed cables have different vibration damping characteristics based on the damping characteristics of water. These differences or variations in strain or vibration damping values ​​can be used by the systems and methods of the disclosed to determine whether parts of the cable are exposed and to determine the precise location of such exposures.

[0038] In one example, DFOS data is recorded from the shore side of single-mode optical fibers that are incorporated into or separated from a submarine cable wet plant but located near submarine cable assets. The DFOS data is collected and (based on the strain detected in the DFOS data) dispersed to estimate the state or change in the state of the submarine cable burial. Next, based on the DFOS data, it is determined whether any portion of the monitored cable is likely to be exposed. If it is determined that one or more portions of the monitored cable are likely to be exposed, a cable risk map is generated. The cable risk map may show the actual geographical location of the potential exposure and the level of risk associated with the exposure. In some embodiments, the cable risk map may include contextual information such as the water depth above the possible cable exposure points, the frequency with which ship routes cross the possible cable exposure points, the frequency of high-risk AIS navigation alerts, forecasts of storm and high wave activity, history of cable strikes in the area, and / or the risk tolerance of the cable owner. Finally, based on the cable risk map, warnings can be issued to the cable owner, cable operator, or cable infrastructure user to consider mitigation strategies such as reburying the cable or avoiding the potential exposure points. Optionally, alerts can be automatically communicated, for example, via an Automatic Identification System (AIS), to warn potential vessels in the area about any location where cables may be exposed.

[0039] These and other aspects of the Disclosure are described in detail in the following sections. Exemplary DAS system

[0040] Generally speaking, a DAS system includes an inquiry unit and a sensing cable. The inquiry unit continuously injects short pulses of optical signals into the sensing cable. As the light passes through the fiber core of the sensing cable, the incident light is scattered in different directions due to spatial variations in the refractive index of the fiber core, generating different types of scattered light (e.g., Rayleigh, Raman, and Brillouin). When the optical fiber is disturbed, distorted, and / or vibrates, the properties of the scattered light (wavelength, light intensity, frequency, etc.) change. By analyzing the specific properties of the returned scattered light, changes in various physical parameters (e.g., axial strain, strain rate, vibration damping) can be revealed. Furthermore, by analyzing the phase information of the Rayleigh scattered light, measurements of dynamic strain (vibration or sound waves) can be obtained.

[0041] Figure 2A shows an exemplary DAS system 200 that can be used in the method of the present disclosure. The DAS system 200 may be optically coupled to one or more optical fibers 202. In this example, it is operably coupled to three optical fibers 202A, 202B, and 202C. However, it will be understood that in other embodiments, it may be coupled to more or fewer optical fibers without departing from the scope of the present disclosure. The optical fibers 202 may be integrated with one or more submarine cable assets, or otherwise in the vicinity of one or more submarine cable assets, and may exist along the entire length of one or more cable assets. In one example, the optical fibers may be bundled together with the cable assets, and the entire bundle may be enclosed in a protective shield. Furthermore, the fibers may be oriented in any orientation to introduce two or more components of motion / deformation to the length of each gauge of subsequent DFOS measurements, or they may even be wound spirally around a central cylinder. Furthermore, existing optical fibers laid for other purposes can also be used for DFOS measurements. Multiple fibers can be joined in series and used for DFOS in a single instrument, or multiple DFOS channels (analyzed by the same or separate DFOS instruments) can be used to record DFOS data within the same perimeter.

[0042] The DAS system 200 includes a coherent optical time-domain reflectometer (C-OTDR) 204. The C-OTDR 204 includes a light source 206 for emitting an optical query field in the form of short optical pulses 207 to be sent to each of the optical fibers 202A, 202B, and 202C.

[0043] The light source 206 is a laser source, and the optical pulses transmitted by the laser source can be simple pulses, chirp pulses, or continuous waves. Furthermore, the optical pulses 207 may be in the infrared or near-infrared frequency range.

[0044] The C-OTDR 204 also includes a photodetector 208 configured to detect optical signals returning from or backscattered by optical fibers 202A, 202B, and 202C. The backscattered signals may include Rayleigh, Brillouin, and / or Raman scattering. Rayleigh scattering is a form of elastic scattering of light that conserves the kinetic energy of the incident particle in the optical fiber where scattering occurs; that is, in this type of scattering, the scattered photon has the same energy as the incident photon. Raman scattering is a form of inelastic scattering of light that does not conserve the kinetic energy of the incident particle. Brillouin scattering is a “photon-phonon” interaction, as the annihilation of the incident photon simultaneously produces a Stokes photon and a phonon. Both Raman and Brillouin scattering produce canned Stokes and anti-Stokes components.

[0045] Generally speaking, the return light 210 is dispersed and scattered in the optical fiber 202. In some embodiments, the photodetector 208 passes the detected optical signal 210 directly to the processing unit 214. In other embodiments, the photodetector 208 may convert the optical signal 210 into an electrical signal, such that the amplitude of the electrical signal 212 is proportional to the reflected light intensity resolved over time. The time scale can be converted to a distance scale relative to the photodetector 208. Figure 2B shows a schematic plot 250 of the amplitude of the optical signal over distance at a particular moment.

[0046] Returning to Figure 2A, the DAS system 200 also includes a processing unit 214, either inside or separately from the C-OTDR 204. The processing unit 214 can be configured to process the optical signal returned from the optical fiber and store the processed data as DFOS data. In some embodiments, the processing unit 214 may be an optical processor that optically analyzes the received optical signal. In other embodiments, the processing unit 214 may be a digital processor that converts the electrical signal received from the photodetector 208 into digital DFOS data and then stores this data. As referred to herein, the DFOS data includes values ​​relating to the state of the optical fiber for a specific time sample at all sensor locations in the fiber.

[0047] In a particular embodiment, the processing unit 214 uses the DFOS data to determine whether the strain is dynamic or static along the optical fiber.

[0048] In certain examples, the input pulse 207 may be transmitted continuously within the optical fiber, the DFOS data may be continuously updated, and dynamic or static strain may also be continuously calculated from the DFOS data. In other examples, the input pulse may be transmitted periodically (e.g., every few hours or days), and the DFOS data may be updated at the same frequency. Dynamic or static strain along the optical fiber can be calculated at the same frequency or at different frequencies (e.g., every few days). In other embodiments, the strain values ​​can be calculated sporadically (e.g., in response to known activity, e.g., storms, earthquakes, cyclones, etc.).

[0049] In addition to determining strain values, the processing unit 214 may also be configured to determine whether any portion of the cable is exposed (for example, based on strain data), determine the geographical location of such potential exposures, and generate a cable risk map. The processing unit 214 may also be configured to automatically generate alerts based on the cable risk map.

[0050] DFOS data or records may be stored in the storage unit 215. The storage unit 215 may include volatile memory, such as random access memory (RAM), for the processing unit 214 to execute instructions, compute, calculate, or process data. The storage unit 215 may include non-volatile memory, such as a hard disk drive, for the processing unit 214 to store data for pre- and post-and / or post-processing retrieval. The processing unit 214 and the storage unit 215 may be distributed across a number of physical units and may include remote storage such as cloud storage, in which case the processing unit 214 and the storage unit 215 may be more generally defined as a cloud computing service.

[0051] Finally, the system 200 also includes a communication interface 217 that communicates with the processing unit 214, which can be used to communicate cable risk maps to cable operators and / or send alert signals to vessels near possible cable exposure points. The communication interface may also be configured to receive requests for cable risk maps from one or more remote mobile terminals or fixed terminals (not shown). Upon receiving a request for a risk map, the processing unit 214 may be configured to generate or retrieve the requested risk map from the storage unit 215 (if it has already been generated and stored).

[0052] Figure 3 is a flowchart illustrating an exemplary method 300 for determining cable exposure according to several aspects of the present disclosure.

[0053] Method 300 begins in step 302, in which a query optical signal 207 is transmitted by a light source to one or more optical fibers 202 integrated with or near one or more submarine cable assets. The query signal may be transmitted in multiple instances. Figure 4A shows an exemplary query optical pulse transmitted by the light source in this step. In particular, Figure 4A shows three time points 252A, 252B, and 252C in which the query optical signal 207 is transmitted within the optical fiber.

[0054] Once the signal is transmitted, the method proceeds to step 304, where a return signal 210 is received from one or more optical fibers 202. Figure 4A also shows the return signals. These return signals 210 are received during observation periods 254A, 254B, and 254C between times 252A, 252B, and 252C. The time at which the return signals are received during the observation period indicates the position of the optical fiber from which the optical signal was backscattered. The later the return signal is received during the observation period, the further away the signal is along the backscattered optical fiber. The characteristics of the returned optical signal (e.g., wavelength, light intensity, frequency, phase, etc.) may vary depending on the type of strain the section of the optical fiber experiences. Therefore, by analyzing the characteristics and timing of the return signals, the processing unit can determine the strain occurring in a particular portion of the optical fiber 202.

[0055] Figure 4B shows the return optical signal 210 from Figure 4A, converted into an electrical signal 212 by the photodetector 208. Each electrical signal 212 has acoustic fluctuations 217. The different fluctuation peaks 217 shown in Figure 4B are due to different return optical signals 210 generated by different types of distortion occurring in one or more sections of the optical fiber.

[0056] The electrical or optical signals received from the optical fiber 202 represent DFOS data, which may be stored in the storage unit 215 by the processing unit 214 in this step. It will be understood that steps 302 and 304 may be repeated continuously, periodically, or at some other frequency. In other cases, these steps may be performed based on one or more trigger conditions—for example, one or more activities detected near the cable (e.g., storms, boat activity, etc.) in response to a request for a cable risk map.

[0057] The frequency bandwidth of DFOS data is 0.001–500 Hz, and it will be understood that this covers a wide frequency range from static or nearly static timescales to the Nyquist sampling frequency of the techniques disclosed herein. In the case of Rayleigh scattering methods such as DAS, the Nyquist frequency can be 1000 Hz for a submarine cable of 50 km in length. Thus, the DFOS measurement bandwidth covers multiple available environmental phenomena or signals generated by active sources in the marine environment.

[0058] For example, DFOS technology can detect disturbances with a wide range of frequencies. These may include, for example, ocean noise (sound waves traveling through a water column), Schorte waves (interface waves traveling between a water column and the seabed), ocean loading (coupled ocean surface gravity waves), or seismic waves (elastic waves traveling through the solid Earth). The DFOS technology described herein can also detect passing ships by changing the frequency of the input pulse so that it is within the ship's frequency range.

[0059] In step 306, the likelihood of exposure of one or more submarine cables is determined from the stored DFOS data. For this purpose, the processing unit first determines the static or dynamic strain on the optical cable along the length of the cable over a specific period of time, based on environmental conditions (e.g., passing ships, waves, etc.). Without departing from the scope of this disclosure, any known technique can be used to determine the strain from the DFOS data.

[0060] Next, the processing unit 214 calculates the power spectral density along the length of the optical fiber. Figure 5A shows the power spectral density of DFOS data associated with a given optical fiber over 8 hours. The horizontal axis of plot 500 represents the position along the sensing optical fiber 202, and the vertical axis represents time (e.g., 8 hours in this example). The color-coded signals in plot 500 represent the values ​​of acoustic intensity and / or distortion. In particular, the gray areas of the plot indicate high signal amplitude (or high distortion), the black areas indicate low signal amplitude (or low distortion), and the white areas indicate intermediate signal amplitude (or intermediate distortion).

[0061] From the spectral power density plot, the processing unit 214 can determine the location of possible exposure points. For example, the white and gray areas of the power spectral map show higher distortion than the black areas and can indicate potential exposure points. For greater accuracy, the processing unit 214 can calculate a statistical trace of the DFOS data calculated from the power spectral density plot. The statistical trace shows the optical distance along the y-axis and the true amplitude of the DFOS signal along the x-axis.

[0062] The true amplitude is calculated by treating the optical fiber cable as an array of sensing points and measuring the amplitude of the backscattered signal at each position along the array. Each sensing point in the array is a short segment of fiber, ranging from 1 to 30 meters in length (also called the gauge length), which is used as an individual strain-sensing channel to generate an array-type measurement of the true amplitude from a simple optical fiber.

[0063] In particular, the true amplitude is estimated using a likelihood function (along the optical fiber) between two points (called the gauge length) where the change in amplitude of the strain data is recorded. Alternatively, the true amplitude can be estimated via a maximum likelihood envelope estimator for the amplitude of the DFOS signal.

[0064] Statistical traces may be used as a substitute for exposure likelihood. Any known low-distortion bands (e.g., ground bands or protected bands, i.e., where cables are within protective enclosures) or high-distortion bands (e.g., rippling bands) may be labeled and masked in the statistical trace. All other portions of the statistical trace that exceed a given true amplitude threshold and minimum width criterion may be labeled as potential exposure points because these points will exhibit higher distortion over long periods than their adjacent areas.

[0065] Figure 5B shows an exemplary statistical trace 504 generated from the PSD plot of Figure 5A. In this example, the ground band, the rippling band, and the protected band are labeled. These bands may be masked. For example, all other parts of the statistical trace that exceed a predefined signal amplitude threshold and minimum width criterion of 10 units may be labeled as potential exposure points. In Figure 5B, there are two points 506, 508 that contain high amplitude signals exceeding 10 units for at least the minimum width (approximately 35 m), and these parts 506, 508 are labeled as potential exposure points.

[0066] In some embodiments, the processing unit 215 can calculate the statistical likelihood to extract the cable burial state from the DFOS data on a single gauge length basis. Alternatively, the maximum statistical envelope movement estimate for the true amplitude of the DAS signal can be calculated. In particular, the processing unit 215 calculates the true amplitude of the recorded acoustic data along the optical fiber cable, specifically around an optical distance of several hundred meters.

[0067] In step 308, it is determined whether any exposure points were identified in step 306. If no exposure points are identified, method 300 terminates. Alternatively, if one or more potential exposure points are identified along the buried submarine cable, the method proceeds to step 310, in which processing unit 214 determines the risks associated with the potential exposures of the identified exposure points. To this end, processing unit 214 determines the geographical location of the exposure points. This can be done by determining the distance of the exposure points along the optical fiber and determining the location of the optical cable and its path along the seabed. In some examples, the location information may be available in storage unit 215 or may be retrieved from an external source.

[0068] Once the geographical location of an exposure point is determined, the processing unit may determine a risk score associated with that exposure point. In particular, it determines the risk of leaving the cable unexposed at that geographical location. This risk score can be calculated based on contextual information about the geographical location, such as the water depth at that location, the frequency with which ship routes cross that geographical area, the frequency of high-risk AIS navigation alerts generated in that area, the forecast of storm and high wave activity in that area, and the history of cable strikes in that area.

[0069] If the processing unit 214 determines that the location of the cable exposure is in a low-risk area, it can associate a low risk score with the exposure site. Conversely, if the processing unit determines that the location of the cable exposure is in a high-risk area (for example, because it is a shallow area, an area with a high frequency of large vessel crossings, or an area with high fishing activity), it can associate a high risk score with the exposure site. It will be understood that the processing unit 214 may utilize heuristic methods based on one or more of the factors identified above when determining the risk associated with a potential exposure site. Alternatively, a machine learning model trained to determine the risk score of an exposure site, taking into account the location of the exposure site and the possible degree of exposure, may be used.

[0070] In step 312, it is determined whether any of the exposure points have a risk score higher than the threshold risk score. If no exposure points have a risk score higher than the threshold risk score, method 300 ends. Alternatively, if even one of the exposure points has a risk score higher than the threshold risk score, the method proceeds to step 314, where processing unit 214 may be configured to issue a warning or alert to the cable owner or cable operator or cable infrastructure user in order to consider mitigation strategies. Optionally, processing unit 214 may also communicate warnings via AIS to alert potential vessels in the area about possible exposure locations and their radius or extension. In some examples, alerts may be communicated via communication interface 217. The alert may include information about the geographical location of the cable exposure and may warn fishing vessels and trawlers to be vigilant in that area.

[0071] In another embodiment, the processing unit 214 can provide a cable risk map to a cable operator, who, upon receiving the cable risk map, can flag exposed locations with live AIS beacons to warn potential vessels in the area, thereby informing fishermen and operators of maritime vessels about the increased risk. Alternatively, other devices can be implemented to flag the coordinates of exposed locations; for example, the location coordinates may be transmitted via the Internet through an Emergency Position Indicating Radio Beacon (EPIRB) alert or other quantitative information device such as a satellite device (i.e., a GPS position tracker).

[0072] In the method 300 described above, baseline measurement may be required to determine exposure locations. For this purpose, when the cable is first laid or buried, a query signal may be transmitted within the sensing optical fiber, and a return signal may be received and processed to determine the baseline distortion and / or vibration damping characteristics of the optical cable when the optical cable is fully buried. This baseline distortion and / or vibration damping can be considered a threshold, and the difference from the baseline can be determined and compared with the real-time distortion and / or vibration damping of the optical cable to identify potential exposure locations. For example, if the amplitude of the signal increases over a long period of time at a particular location from a threshold (e.g., baseline level), the processing system can determine that the location is a possible exposure location.

[0073] Baseline readings can also be used to identify intrinsic low-distortion or high-distortion bands (e.g., ground bands or shore wave bands) which can later be used to label statistical plots and mask those bands.

[0074] In some cases, a DFOS method 300 for determining exposure points may be used in conjunction with the DTS method described above to improve the accuracy of the detection system and method.

[0075] Figure 6A illustrates this example. In this figure, a PSD plot 602 based on DFOS data is compared with DTS data 604. Two exposure points 606 and 608, 35m wide, are identified 8km from the coast based on DFOS data. The DTS data shows a decrease in perceived temperature at the same locations 610 and 612, demonstrating that exposure points can be detected using DTS and DFOS data together.

[0076] Mapping cable exposure risks as described herein offers several advantages. A key advantage is that, after identifying exposure events as shown in Figure 5, cable operators can flag these locations to prevent or minimize the risk of attacks on the cables.

[0077] Furthermore, the above-described system and method can, alternatively, record acoustic data using the DFOS telecommunications receiver statistics themselves, without using dedicated equipment at one end, to extract information about the fiber's state, such as polarization characteristics or time of flight from one end to the other. This can also convey information about the rate of change over time of the fiber's length, or the stress or strain state of the fiber at a point or along its length.

[0078] It will be understood that the present invention, as disclosed and defined herein, extends to all alternative combinations of two or more individual features referred to or evident from the text or drawings. All of these different combinations constitute various alternative embodiments of the present invention.

[0079] As used herein, unless the context requires otherwise, the term “comprise” and its variations such as “comprising,” “comprises,” and “comprised” are not intended to exclude further additives, ingredients, integers, or processes.

Claims

1. A method for determining the location of exposure along an underground submarine cable, wherein the method Transmitting an inquiry signal to a sensing optical fiber that extends the length of the aforementioned submarine cable, Receiving a backscattered signal from the sensing optical fiber in response to the aforementioned inquiry signal, A method comprising processing the backscattered signal to determine at least one of temporal and spatial variations of strain along the sensing optical fiber, wherein the determined at least one spatial and temporal variation of strain indicates an exposure point and its location along the length of the buried submarine cable.

2. The method according to claim 1, wherein determining the spatial and / or temporal variation of the distortion includes determining the true amplitude and duration variation of the backscatter signal at positions along the length of the photosensitive fiber.

3. The method of claim 2, further comprising detecting the exposure location by determining that the true amplitude of the backscattered signal at a position along the length of the light-sensing fiber exceeds a threshold and remains above the threshold for a threshold duration.

4. The method according to claim 3, wherein the threshold is determined based on the value of baseline strain at a corresponding position along the length of the optical sensing fiber, the value of baseline strain is determined when the submarine cable is buried.

5. The method according to claim 3, further comprising determining the position of the exposure point based on the timing of receiving the backscatter signal.

6. The method according to any one of claims 3 to 5, further comprising determining a risk score associated with the detected exposure location.

7. Determining the aforementioned risk score means Determining the geographical location of the exposure point, and The method according to claim 6, comprising determining a risk score associated with the geographic location based on contextual data.

8. The method according to any one of claims 6 to 7, further comprising generating an alert if it is determined that the risk score of the detected exposure location exceeds a threshold risk score.

9. The method according to any one of claims 6 to 8, further comprising generating a cable risk map based on the detected exposure locations, and communicating the cable risk map to the operator of the cable buried on the seabed.

10. A distributed fiber optic sensing (DFOS) system, A light source configured to transmit an inquiry signal to a sensing optical fiber that extends the length of the aforementioned submarine cable, A photodetector configured to receive a backscattered signal from the sensing optical fiber in response to the aforementioned query signal, A DFOS system comprising a processing unit configured to process the backscattered signal in order to determine the temporal and / or spatial variation of the strain along the sensing optical fiber, wherein the determined spatial and / or temporal variation of the strain indicates the exposure points and their positions along the length of the buried submarine cable.

11. The DFOS system according to claim 10, wherein, in order to determine the spatial and / or temporal variation of the distortion, the processing unit is further configured to determine the true amplitude and duration variation of the backscatter signal at positions along the length of the photosensitive fiber.

12. The DFOS system according to claim 11, wherein the processing unit is further configured to detect the exposure location by determining that the true amplitude of the backscattered signal at a position along the length of the light sensing fiber exceeds a threshold and remains above the threshold for a threshold duration.

13. The DFOS system according to claim 12, wherein the threshold is determined based on the value of baseline strain at a corresponding position along the length of the optical sensing fiber, and the value of baseline strain is determined when the submarine cable is buried.

14. The DFOS system according to claim 12, wherein the processing unit is further configured to determine the position of the exposure point based on the timing of receiving the backscatter signal.

15. The DFOS system according to any one of claims 12 to 14, wherein the processing unit is further configured to determine a risk score associated with the detected exposure location.

16. Determining the aforementioned risk score means Determining the geographical location of the exposure point, and The DFOS system according to claim 15, comprising determining a risk score associated with the geographic location based on contextual data.

17. The DFOS system according to any one of claims 15 to 16, wherein the processing unit is further configured to generate an alert when it determines that the risk score of the detected exposure point exceeds a threshold risk score.

18. The DFOS system according to any one of claims 15 to 16, wherein the processing unit is further configured to generate a cable risk map based on the detected exposure locations and to communicate the cable risk map to the operator of the submarine buried cable.

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