Geolocating hidden infrastructure

Abstract

The present invention relates to a geolocating system and geolocating method for geolocating a fibre optic cable, wherein a first sensor of a fibre optic sensing system is positioned at a first sensin

Description

TECHNICAL FIELD The present invention relates to geolocating hidden infrastructure. BACKGROUND Hidden infrastructures are structures that are not easily visible and / or easily accessible. For example, a hidden infrastructure may be a structure buried below ground, positioned underwater, or encapsulated within a wall. Examples of hidden infrastructures include pipelines, power cables and communication cables. The geolocation of a hidden infrastructure may not be known, may become lost or may change over time. For example, the geolocation of a subsea power cable extending along, or buried under, a seabed may vary due to tidal flow forces and / or shifting sands. A large industrial site may comprise a complex network of buried power cables, and the exact geolocation of each buried power cable along its length may not be known. By not knowing the geolocation of a hidden infrastructure, the monitoring and maintenance of the hidden infrastructure may be problematic, cause increased costs, and / or take a longer time. The present invention seeks to solve the geolocation problems associated with hidden infrastructures. SUMMARY OF THE INVENTION A first aspect of the invention relates to a method for geolocating a fibre optic cable, wherein a first sensor of a fibre optic sensing system is positioned at a first sensing point of the fibre optic cable. It follows that the first geolocation of the first sensing point of the fibre optic cable where the first sensor is positioned corresponds to the geolocation of the sensor. The first sensor is configured to detect a first known characteristic signal at a first detection time, wherein the first known characteristic signal is generated by a first known background event at a first known generation time and at a first known background geolocation, and wherein the first known characteristic signal has a first known propagation speed between the first background geolocation and the first sensor. The method may comprise detecting, at the first sensor of the fibre optic sensing system, the first known characteristic signal at the first detection time. The method may comprise receiving, at an input of a geolocating system, first detecting time data indicative of the first detection time of the first known characteristic signal by the first sensor. The method may comprise receiving, at the input, first geolocation data indicative of the first generation time and the first known background geolocation of the first known characteristic signal. The method may comprise receiving, at the input, first detecting time data indicative of the first detection time of the first known characteristic signal by the first sensor. The first geolocation data received at the input may comprise first coincident generation time data corresponding to the first generation time of the first known characteristic signal, and first coincident known background geolocation data corresponding to the first known background geolocation of the first known characteristic signal. Alternatively, the first geolocation data may comprise first intermittent time data and first intermittent geolocation data, and the method may comprise deriving, at a processor of the geolocating system, the first known generation time of the first known characteristic signal and the first known background geolocation of the first known characteristic signal by interpolating the first intermittent time data, the first intermittent geolocation data and the first generation time. The method may comprise determining, by the processor, a first offset time of the first sensor, wherein the first offset time is the time difference between the first known generation time and the first detection time. The method may comprise determining, by the processor, the first offset time of the first sensor by correlating the first known generation time and the first detection time. The method may comprise identifying, by the processor, if the first offset time for the first sensor is substantially zero. When the first offset time is substantially zero, first sensor is deemed to be at substantially the same geolocation as the first known background geolocation of the first known characteristic signal. Therefore, when the first offset time is identified as being substantially zero, the method may comprise estimating, by the processor, the first geolocation of the first sensor (and thereby the first geolocation of the first sensing point of the fibre optic cable where the first sensor is positioned) corresponds to the first known background geolocation. The method may comprise identifying, by the processor, if the first offset time for the sensor falls within a first resolution time range of the first sensor, wherein the first resolution time range is based on a first known propagation speed of the first known characteristic signal, and spatial resolution of the first sensor. When the first offset time falls within the resolution time range, the first sensor is deemed to be sufficiently close to the first known background geolocation of the first known characteristic signal for the first sensor to have the same geolocation as the first known background geolocation of the first known characteristic signal. Therefore, when the first offset time is identified as falling within the first resolution time range, the method may comprise estimating, by the processor, the first geolocation of the first sensor (and thereby the first geolocation of the first sensing point of the fibre optic cable where the first sensor is positioned) corresponds to the first known background geolocation. When the first offset time is identified as falling outside the first resolution time range, the method may comprise estimating, by the processor, a first radial distance of the first sensor, wherein the first radial distance is a direct distance between the first known background geolocation and the first sensor, and wherein the first radial distance is based on the first offset time and the first known propagation speed of the first known characteristic signal. The method may comprise estimating the first radial distance of the first sensor using the equation v=d / t wherein v is the first known propagation speed of the first known characteristic signal, d is the first radial distance of the first sensor and t is the first offset time of the first sensor. If the sensor has a first known depth, the method may comprise identifying if the first radial distance of the first sensor corresponds to the first known depth. When the first radial distance corresponds to the first known depth, the first sensor is deemed to be directly below the first known background geolocation, and spaced apart by the first radial distance. Therefore, when the first radial distance is identified as corresponding to the first known depth of the sensor, the method may comprise estimating, by the processor, the first geolocation of the first sensor (and thereby the first geolocation of the first sensing point of the fibre optic cable where the first sensor is positioned) corresponds to the first known background geolocation. When the direction (bearing) between the first known background event and the sensor is unknown, the first geolocation of the sensor is deemed to have a geolocation at the radial distance, in any direction (bearing) in two-dimensional space, from the first known background geolocation. Therefore, the method may comprise estimating, by the processor, the first geolocation of the first sensor (and thereby the first geolocation of the first sensing point of the fibre optic cable where the first sensor is positioned) corresponds to a circumferential geolocation region of multiple possible geolocations, wherein the first sensor is geolocated at one of the possible geolocations within the circumferential geolocation region, and wherein the circumferential geolocation region is based on the first radial distance and the first known background geolocation. To narrow the geolocation, the method may comprise estimating, by the processor, the first geolocation of the first sensor (and thereby the first geolocation of the first sensing point of the fibre optic cable where the first sensor is positioned) corresponds to one or more circumferential segment geolocation region of multiple possible geolocations, wherein the first sensor is geolocated one of the possible geolocations within the one or more circumferential segment geolocation region, and wherein the one or more circumferential segment geolocation region is based on the circumferential geolocation region and segmenting geolocation data. The method may comprise estimating, by the processor, the circumferential segment geolocation region by correlating the circumferential geolocation region and segmenting geolocation data to segment the circumferential geolocation region. Likewise, when the direction (bearing) between the first known background event and the sensor is unknown, the first geolocation of the sensor is deemed to have a geolocation at the radial distance, in any direction (bearing) in three-dimensional space, from the first known background geolocation. Therefore, the method may comprise estimating, by the processor, the first geolocation of the first sensor (and thereby the first geolocation of the first sensing point of the fibre optic cable where the first sensor is positioned) corresponds to a surface geolocation region of multiple possible geolocations, wherein the first sensor is geolocated one of the possible geolocations within the surface geolocation region, and wherein the surface geolocation region is based on the first radial distance and the first known background geolocation. To narrow the geolocation, the method may comprise estimating, by the processor, the first geolocation of the first sensor (and thereby the first geolocation of the first sensing point of the fibre optic cable where the first sensor is positioned) corresponds to one or more surface segment geolocation region of multiple possible geolocations, wherein the first sensor is geolocated at one of the possible geolocations within the one or more surface segment geolocation region, and wherein the one or more surface segment geolocation region is based on the surface geolocation region and segmenting geolocation data. The method may comprise estimating, by the processor, the surface segment geolocation region by correlating the surface geolocation region and segmenting geolocation data to segment the surface geolocation region. Multiple sensors of the fibre optic sensing system may be positioned at respective sensing points along a length of the fibre optic cable. The multiple sensors may be configured to detect the first known characteristic signal. Alternatively, the multiple sensors may be configured to detect any first known characteristic signal from a first set of N first known characteristic signals generated at N known generation times and at N known background geolocations by N background events, wherein the N known characteristic signals have N propagation speeds, wherein N= ni, n2...nn. The geolocating method may be repeated for each sensor, to estimate a geolocation for each sensor (and thereby a geolocation of each sensing point along the length of the fibre optic cable where the respective sensor is positioned), wherein the multiple sensors detect the first known characteristic signal / any first known characteristic signal. The first sensor may be configured detect a second known characteristic signal at a second detecting time, wherein the second known characteristic signal is generated by a second known background event at a second known generation time and at a second known background geolocation, and wherein the second known characteristic signal has a second known propagating speed, wherein the first known background geolocation and the second known background geolocation are different. The method may comprise detecting, at the first sensor of the fibre optic sensing system, the second known characteristic signal at the second detection time. The method may comprise receiving, at the input, second geolocation data indicative of the second generation time and the second known background geolocation of the second known characteristic signal. The method may comprise receiving, at the input, second detecting time data indicative of the second detection time of the second known characteristic signal by the first sensor. The second geolocation data received at the input may comprise second coincident generation time data corresponding to the second generation time of the second known characteristic signal, and second coincident known background geolocation data corresponding to the second known background geolocation of the second known characteristic signal. Alternatively, the second geolocation data may comprise second intermittent time data and second intermittent geolocation data, and the method may comprise deriving, at the processor, the second known generation time of the second known characteristic signal and the second known background geolocation of the second known characteristic signal by interpolating the second intermittent time data, the second intermittent geolocation data and the second generation time. The method may comprise determining, by the processor, a second offset time of the first sensor, wherein the second offset time is the time difference between the second known generation time and the second detection time. The method may comprise determining, by the processor, the second offset time of the first sensor by correlating the second known generation time and the second detection time. The method may comprise identifying, by the processor, if the second offset time for the first sensor falls within a second resolution time range of the first sensor, wherein the second resolution time range is based on the second known propagation speed and spatial resolution of the first sensor. When the second offset time is identified as falling outside the second resolution time range, the method may comprise estimating, by the processor, a second radial distance of the first sensor, wherein the second radial distance is a direct distance between the second known background geolocation and the first sensor, and wherein the second radial distance is based on the second offset time and the second known propagation speed. The method may comprise estimating the second radial distance of the first sensor using the equation v=d / t wherein v is the second known propagation speed of the second known characteristic signal, d is the second radial distance of the first sensor and t is the second offset time of the first sensor. The method may comprise estimating, by the processor, the first geolocation of the first sensor (and thereby the first geolocation of the first sensing point of the fibre optic cable where the first sensor is positioned) corresponds to one or more geolocation determined by reverse multilateration of the first known background geolocation, the second known background geolocation, the first radial distance and the second radial distance. For example, the processor may estimate the first geolocation of the first sensor (and thereby sensing point of the fibre optic cable) corresponds to two possible geolocations determined by reverse multilateration, wherein the first sensor is geolocated at one of the two possible geolocations. For example, when the distance between the known background geolocations is the same as the sum of the two radial distances, the processor may estimate the first geolocation of the first sensor (and thereby the sensing point of the fibre optic cable) corresponds to a geolocation determined by reverse multilateration. Multiple sensors of the fibre optic sensing system positioned at respective sensing points along a length of the fibre optic cable may be configured to detect the second known characteristic signal. Alternatively, the multiple sensors may be configured to detect any second known characteristic signal from a second set of M second known characteristic signals generated at M known generation times and at M known background geolocations by M background events, wherein the M known characteristic signals have M propagation speeds, wherein M= rm, m2...mm. The geolocating method may be repeated for each sensor, by the processor, to estimate a geolocation for each sensor (and thereby a geolocation of each sensing point along the length of the fibre optic cable where the respective sensor is positioned), wherein the multiple sensors detect the first known characteristic signal / any first known characteristic signal and the second known characteristic signal / any second known characteristic signal. The first sensor may be configured detect a third known characteristic signal at a third detecting time, wherein the third known characteristic signal is generated by a third known background event at a third known generation time and at a third known background geolocation, and wherein the third known characteristic signal has a third known propagating speed, wherein the first known background geolocation, the second known background geolocation and the third known background geolocation are different. The method may comprise detecting, at the first sensor of the fibre optic sensing system, the third known characteristic signal at the third detection time. The method may comprise receiving, at the input, third geolocation data indicative of the third generation time and the third known background geolocation of the third known characteristic signal. The method may comprise receiving, at the input, third detecting time data indicative of the third detection time of the third known characteristic signal by the first sensor. The third geolocation data received at the input may comprise third coincident generation time data corresponding to the third generation time of the third known characteristic signal, and third coincident known background geolocation data corresponding to the third known background geolocation of the third known characteristic signal. Alternatively, the third geolocation data may comprise third intermittent time data and third intermittent geolocation data, and the method may comprise deriving, at the processor, the third known generation time of the third known characteristic signal and the third known background geolocation of the third known characteristic signal by interpolating the third intermittent time data, the third intermittent geolocation data and the third generation time. The method may comprise determining, by the processor, a third offset time of the first sensor, wherein the third offset time is the time difference between the third known generation time and the third detection time. The method may comprise determining, by the processor, the third offset time of the first sensor by correlating the third known generation time and the third detection time. The method may comprise identifying, by the processor, if the third offset time for the first sensor falls within a third resolution time range of the first sensor, wherein the third resolution time range is based on the third known propagation speed and spatial resolution of the first sensor. When the third offset time is identified as falling outside the third resolution time range, the method may comprise estimating, by the processor, a third radial distance of the first sensor, wherein the third radial distance is a direct distance between the third known background geolocation and the first sensor, and wherein the third radial distance is based on the third offset time and the third known propagation speed. The method may comprise estimating the third radial distance of the first sensor using the equation v=d / t wherein v is the third known propagation speed of the third known characteristic signal, d is the third radial distance of the first sensor and t is the third offset time of the first sensor. The method may comprise estimating, by the processor, the first geolocation of the first sensor (and thereby the first geolocation of the first sensing point of the fibre optic cable where the first sensor is positioned) corresponds to one or more geolocation determined by reverse multilateration of the first known background geolocation, the second background geolocation, the third known background geolocation, the first radial distance, the second radial distance and the third radial distance. For example, the processor may estimate the geolocation of the sensor (and thereby the sensing point of the fibre optic cable) corresponds to a two-dimensional geolocation determined by reverse multilateration. For example, the processor may estimate the geolocation of the sensor (and thereby the sensing point of the fibre optic cable) corresponds to two possible three-dimensional geolocations determined by reverse multilateration, wherein the sensor (and thereby the sensing point of the fibre optic cable) is geolocated at one of the two possible three-dimensional geolocations. Multiple sensors of the fibre optic sensing system positioned at respective sensing points along a length of the fibre optic cable may be configured to detect the third known characteristic signal. Alternatively, the multiple sensors may be configured to detect any third known characteristic signal from a third set of P third known characteristic signals generated at P known generation times and at P known background geolocations by P background events, wherein the P third known characteristic signals have P propagation speeds, wherein P= pi, p2...pP. The geolocating method may be repeated for each sensor, by the processor, to estimate a geolocation for each sensor (and thereby a geolocation of each sensing point along the length of the fibre optic cable where the respective sensor is positioned), wherein the multiple sensors detect the first known characteristic signal / any first known characteristic signal, the second known characteristic signal / any second known characteristic signal and the third known characteristic signal / any third known characteristic signal. The first sensor may be configured detect a fourth known characteristic signal at a fourth detecting time, wherein the fourth known characteristic signal is generated by a fourth known background event at a fourth known generation time and at a fourth known background geolocation, and wherein the fourth known characteristic signal has a fourth known propagating speed, wherein the first known background geolocation, the second known background geolocation, the third known background geolocation, and the fourth known background geolocation are different. The method may comprise detecting, at the first sensor of the fibre optic sensing system, the fourth known characteristic signal at the fourth detection time. The method may comprise receiving, at the input, fourth geolocation data indicative of the fourth generation time and the fourth known background geolocation of the fourth known characteristic signal. The method may comprise receiving, at the input, fourth detecting time data indicative of the fourth detection time of the fourth known characteristic signal by the first sensor. The fourth geolocation data received at the input may comprise fourth coincident generation time data corresponding to the fourth generation time of the fourth known characteristic signal, and fourth coincident known background geolocation data corresponding to the fourth known background geolocation of the fourth known characteristic signal. Alternatively, the fourth geolocation data may comprise fourth intermittent time data and fourth intermittent geolocation data, and the method may comprise deriving, at the processor, the fourth known generation time of the fourth known characteristic signal and the fourth known background geolocation of the fourth known characteristic signal by interpolating the fourth intermittent time data, the fourth intermittent geolocation data and the fourth generation time. The method may comprise determining, by the processor, a fourth offset time of the first sensor, wherein the fourth offset time is the time difference between the fourth known generation time and the fourth detection time. The method may comprise determining, by the processor, the fourth offset time of the first sensor by correlating the fourth known generation time and the fourth detection time. The method may comprise identifying, by the processor, if the fourth offset time for the first sensor falls within a fourth resolution time range of the first sensor, wherein the fourth resolution time range is based on the fourth known propagation speed and spatial resolution of the first sensor. When the fourth offset time is identified as falling outside the fourth resolution time range, the method may comprise estimating, by the processor, a fourth radial distance of the first sensor, wherein the fourth radial distance is a direct distance between the fourth known background geolocation and the first sensor, and wherein the fourth radial distance is based on the fourth offset time and the fourth known propagation speed. The method may comprise estimating the fourth radial distance of the first sensor using the equation v=d / t wherein v is the fourth known propagation speed of the fourth known characteristic signal, d is the fourth radial distance of the first sensor and t is the fourth offset time of the first sensor. The method may comprise estimating, by the processor, the first geolocation of the first sensor (and thereby the first geolocation of the first sensing point of the fibre optic cable where the first sensor is positioned) corresponds to a three-dimensional geolocation determined by reverse multilateration of the first known background geolocation, the second background geolocation, the third known background geolocation, the fourth known background geolocation, the first radial distance, the second radial distance, the third radial distance and the fourth radial distance. Multiple sensors of the fibre optic sensing system positioned at respective sensing points along a length of the fibre optic cable may be configured to detect the fourth known characteristic signal. Alternatively, the multiple sensors may be configured to detect any fourth known characteristic signal from a fourth set of Q fourth known characteristic signals generated at Q known generation times and at Q known background geolocations by Q background events, wherein the Q fourth known characteristic signals have Q propagation speeds, wherein Q= qi, q2...qq. The geolocating method may be repeated for each sensor, by the processor, to estimate a geolocation for each sensor (and thereby a geolocation of each sensing point along the length of the fibre optic cable where the respective sensor is positioned), wherein the multiple sensors detect the first known characteristic signal / any first known characteristic signal, the second known characteristic signal / any second known characteristic signal, the third known characteristic signa / any fourth known characteristic signa and the fourth known characteristic signal / any fourth known characteristic signal. A predetermined time period after estimating the first geolocation of the first sensor, the geolocating method may be repeated, by the processor, to estimate a second geolocation of the first sensor. The method may comprise identifying, by the processor, if the first sensor has moved during the predetermined time period based on the first geolocation and the second geolocation of the first sensor. Any movement of the first sensor is the difference between the first geolocation and the second geolocation. The method may comprise estimating, by the processor, a geolocation of a second sensor positioned at a second sensing point of the fibre optic cable (and thereby the geolocation of the second sensing point of the fibre optic cable where the second sensor is positioned) by interpolating the first geolocation of the first sensor and the known geolocation of one or more known sensors positioned a known sensing points on the fibre optic cable. The fibre optic cable may be a hidden infrastructure. The fibre optic cable may be co-located with a further hidden infrastructure, wherein the geolocation of the hidden infrastructure adjacent the first sensing point of the fibre optic cable corresponds to the first geolocation of the first sensor. A second aspect of the invention relates to a geolocating system for geolocating a fibre optic cable, wherein a first sensor of a fibre optic sensing system is positioned at a first sensing point of the fibre optic cable, wherein a geolocation of the first sensing point of the fibre optic cable corresponds to a geolocation of the first sensor, wherein the first sensor is configured to detect a first known characteristic signal at a first detecting time, wherein the first known characteristic signal is generated by a first known background event at a first known generation time and at a first known background geolocation, and wherein the first known characteristic signal has a first known propagating speed, the geolocating system comprising a processor configured to carry out the geolocating method of the first aspect of the invention. The first sensor may be configured to detect a second known characteristic signal at a second detecting time, wherein the second known characteristic signal is generated by a second known background event at a second known generation time and at a second known background geolocation, and wherein the second known characteristic signal has a second known propagating speed, wherein the first known background geolocation and the second known background geolocation are different. The first sensor may be configured to detect a third known characteristic signal at a third detecting time, wherein the third known characteristic signal is generated by a third known background event at a third known generation time and at a third known background geolocation, and wherein the third known characteristic signal has a third known propagating speed, wherein the first known background geolocation, the second known background geolocation and the third known background geolocation are different. The first sensor may be configured to detect a fourth known characteristic signal at a fourth detecting time, wherein the fourth known characteristic signal is generated by a fourth known background event at a fourth known generation time and at a fourth known background geolocation, and wherein the fourth known characteristic signal has a fourth known propagating speed, wherein the first known background geolocation, the second known background geolocation, the third known background geolocation and the fourth known background geolocation are different When multiple sensors are positioned at respective sensing points along a length of the fibre optic cable, the processor may be configured to repeat the geolocating method of the first aspect of the invention. A third aspect of the invention relates to a geolocating apparatus for geolocating a fibre optic cable comprising: a fibre optic sensing system comprising a first sensor positioned at a first sensing point of the fibre optic cable, wherein a geolocation of the first sensing point of the fibre optic cable corresponds to a geolocation of the first sensor, wherein the first sensor is configured to detect a first known characteristic signal at a first detecting time, wherein the first known characteristic signal is generated by a first known background event at a first known generation time and at a first known background geolocation, and wherein the first known characteristic signal has a first known propagating speed; and a geolocating system of the second aspect of the invention configured to carry out the geolocating method of the first aspect of the invention. The first sensor may be configured to detect a second known characteristic signal at a second detecting time, wherein the second known characteristic signal is generated by a second known background event at a second known generation time and at a second known background geolocation, and wherein the second known characteristic signal has a second known propagating speed, wherein the first known background geolocation and the second known background geolocation are different. The first sensor may be configured to detect a third known characteristic signal at a third detecting time, wherein the third known characteristic signal is generated by a third known background event at a third known generation time and at a third known background geolocation, and wherein the third known characteristic signal has a third known propagating speed, wherein the first known background geolocation, the second known background geolocation and the third known background geolocation are different. The first sensor may be configured to detect a fourth known characteristic signal at a fourth detecting time, wherein the fourth known characteristic signal is generated by a fourth known background event at a fourth known generation time and at a fourth known background geolocation, and wherein the fourth known characteristic signal has a fourth known propagating speed, wherein the first known background geolocation, the second known background geolocation, the third known background geolocation and the fourth known background geolocation are different. When multiple sensors are positioned at respective sensing points along a length of the fibre optic cable, the geolocating system may be configured to repeat the geolocating method of the first aspect of the invention. A fourth aspect of the invention relates to a non-transitory computer readable medium comprising instructions, which when executed by a computer cause the computer to carry out the geolocating method of the first aspect invention. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figures 1a to 1d relate to a first example of geolocating a fibre optic cable buried underground according to the geolocating invention; Figures 2a-2c relates to a second example of geolocating a fibre optic cable buried underground according to the geolocating invention; Figures 3a to 3h relate to a third example of geolocating a fibre optic cable buried under a seabed according to the geolocating invention; Figures 4a and 4b relate to examples of geolocating methods for a sensor detecting a characteristic signal according to the geolocating invention; Figure 5 relates to a fourth example of geolocating a fibre optic cable according to the geolocating invention; Figure 6a-6c relate to a fifth example of geolocating a fibre optic cable according to the geolocating invention; Figures 7a-7c relate to examples of geolocating methods for a sensor detecting multiple characteristic signals according to the geolocating invention. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to geolocating hidden infrastructure. The geolocation may be two-dimensional or three-dimensional geolocating co-ordinates. For example, the geolocation may comprise longitude and latitude co-ordinates, or any other suitable identifying coordinates. In the geolocating invention, the hidden infrastructure may be a fibre optic cable or other hidden infrastructure co-located with a fibre optic cable. Fibre optic sensing is a well-known technique using a fibre optic sensing system where a fibre optic cable acts as a sensing element to detect behavioural changes at one or more intrinsic sensor positioned on the fibre optic cable. The geolocating invention relates to an apparatus, and associated method, for geolocating a fibre optic cable by geolocating one or more sensor positioned at one or more respective sensing point of the fibre optic cable. The geolocation of a sensor positioned at a sensing point of the fibre optic cable is equivalent to geolocation of the sensing point of the fibre optic cable. Therefore, by geolocating one or more sensor, the geolocation of the respective sensing point of the fibre optic cable where the one or more sensor is positioned is thereby known. It follows that by geolocating all the sensors positioned at sensing points along a length of the fibre optic cable, the geolocation of the length of the fibre optic cable is thereby known. Fibre optic cables are a well-known type of hidden infrastructure that are widely used on-shore and off-shore. Hence, the invention allows for the geolocating of a hidden fibre optic cable. Figures 1a-1d relate to a first example of geolocating a hidden fibre optic cable (FOC) buried underground (UG). Figures 2a-2c relate to a second example of geolocation a hidden fibre optic cable (FOC) buried underground (UG). Figures 3a-3h relate to a third example of geolocating a hidden fibre optic cable (FOC) buried under a seabed (SB). As mentioned above, a fibre optic cable may be co-located with other hidden infrastructure. As such, the geolocation of the fibre optic cable corresponds to the geolocation of the co-located hidden infrastructure. Therefore, by geolocating the fibre optic cable, the invention allows for the geolocating of the hidden infrastructure co-located with the fibre optic cable. Embodiments of the invention geolocate the fibre optic cable using at least one background event that occurs proximate the fibre optic cable. The known background event is deemed to be a “background” type of event because it occurs within the background environment of the fibre optic cable, and because the known background event is unrelated (independent) and separate to the functioning of the fibre optic cable. The known background event may be an environmental (natural) event or it may be a man-made event. The known background event may be a static background event or a moving background event. The known background event generates a known characteristic signal at a known generation time and at a known background geolocation. The known characteristic signal propagates from the known background event to the one or more sensor positioned at the one or more respective sensing point of the fibre optic cable with a known propagation speed. The known characteristic signal is detectable at the one or more sensor by fibre optic sensing. Embodiments of the method invention may comprise detecting, at a sensor positioned at a sensing point of a fibre optic cable, a known characteristic signal, wherein the known characteristic signal is generated by a known background event at a known generation time and at a known background geolocation, wherein the known characteristic signal propagates from the known background event to the sensor with a known propagation speed, and wherein the sensor detects the known characteristic signal at a detection time. Multiple sensors positioned at respective sensing points along the fibre optic cable may detect the same known characteristic signal generated by the same known background event. It will be understood that the multiple sensors will each detect the same known characteristic signal at a different detection time due to the arrangement of the multiple sensors at the respective sensing points along the fibre optic cable and the propagation path of the known characteristic signal. Embodiments of the method invention may comprise detecting, at multiple sensors positioned at respective sensing points on the fibre optic cable, a known characteristic signal, wherein the known characteristic signal is generated by a known background event at a known generation time, at a known background geolocation, wherein the known characteristic signal has a known propagations speed, and wherein each sensor detects the known characteristic signal at a respective detection time. Alternatively, multiple sensors positioned at respective sensing points along the fibre optic cable may detect any known characteristic signal from a set of known characteristic signals. For example, if a set of first known characteristic signals comprises N first known characteristic signals generated by N background events at N known generation times, at N known background geolocations, and at N known propagating speeds, where N= ni, n2...nn, each of the multiple sensors may detect a first known characteristic signal from any of the set of N first known characteristic signals. For example, a fibre optic cable may comprise sensors Si - Ss9 positioned at respective sensing points along the length of the fibre optic cable, whereby sensors S1-S10 may detect the ni first known characteristic signal from the set of N known characteristic signals and sensors Sn-Ss9 may detect the n2 first known characteristic signal from the set of N known characteristic signals (where the ni first known characteristic signal is generated by the ni known background event at the ni known generation time and at the ni known background geolocation, and has the ni known propagation speed, and where the n2 first known characteristic signal is generated by the n2 known background event at the n2 known generation time and the n2 at known background geolocation, and has the n2 known propagation speed). The first known characteristic signals in the set may be the same type of characteristic signals generated by the same type of background event. Embodiments of the method invention may comprise detecting, at multiple sensors positioned at respective sensing points along the fibre optic cable, any known characteristic signal from a set of known characteristic signals, wherein each sensor detects the respective known characteristic signal at a respective detection time. Embodiments of the apparatus invention may comprise a fibre optic sensing system comprising one or more sensor positioned at one or more respective sensing point of a fibre optic cable, wherein the one or more sensor is configured to detect one or more known characteristic signal. For example, the apparatus may comprise a fibre optic sensing system comprising a sensor positioned at a sensing point of a fibre optic cable, wherein the sensor is configured to detect a known characteristic signal at a detection time. The apparatus may comprise a fibre optic sensing system comprising multiple sensors positioned at respective sensing points along a fibre optic cable, wherein each sensor is configured to detect a known characteristic signal at a respective detection time. The apparatus may comprise a fibre optic sensing system comprising multiple sensors positioned at respective sensing points along a fibre optic cable, wherein each sensor may detect any known characteristic signal from a set of known characteristic signals at a respective detection time. The fibre optic sensing system may comprise any suitable fibre optic sensing system. The fibre optic sensing system may be a computer driven (operable) fibre optic sensing system. The fibre optic sensing system may comprise a distributed fibre optic sensing system whereby the fibre optic cable acts as a sensing element with multiple sensors arranged at continuous and sequential sensing points along the length of the fibre optic cable. For example, the distributed fibre optic sensing system may comprise a distributed acoustic sensing system (DAS) based on Rayleigh scattering, a distributed temperature sensing system (DTS) based on Brillouin or Raman scattering, a distributed temperature and strain sensing system (DTSS) based on Brillouin scattering, or a distributed strain sensing system (DSS) based on Brillouin scattering. Alternatively, the fibre optic sensing system may comprise a discrete fibre optic sensing system whereby sensors are incorporated in the fibre optic cable at sensing points in spaced positions along the length of the fibre optic cable. For example, the discrete fibre optic sensing system may be a fibre Bragg grating (FBG) sensing system. In the geolocating examples shown in Figures 1 a-1 d, 2a-2c and 3a-3h, each fibre optic cable is part of a distributed acoustic fibre optic sensing system where multiple sensors (S) are continuously and sequentially arranged at sensing points along the length of the fibre optic cable. A known characteristic signal generated by a known background event is detectable by one or more sensor positioned at a one or more respective sensing point of the fibre optic cable because it causes a detectable behavioural change at the sensor. For example, the known characteristic signal may cause a detectable behavioural change in the strain, stress, temperature at the sensor. In turn, the detectable behaviour change at the sensor causes a detectable change in the returning light signal from sensor. For example, the detectable behaviour change in the sensor may cause a change in the frequency, phase, wavelength and / or amplitude of the returning light signal from the sensor. The fibre optic sensing system is configured to monitor the change in the returning light signal to identify if one or more sensor has detected the known characteristic signal and the respective detection time. For example, the fibre optic sensing system may comprise an interrogator coupled to the fibre optic cable, wherein the interrogator is configured to monitor change in the returning light signal to identify if one or more sensor has detected a known characteristic signal at a respective detection time. A known characteristic signal generated by a known background event and detectable by fibre optic sensing of the fibre optic cable may be an acoustic signal, thermal signal, vibrational signal, seismic signal and / or any other suitable characteristic signal. For example, the known background event may be a lightning strike at a known background geolocation and at a known generation time, whereby the lightning strike produces a characteristic acoustic signal with a known propagation speed that is detectable by fibre optic sensing. The known background event may be an earthquake in a known background geolocation and at a known time, whereby the earthquake produces a characteristic seismic signal with a known propagation speed that is detectable by fibre optic sensing. The known background event may be a solar heating at a known background geolocation and at a known time, whereby the solar heating produces a characteristic thermal signal with a known propagation speed that is detectable by fibre optic sensing. The known background event may be a moving on-shore vehicle or a moving offshore vessel at a known background geolocation and known time, whereby the movement generates a characteristic vibration signal and / or an acoustic signal at a known propagation speed that is detectable by fibre optic sensing. The characteristic vibration signal may be a wake (water wave) with a known propagation speed caused by the movement of a vessel on / in water. The known background event may be an object at a known background geolocation that produces a characteristic acoustic signal at a known time, whereby the acoustic signal has a known propagation speed and is detectable by fibre optic sensing. In the first geolocating example depicted in Figures 1a-1d, and the second geolocating example depicted in Figures 2a-2c, a gun (G) is fired to generate a known characteristic acoustic signal (AS) at a known background geolocation and at a known generation time. The acoustic signal propagates from the fired gun and through the ground to the hidden fibre optic cable buried underground at a known propagation speed. For a shallow buried fibre optic cable, it is estimated that the average propagation speed of the acoustic signal from the fired gun to the fibre optic cable is approximately the speed of sound in air at 330m / s. In the third geolocating example depicted in Figures 3a-3h, a ship (SH) generates a known characteristic acoustic signal (AS) at a known background geolocation and at a known generation time. The acoustic signal propagates from the ship and through water to the hidden fibre optic cable buried under the seabed at a known propagation speed. For a shallow buried fibre optic cable, it is estimated that the average propagation speed of the acoustic signal from the ship to the fibre optic cable is approximately the speed of sound in water at 1500m / s. By fibre optic sensing a known characteristic signal generated by a known background event, the geolocating invention can avoid the need to use additional devices to generate a specific detectable signal at specific geolocations proximal the fibre optic cable, which is labour intensive and expensive. For example, by fibre optic sensing a known characteristic signal generated by a known background event, the geolocating invention avoids the conventional methods of using electromagnetic locators, radar locators, laser locators, that must be specifically transported onsite to scan for a fibre optic cable. Hence, known characteristic signal generated by a known background event may be considered a “characteristic signal of opportunity”, and the geolocating invention takes advantage of the available characteristic signals of opportunity to geolocate a fibre optic cable. The known background geolocation, known generation time and known propagation speed of a known characteristic signal generated by the known background event may be derived from data records of one or more background event monitoring devices relating to the known background event. As such, depending on the type of known background event and type of known characteristic signal, the known background geolocation where the characteristic signal is generated by the known background event, the known generation time of the characteristic signal and the known propagating time of the characteristic signal may be derived from meteorological data, seismic sensing data, vehicle timetable data, global positioning system (GPS) data, automatic identification system (AIS) data and / or any other suitable monitoring or tracking data for known background events. For example, the known background geolocation of a lightning strike, generation time of a characteristic acoustic signal by the lightning strike and propagation speed of the acoustic signal may be derived from meteorological data of local meteorological monitoring devices. The known background geolocation of a ship at a certain time, and thereby the generation time of a characteristic acoustic signal of the ship and the propagation speed of the acoustic signal at that certain time, may be derived from the AIS data of an AIS monitoring device mounted on the ship. Embodiments of the method invention may comprise receiving, at a geolocating system, known background event data for one or more known characteristic signal detected by one or more sensor positioned at one or more respective sensing point of the fibre optic cable. For the one or more characteristic signal, the known background event data is indicative of the known background event generating the known characteristic signal, the known background geolocation where the characteristic signal is generated, the known generation time when the known characteristic signal is generated and the known propagating speed of the known characteristic signal. Embodiments of the apparatus invention may comprise any suitable geolocating system. The geolocating system may be a computer driven (operable) geolocating system. In embodiments of the apparatus invention, the geolocating system may comprise an input configured to receive known background event data for one or more known characteristic signal. The input may be configured to receive known background event data from one or more background event monitoring device. The input may be configured to receive known background event data according to any suitable communication technique. The input may comprise a receiver arranged in communication (for example, in wired communication or in wireless communication) with a remote transmitter to receive known background event data transmitted by the transmitter. The input may be configured to receive a computer readable medium to receive known background event data uploaded from the computer readable medium. The input may comprise a manually usable interface to receive known background event data manually inputted via the interface. The geolocating system may comprise a memory configured to store the received known background event data for one or more known background event. Embodiments of the method invention may comprise receiving, at the geolocating system, detection time data for one or more sensor positioned at one or more respective sensing point of the fibre optic cable. The detection time data is indicative of the time when the known characteristic signal is detected by the one or more sensor positioned at one or more respective sensing point along the fibre optic cable. In embodiments of the apparatus invention, the geolocating system may comprise an input configured to receive the detection time data. The input may be configured to receive detection time data from the fibre optic sensing system. The geolocating system may be remotely located with respect to the fibre optic sensing system. The input may be configured to receive detection time data according to any suitable communication technique. The input may comprise a receiver arranged in communication (for example, in wired communication or in wireless communication) with a remote transmitter of the fibre optic sensing system to receive detection time data transmitted by the transmitter. The geolocating system may comprise a memory configured to stored received detection time data. The input to receive known background event data and the input to receive detection time data may be the same input or different inputs of the geolocating system. The memory to store known background event data and the memory to store detection time data may be same memory or different memories of the geolocating system. Embodiments of the method invention may comprise processing known background event data and optional detection time data to derive the known background geolocation and / or the known generation time of the known characteristic signal generated by the known background event. In embodiments of the apparatus invention, the geolocating system may comprise a processor configured to derive the known background geolocation and / or the known generation time of the known characteristic signal from the background event data and optional detection time data. One or more background event monitoring device may be configured to coincidently record the device geolocation and the device time at the same geolocation as a known background event generates a known characteristic signal. As such, the recorded coincident geolocation data of the coincident monitoring device is equivalent to the known background geolocation where the known characteristic signal is generated by the known background event, and the recorded coincident time data of the coincident monitoring device is equivalent to the known generation time of the known characteristic signal. For example, in the first geolocating example shown in Figures 1a-1d and the second geolocating example shown in Figures 2a-2c, a coincident GPS device (GPS) is arranged adjacent the gun (G) and configured to record a GPS geolocation and GPS time when the gun is fired and generates the known characteristic acoustic signal (AS). Hence, the recorded GPS geolocation is the same as the known background geolocation where the acoustic signal is generated by the fired gun, and the recorded GPS time is the same as the known generation time (To) when the known characteristic acoustic signal is generated by the fired gun. The known characteristic acoustic signal is then detected by one or more sensor at one or more respective sensing point of the fibre optic cable. Embodiments of the method invention may comprise receiving, at the input of the geolocating system, known background event data, wherein the known background event data comprises coincident geolocation data equivalent to a known background geolocation where a known characteristic signal is generated by a known background event, and coincident time data equivalent to a known generation time when the known characteristic signal is generated by the known background event. Embodiments of the method invention may comprise, deriving, by the processor of the geolocating system, the known background geolocation from the coincident geolocation data and the known generation time from the coincident time data. In embodiments of the apparatus invention, the processor of the geolocating system may be configured to derive the known background geolocation from the coincident geolocation data and the known generation time from the coincident time data. Alternatively, one or more background event monitoring device may be configured to intermittently record the geolocation of the monitoring device / known background event at intermittent time periods, whereby the known background event generates a known characteristic signal at a different geolocation and at a different time to the intermittently recorded data. The known characteristic signal has a known propagation speed and is detected by one or more sensor positioned at one or more respective sensing point of the fibre optic cable at a respective detection time. For example, in the third geolocating example shown in Figures 3a-3h, an AIS device (not shown) mounted on the moving ship may intermittently record the AIS device / ship geolocation and time of the AIS device / ship after each predetermined time period (e.g. every 1 minute). Meanwhile, the ship generates a known characteristic acoustic signal, and one or more sensor positioned on the subsea fibre detects the characteristic acoustic signal at a respective detection time. In another example, a GPS device mounted on a moving road vehicle may intermittently record the geolocation of the GPS device / vehicle and the time of the GPS device / vehicle after each predetermined time period (e.g. every 1 minute). The moving vehicle may generate a known characteristic acoustic signal when it interacts with a known object (e.g. a pothole in the road etc.) and the known characteristic acoustic signal is detected by one or more sensor positioned on the buried fibre optic cable at a respective detection time. As such, embodiments of the method invention may comprise receiving, at the input of the geolocating system, known intermittent background event data comprising intermittent geolocation data and intermittent time data of the background event, and detection time data of the known characteristic signal detected by one or more sensor of a fibre optic cable. The method invention may further comprise deriving, by the processor of the geolocating system, the known background geolocation where the detected known characteristic signal is generated by the known background event and the known generation time when the detected known characteristic signal is generated by the known background event based on the intermittent geolocation data, intermittent time data, and detection time data. In embodiments of the apparatus invention, the processor of the geolocating system may be configured to derive the known background geolocation and the known generation time by interpolating the intermittent geolocation data, intermittent time data and detecting time data. Embodiments of the method invention may comprise determining, by the processor of the geolocating system, an offset time for a sensor positioned at a sensing point of the fibre optic cable, wherein the offset time is the time for a known characteristic signal to propagate from the known background geolocation where the characteristic signal is generated to the sensor (the time difference between the generation time of the known characteristic signal and the detection time of the known characteristic signal at the sensor). In embodiments of the apparatus invention, the processor of the geolocating system may be configured to determine the offset time of the sensor by correlating the detection time of the known characteristic signal at the sensor with the generation time of the known characteristic signal. When multiple sensors positioned at respective sensing points along the fibre optic cable detect the same known characteristic signal, embodiments of the method invention may comprise determining, by the processor of the geolocating system, the offset time for each sensor. In embodiments of the apparatus invention, the processor may be configured to determine the offset time for each sensor by correlating the detection time of the known characteristic signal at each sensor with the generation time of the known characteristic signal. From the offset times of the multiple sensors positioned along the fibre optic cable, the sensor with the shortest offset time will be closest to the background event. The shape of the fibre optic cable may be estimated from the detection times / offset times of each sensor. For example, it may be estimated if the fibre optic cable has a generally linear shape or non-linear shape from the detection times / offset times of each sensor positioned on the fibre optic cable. When multiple sensors positioned at respective sensing points along the fibre optic cable each detect any known characteristic signal from a set of known characteristic signals, embodiments of the method invention may comprise determining, by the processor of the geolocating system, the offset time for each sensor, wherein the offset time is the difference between the detection time and the generation time of the known characteristic signal. In embodiments of the apparatus invention, the processor of the geolocating system may be configured to determine the offset time for each sensor by correlating the detection time of the known characteristic signal at each sensor with the generation time of the respective known characteristic signal. In the first geolocating example shown in Figures 1 a-1 d, the acoustic signal is generated by firing the gun (G) at the generation time (To) and at the known background geolocation, and the acoustic signal propagates from the gun to the fibre optic cable buried underground. As shown in Figure 1b, the acoustic signal is detected by multiple sensors (S) positioned at continuous and sequential sensing points along the fibre optic cable. The time of detecting the acoustic signal is recorded for each sensor, and the graph in Figure 1c depicts the detection times of the acoustic signal for each sensor. The offset time (ST) between the generation time of the acoustic signal by the fired gun and the detection time of the acoustic signal at each sensor is determined by the processor of the geolocating system using the correlation technique. In this geolocating example, the graph in Figure 1c has a curved V shape, where the apex (A) is curved and corresponds to the shortest offset time for the sensors. The sensor (SXo) with the shortest offset time is identified as being at distance Xo along the length of the fibre optic cable, and the sensor (SXo) is deemed to have a geolocation closest to the known background geolocation where the acoustic signal is generated by the fired gun. Hence, the geolocation of the sensing point of the fibre optic cable at distance Xo is also closest to the known background geolocation. The symmetrical V shape of the graph in Figure 1c is due to a regular variation in the detection time by the sensors positioned at the regular sensing points along the fibre optic cable, and this is indicative that the fibre optic cable where the detecting sensors are positioned has a generally linear shape as it extends underground. In the second geolocating example shown in Figures 2a-2c, the acoustic signal is generated by firing the gun (G) at the generation time (To) and at the known background geolocation, and the acoustic signal propagates from the gun to the fibre optic cable buried underground. The acoustic signal is detected by multiple sensors (S) positioned at continuous and sequential sensing points along the fibre optic cable, as shown in Figure 2b. The time of detecting the acoustic signal is recorded for each sensor, and the graph in Figure 2c depicts the detection times of the acoustic signal for each sensor. The offset time (ST) between the generation time of the acoustic signal by the fired gun and the detection time of the acoustic signal at each sensor is determined by the processor of the geolocating system using the correlation technique. In this geolocating example, the shape of the graph in Figure 2c has a sharp V shape, where the apex (A) is pointed and corresponds to a sensor (SXo) with a substantially zero offset time. The sensor (SXo) with substantially zero offset time is identified as being at distance Xo along the length of the fibre optic cable. The symmetrical V shape of the graph in Figure 2c is due to a regular variation in the detection time by the sensors positioned at the regular sensing points along the fibre optic cable, and this is indicative that, in this example, the fibre optic cable where the detecting sensors are positioned has a generally linear shape as it extends underground. In the third geolocating example shown in Figures 3a-3h, the acoustic signal is generated by the ship at generation time (To) and at the known background geolocation, and the acoustic signal propagates from the ship to the fibre optic cable buried under the seabed. As shown in Figure 3b, the acoustic signal is detected by multiple sensors (S) positioned at sensing points along the fibre optic cable. The time of detecting the acoustic signal is recorded for each sensor, and the graph in Figure 3c depicts the detection times for each sensor positioned along the fibre optic cable. The non-regular variation in the detection time by the sensors positioned along the fibre optic cable is indicative that the fibre optic cable where the detecting sensors are positioned has a non-linear shape as it extends buried under the seabed. The offset time (ST) between the generation time of the acoustic signal and the detection time of the acoustic signal at each sensor is derived by the processor of the geolocating system using the correlation technique. The lowest point on the graph (A) shows that the sensor at a sensing point at distance (Xo) along the length of the fibre optic cable has the shortest offset time (ST) and is thereby deemed to have a geolocation closest to the known background geolocation where the acoustic signal is generated by the ship. It follows that the geolocation of the sensing point of the fibre optic cable at distance (Xo) is also closest to the known background geolocation. Other detecting sensors (SX0-1, SX0+1 etc) positioned at sensing points along the fibre optic cable are identified relative to closest sensor (SXo). Embodiments of the method invention may comprise identifying, by the processor of the geolocating system, if an offset time for a sensor positioned at the sensing posing along the fibre optic cable is substantially zero. In embodiments of the apparatus invention, the processor of the geolocating system may be configured to identify if the offset time of the sensor is substantially zero. If there are multiple sensors positioned at sensing points along the fibre optic cable, the geolocating method may be repeated to identify if the offset time for each sensor is substantially zero. If the processor identifies an offset time for a sensor positioned at a sensing point of the fibre optic cable is substantially zero, the sensor is deemed to be at substantially the same geolocation where the known background event generates the known characteristic signal. As such, the geolocation of the sensor corresponds to the known background geolocation where the known characteristic signal is generated by the known background event. It follows that the geolocation of the sensing point of the fibre optic cable where the sensor is positioned corresponds to the known background geolocation. Embodiments of the method invention, when the processor determines the offset time is substantially zero, may comprise estimating the geolocation of the sensor (and thereby the geolocation of the sensing point of the fibre optic cable where the sensor is positioned) corresponds to the known background geolocation. In embodiments of the apparatus invention, when the processor of the geolocating system determines the offset time is substantially zero, the processor may be configured to estimate the geolocation of the sensor (and thereby the geolocation of the sensing point of the fibre optic cable where the sensor is positioned) corresponds to the known background geolocation. If the processor identifies the offset time of multiple sensors positioned at respective sensing points on the fibre optic cable is substantially zero, the geolocating method may be repeated to estimate the geolocation of each sensor (and thereby the geolocation of each sensing point of the fibre optic cable where the respective sensors are positioned) corresponds to the known background geolocation. In the second geolocating example shown in Figures 2a-2c, the processor of the geolocating system identifies the offset time for the sensor (SXo) is substantially zero (the sensor at position Xo on the fibre optic cable). As such, the processor estimates the geolocation of the sensor (SXo) with the substantially zero offset time corresponds to the known background geolocation of the fired gun, and thereby the geolocation of position Xo on the fibre optic cable is estimated to be the known background geolocation of the fired gun. Embodiments of the method invention may comprise identifying, by the processor of the geolocating system, if an offset time for a sensor positioned at the sensing point along the fibre optic cable falls within a resolution time range of the sensor. In embodiments of the apparatus invention, the processor of the geolocating system may be configured to identify if the offset time for the sensor falls within the resolution time range of the sensor by comparing the offset time with the resolution time range. If there are multiple sensors positioned at sensing points along the fibre optic cable, the geolocating method may be repeated to identify if the offset time for each sensor falls within the resolution time. The resolution time range of the sensor may depend on the known propagation speed of the known characteristic signal propagating from the known geolocation to the sensor, and spatial resolution of the sensor. When multiple sensors are positioned at respective sensing points on the fibre optic cable, the resolution time range of each sensor may also depend on the separation spacing of the sensors. If the processor identifies an offset time for a sensor positioned at a sensing point of the fibre optic cable falls within the resolution time range of the sensor, the sensor is deemed to be sufficiently close to the known background event such that the geolocation of the sensor corresponds to the known background geolocation where known characteristic signal is generated by the known background event. When the offset time for the sensor falls within the resolution time range, embodiments of the method invention may comprise estimating, by the processor of the geolocating system, the geolocation of the sensor corresponds to the known background geolocation. Given that the geolocation of the sensor is estimated to be the known background geolocation of the background event, the geolocation of the sensing point of the fibre optic where the sensor is positioned is likewise estimated to be the known background geolocation. In embodiments of the apparatus invention, when the processor of the geolocating system identifies the offset time falls within the resolution time range, the processor may be configured to estimate the geolocation of the sensor (and thereby the geolocation of the sensing point of the fibre optic cable where the sensor is positioned) corresponds to the known background geolocation. If the processor identifies the offset time of multiple sensors positioned at respective sensing points on the fibre optic cable falls within with the resolution time range for each sensor, the geolocating method may be repeated to estimate the geolocation of each sensor (and thereby the geolocation of each sensing point of the fibre optic cable where the respective sensors are positioned) corresponds to the known background geolocation. In the first geolocating example shown in Figures 1 a-1d, the processor of the geolocating system identifies the offset time for the sensor (SXo) with the shortest offset time (the sensor at position Xoon the fibre optic cable) falls with the resolution time range for the sensor. As such, the processor estimates the geolocation of the sensor (SXo) with the shortest offset time corresponds to the known background geolocation of the fired gun, and thereby the geolocation of position Xoon the fibre optic cable is estimated to be the known background geolocation of the fired gun. If the processor identifies an offset time for a sensor positioned at a sensing point of the fibre optic cable falls outside the resolution time range of the sensor, the sensor is deemed to be at a spaced distance from the known background geolocation where the characteristic signal is generated by the background event. In other words, the geolocation of the sensor is deemed to be different to the known background geolocation of the known characteristic signal generated by the background event. When the offset time for the sensor falls outside the resolution time range, embodiments of the method invention may comprise estimating, by the processor of the geolocating system, a radial distance of the sensor. The radial distance is a direct distance between the known background geolocation of the characteristic signal generated by the background event and the sensor. In embodiments of the apparatus invention, when the processor identifies the offset time falls outside the resolution time range, the processor of the geolocating system may be configured to estimate the radial distance for the sensor based on the known propagation speed of the known characteristic signal and the calculated offset time. For example, the processor may be configured to estimate the radial distance of the one or more sensor using the equation v= d / t where v is the known propagation speed of the characteristic signal detected by the sensor, d is the radial distance of the sensor and t is the offset time of the sensor. If the processor identifies the offset time of multiple sensors positioned at respective sensing points on the fibre optic cable falls outside the resolution time range for each sensor, the geolocating method may be repeated to estimate the radial distance for each sensor. In the first geolocating example shown in Figures 1 a-1 d, for each sensor that is identified as having an offset that falls outside the resolution time range of the respective sensor, Figure 1b shows the processor of the geolocating system estimates the radial distance (R) for each respective sensor (the direct distance between known background geolocation where acoustic signal is generated by the gun and each respective sensor). The processor of the geolocating system estimates the radial distance using the equation v= d / t where v is the known propagation speed of the acoustic signal detected by each sensor, d is the radial distance for each respective sensor, and t is the offset time for each respective sensor. In the third geolocating example shown in Figures 3a-3h, the processor of the geolocating system identifies the offset time for each sensor falls out the resolution time range of each respective sensor. As shown in Figure 3d, the processor of the geolocating system estimates the radial distance (R) for each respective sensor (the direct distance between the known background geolocation of the ship and each respective sensor) using the equation v= d / t where v is the known propagation speed of the acoustic signal detected by each sensor, d is the radial distance for each respective sensor, and t is the offset time for each respective sensor. A sensor may be arranged at a known depth with respect to the known background geolocation where the known characteristic signal is generated. For example, if a fibre optic cable is arranged on a seabed, the known depth may be an approximate depth of the sea. If a fibre optic cable is buried underground in a road, the known depth may be a conventional depth for burying a fibre optic cable in a road. If a sensor of the fibre optic cable has a known depth, embodiments of the method invention may comprise identifying, by the processor of the geolocating system, if the radial distance of the sensor corresponds to the known depth of the sensor. In embodiments of the apparatus invention, the processor of the geolocating system may be configured to identify if radial distance of the sensor corresponds to the known depth of the sensor. If there are multiple sensors positioned at sensing points along the fibre optic cable, the geolocating method may be repeated to identify if the radial distance for each sensor corresponds to the known depth. If the processor identifies the radial distance of a sensor corresponds to the known depth of the sensor, the known background geolocation of the known characteristic signal is deemed to be directly above (substantially perpendicular to) the sensor, and spaced apart by the radial distance. As such, the geolocation of the sensor (and thereby the sensing point on the fibre optic cable where the sensor is positioned) corresponds to the known background geolocation where the known characteristic signal is generated. Embodiments of the method invention, when the processor determines the radial distance of the sensor corresponds to the known depth of the sensor, may comprise estimating the geolocation of the sensor (and thereby the sensing point of the fibre optic cable) corresponds to the known background geolocation. In embodiments of the apparatus invention, when the processor determines the radial distance of the sensor corresponds to the known depth of the sensor, the processor may be configured to estimate the geolocation of the sensor (and thereby the sensing point of the fibre optic cable) corresponds to the known background geolocation. If the processor identifies the radial distance of multiple sensors positioned at respective sensing points along the fibre optic cable corresponds to the known depth of each sensor, the geolocating method may be repeated to estimate the geolocation of each sensor (and thereby each respective sensing point on the fibre optic cable) corresponds to the known background geolocation. If the direction (bearing) between the known characteristic signal generated by the known background event and a sensor is unknown, the sensor may be geolocated at the radial distance, in any direction (bearing) in two-dimensional space and in three-dimensional space, from the known background geolocation. As such, the geolocation of the sensor in two-dimensional space (and thereby the sensing point of the fibre optic cable where the sensor is positioned) may correspond to a circumferential geolocation region of multiple possible geolocations, wherein the sensor (and thereby the sensing point of the fibre optic cable) is geolocated at any of the multiple possible geolocations within the circumferential geolocation region. Likewise, the geolocation of the sensor in three-dimensional space (and thereby the sensing point of the fibre optic cable where the sensor is positioned) may correspond to a surface geolocation region of multiple possible geolocations, wherein the sensor (and thereby the sensing point of the fibre optic cable) is geolocated at any of the multiple possible geolocations within the surface geolocation region. Embodiments of the method invention may comprise estimating, by the processor of the geolocating system, the circumferential geolocation region for a sensor (and thereby the sensing point of the fibre optic cable where the sensor is positioned) relative to the known background geolocation. The circumferential geolocation region has a circular circumference shape in two-dimensional space defined by the radial distance with the known background geolocation at the centre. The circumferential geolocation region comprises the multiple possible geolocations where the sensor may be geolocated at the radial distance, in any direction in two-dimensional space, from the known background geolocation. In embodiments of the apparatus invention, the processor of the geolocating system may be configured to estimate the two-dimensional circumferential geolocation region based on the radial distance of the sensor and the known background geolocation. For example, the processor may be configured to estimate the circumferential geolocation region based on the radial distance of the sensor and the known background geolocation using a conventional two-dimensional coordinate geometry technique. If there are multiple sensors positioned at respective sensing points along the fibre optic cable, this embodiment of the geolocating method may be repeated to estimate a circumferential geolocation for each sensor (and thereby each respective sensing point of the fibre optic cable). Embodiment of the method invention may comprise estimating, by the processor of the geolocating system, the surface geolocation region for a sensor (and thereby the sensing point of the fibre optic cable where the sensor is positioned) relative to the known background geolocation. The surface geolocation region has a surface shape in three-dimensional space defined by the radial distance with the known background geolocation at the centre. The surface geolocation region comprises the multiple possible geolocations where the sensor may be geolocated at the radial distance, in any direction in three-dimensional spaced, from the known background geolocation. In embodiments of the apparatus invention, the processor of the geolocating system may be configured to estimate the three-dimensional surface geolocation region based on the radial distance of the sensor and the known background geolocation. For example, the processor may be configured to estimate the surface geolocation region based on the radial distance of the sensor and the known background geolocation using a conventional three-dimensional coordinate geometry technique. Given that the radial distance between the known background geolocation and sensor is dependent of the propagation speed of the known characteristic signal, the surface shape of the surface geolocation region depends on the propagation speed of the known characteristic signal detected by the sensor. For example, if the propagation speed of the known characteristic signal is constant then the surface geolocation region has a generally spherical surface shape in three-dimensional space defined by the radial distance with the known background geolocation at the centre. Alternatively, if the propagation speed varies then the surface geolocation region has a non-spherical, non-regular surface shape in three-dimensional space. If there are multiple sensors positioned at respective sensing points along the fibre optic cable, this embodiment of the geolocating method may be repeated to estimate a surface geolocation for each sensor (and thereby each respective sensing point of the fibre optic cable). In the first geolocating example shown in Figures 1 a-1 d, Figure 1d depicts the geolocation of a particular sensor positioned at a particular sensing point of the fibre optic cable corresponds to a two-dimensional circumferential geolocation region (CGR) of multiple possible geolocations, where the sensor is geolocated at any of the possible geolocation within the circumferential geolocation region. It follows that the geolocation of the particular sensing point of the fibre optic cable where the sensor is positioned corresponds to the two-dimensional circumferential geolocation region, where the sensing point of the fibre optic cable is geolocated at any of the possible geolocations within the circumferential geolocation region. The circumferential geolocation region is a two-dimensional circular circumference shape because the sensor is estimated to be geolocated at the radial distance (R), in any direction (bearing) in two-dimensional space, from the known background geolocation where the gun (G) is fired and generates the acoustic signal. The processor for the geolocating system estimates the circumferential geolocation region based on the known background geolocation of the acoustic signal generated by the gun and the radial distance of the sensor. In the third geolocating example shown in Figures 3a-3h, Figure 3f depicts the geolocation of a particular sensor positioned at a particular sensing point of the fibre optic cable (and thereby the geolocation of the particular sensing point of the fibre optic cable where the sensor is positioned) corresponds to a three-dimensional surface geolocation region (SGR) of multiple possible geolocations, where the sensor (and thereby the sensing point of the fibre optic cable where the sensor is positioned) is geolocated at any of the possible geolocation within the surface geolocation region. The geolocation region of the sensor is three-dimensional because the sensor is estimated to be geolocated at the radial distance, in any direction (bearing) in three-dimensional space, from the known background geolocation where the ship generates the acoustic signal. In this example, the surface geolocation region has a generally spherical shape. The processor of the geolocating system estimates the spherical surface geolocation region for the sensor based on the known background geolocation where the acoustic signal is generated by the ship and the radial distance of the sensor. If the geolocation of the sensor can be narrowed down within a circumferential geolocation region, the geolocation of the sensor (and thereby the sensing point of the fibre optic cable where the sensor is positioned) may correspond to one or more circumferential segment geolocation region of multiple possible geolocations, wherein the sensor (and thereby the sensing point of the fibre optic cable) is geolocated at any of the possible geolocations within the one or more circumferential segment geolocation region. Likewise, if the geolocation of the sensor can be narrowed down within a surface geolocation region, the geolocation of the sensor (and thereby the sensing point of the fibre optic cable where the sensor is positioned) may correspond to one or more surface segment geolocation region of multiple possible geolocations, wherein the sensor (and thereby the sensing point of the fibre optic cable) is geolocated at any of the possible geolocations within the one or more surface segment geolocation region. When a circumferential geolocation region and / or a surface geolocation region for a sensor is estimated, embodiments of the method invention may comprise receiving, at the input of the geolocating system, segmenting geolocation data relevant to the sensor wherein the segmenting geolocation data is indicative of one or more segment of the surface geolocation region in which the sensor may be geolocated. The segmenting geolocation data may comprise a known geolocation of one or more known sensor positioned on the fibre optic cable, whereby the known geolocation of the one or more sensor indicates one or more segment of the circumferential geolocation region, and / or one or more segment of the surface geolocation region, where the one or more detecting sensor is geolocated. For example, the segmenting geolocation data may comprise the known geolocation of a first sensor and last sensor positioned on the fibre optic cable. The known geolocations of multiple known sensors positioned on the fibre optic cable may indicate a general route of the fibre optic cable in a segment of the circumferential geolocation region and / or surface geolocation region. The segmenting geolocation data may comprise a known geolocation of a known object within the circumferential geolocation region and / or the surface geolocation region where the fibre optic cable cannot be geolocated. For example, the segmenting geolocation data may comprise the geolocation of a man-made object, obstruction or environmental feature (such as an infrastructure, mountain etc.) within the circumferential geolocation region and / or the surface geolocation region where the fibre optic cable cannot be geolocated. The segmenting geolocation data may comprise a known geolocation of a known object within the circumferential geolocation region and / or the surface geolocation region that is known to be proximal the fibre optic cable. For example, the segmenting geolocation data may comprise the geolocation of a known pipeline within the circumferential geolocation region and / or the surface geolocation region that is known to be proximal to the fibre optic cable. In embodiments of the apparatus invention, the input of the geolocating system be configured to receive segmenting geolocating data for the sensor. In embodiments of the apparatus invention, the memory of the geolocating system may be configured to store the received segmenting geolocation data. To enhance the geolocating of a sensor positioned at a sensing point of the fibre optic cable (and thereby the geolocating of the sensing point of the fibre optic cable where the sensor is positioned), embodiments of the method invention may comprise segmenting, using the processor of the geolocating system, the circumferential geolocation region for the sensor relative to the known background geolocation to form one or more circumferential segment geolocation region for the sensor, wherein the sensor (and thereby the sensing point of the fibre optic cable) is geolocated at any of the possible geolocations within the one or more circumferential segment geolocation region. In embodiments of the apparatus invention, the processor of the geolocating system may be configured to segment the circumferential geolocation region for the sensor to form the circumferential segment geolocation region by correlating the circumferential geolocation region for the sensor and segmenting geolocation data for the sensor. Correlating the circumferential geolocation region with the segmenting geolocation data has a segmenting effect that reduces the circumferential geolocation region to the one or more circumferential segment geolocation region, wherein the sensor (and thereby the sensing point of the fibre optic cable) is geolocated at any geolocation within the one or more circumferential segment geolocation region. Correlating may form the one or more circumferential segment geolocation region by excluding non-sensible geolocations of the sensor within the circumferential geolocation region. Correlating may form the one or more circumferential segment geolocation region to overcome the left-right ambiguity of the fibre optic cable relative to the known background event. If multiple sensors are positioned at respective sensing points of the fibre optic cable, this embodiment of the geolocating method may be repeated to estimate one or more circumferential segment geolocation region for each sensor (and thereby each respective sensing point of the fibre optic cable). Likewise, to enhance the geolocating of a sensor positioned at a sensing point of the fibre optic cable (and thereby the geolocating of the sensing point of the fibre optic cable where the sensor is positioned), embodiments of the method invention may comprise segmenting, using the processor of the geolocating system, the surface geolocation region for the sensor to form one or more a surface segment geolocation region for the sensor relative to the known background geolocation, wherein the sensor (and thereby the sensing point of the fibre optic cable) is geolocated at any of the possible geolocations within the one or more surface segment geolocation region. In embodiments of the apparatus invention, the processor of the geolocating system may be configured to segment the surface geolocation region of the sensor to form the one or more surface segment geolocation region by correlating the surface geolocation region for the sensor and segmenting geolocation data for the sensor. Correlating the surface geolocation region with the segmenting geolocation data has a segmenting effect that reduces the surface geolocation region to the one or more surface segment geolocation region, wherein the sensor (and thereby the sensing point of the fibre optic cable) is geolocated at any geolocation within the one or more surface segment geolocation region. Correlating may form the one or more surface segment geolocation region by excluding non-sensible geolocations of the sensor within the surface geolocation region. Correlating may form the one or more surface segment geolocation region to overcome the left-right ambiguity of the fibre optic cable relative to the known background event. If multiple sensors are positioned at respective sensing points of the fibre optic cable, this embodiment of the geolocating method may be repeated to estimate one or more surface segment geolocation region for each sensor (and thereby each respective sensing point of the fibre optic cable). In the third geolocating example shown in Figures 3a-3h, Figure 3g shows how the processor of the geolocating system segments the surface geolocation region of the particular sensor depicted in Figure 3f to form a surface segment geolocation region (SSGR). In this example, a known geolocation of a known sensor positioned on the fibre optic cable indicates the fibre optic cable is subsea, below the ship in the water, and so any geolocation above the ship is a non-sensible geolocation for a sensor positioned in the surface geolocation region. The processor correlates the known geolocation of the known sensor with the surface geolocation region to exclude the non-sensible geolocations above the ship and form the surface segment geolocation region. In this example as shown in Figure 3g, the surface segment geolocation region is a substantially semi-spherical geolocation region under water. The semi-spherical geolocation region comprises multiple possible geolocations of the sensor. As such, the processor estimates the geolocation of the particular sensor corresponds to the semi-spherical geolocation region as shown in Figure 3g, wherein the particular sensor is geolocated at any of the possible geolocations within the semi-spherical geolocation region. Figure 3h shows how the processor of the geolocating system may further segment the surface geolocation region of the particular sensor depicted in Figure 3f to form a surface segment geolocation region (SSGR). In this example, known geolocations of one or more known sensors positioned the fibre optic cable indicate that the fibre optic cable is buried below the seabed, and so any geolocation above the seabed is a non-sensible geolocation for the sensor. The processor correlates the known geolocations of the known sensor with the surface geolocation region to exclude the non-sensible geolocations above the seabed and form the surface segment geolocation region as shown in Figure 3h. The surface segment geolocation region below the seabed comprises multiple possible geolocations of the sensor. As such, the processor estimates the geolocation of the particular sensor corresponds to the surface segment geolocation region as shown in Figure 3h, wherein the particular sensor is geolocated at any of the possible geolocation within the surface segment geolocation region. Figures 4a and 4b depicts an example of a geolocating method for a sensor positioned at a sensing point of a fibre optic cable as described above, whereby the sensor detects a known characteristic signal generated by a known background event at a known generation time and at a known background geolocation, and the known characteristic signal has a known propagation speed. The geolocation of the sensor corresponds to the geolocation of the sensing point of the fibre optic cable where the sensor is positioned. In step 1, a processor of a geolocating system determines an offset time by correlating a known generation time of a known characteristic signal with a detection time of the known characteristic signal. In step 2a, if the processor identifies the offset time is substantially zero, the processor estimates the geolocation of the sensor corresponds to a known background geolocation where the known characteristic signal is generated by a known background event. In alternative step 2b, if the processor identifies the offset time falls within a resolution time range of the sensor, the processor estimates the geolocation of the sensor corresponds to the known background geolocation. In alternative step 2c, if the processor identifies the offset time falls outside the resolution time range of the sensor, the processor estimates a radial distance between the known background geolocation and the sensor using v=d / t where v is the known propagation speed of the characteristic signal detected by the sensor, d is the radial distance of the sensor and t is the offset time of the sensor. Optionally in step 2d, if the processor estimates the radial distance corresponds to a known depth of the sensor, the processor estimates the geolocation of the sensor corresponds to the known background geolocation. In step 3a, the processor determines a circumferential geolocation region of multiple possible geolocations for the sensor based on the radial distance and the known background geolocation, and the processor estimates the geolocation of the sensor corresponds to the circumferential geolocation region of the multiple possible geolocations, wherein the geolocation of the sensor is any one of the possible geolocations within the circumferential region. Optionally, in step 3b, the processor determines one or more circumferential segment geolocation for the sensor by correlating the circumferential geolocation region and geolocation segmenting data, and the processor estimates the geolocation of the sensor corresponds to the one or more circumferential segment geolocation region of multiple possible geolocations, wherein the geolocation of the sensor is any one of the possible geolocations within the one or more circumferential segment geolocation region. Additionally, or alternatively, in step 4a the processor determines a surface geolocation region of multiple possible geolocations for the sensor based on the radial distance and the known background geolocation, and the processor estimates the geolocation of the sensor corresponds to the surface geolocation region, wherein the geolocation of the sensor is any one of the possible geolocations within the surface geolocation region. Optionally, in step 4b, the processor determines one or more surface segment geolocation for the sensor by correlating the surface geolocation region and geolocation segmenting data, and the processor estimates the geolocation of the sensor corresponds to the one or more surface segment geolocation region, wherein the geolocation of the sensor is any one of the possible geolocations within the one or more surface segment geolocation region. If there are multiple sensors positioned a respective sensing points of the fibre optic cable, the geolocating method may be repeated to estimate the geolocation for each sensor (and thereby each respective sensing point of the fibre optic cable). If the sensor is configured to detect multiple different known characteristic signals, embodiments of the method invention may comprise detecting, at the sensor, the multiple different known characteristic signals at respective different detection times. In embodiments of the apparatus invention, the sensor of the fibre optic sensing system may be configured to detect multiple different known characteristic signals at respective different detection times. The different known characteristic signals are unambiguously distinguishable. For example, the different known characteristic signals may be generated at different known background geolocations by the background event and at different generations times by the background event. The different known characteristic signals may be the same type of known characteristic signal or different types of characteristic signal. The different known characteristic signals may be generated by the same type of background event or a different type of background event. For example, a sensor may detect a first known characteristic signal at a first detection time where the first known characteristic signal is generated at a first known background geolocation by a first known background event at a first known generation time. The same sensor may further detect a second known characteristic signal at a second generation time where the second known characteristic signal is generated at a second known background geolocation by a second known background event at a second known generation time. The same sensor may further detect a third known characteristic signal at a third detection time, where the third known characteristic signal is generated at a third known background geolocation by a third known background event at a third known generation time. The same sensor may further detect a fourth known characteristic signal at a fourth detection time, where the fourth known characteristic signal is generated by a fourth known background geolocation by a fourth known background event at a fourth known generation time, where the first, second, third and fourth known background geolocations are different and the first, second, third and fourth known generation times are different. For example, the sensor may be configured to detect any known characteristic signal from multiple sets of different known characteristic signals. For example, from a set of N first known characteristic signals where N = ni, n2...nn, a sensor Si may detect ni first known characteristic signal generated at ni known background geolocation by ni known background event at ni known generation time. From a set of M second known characteristic signals where M= rm, m2...mm, the same sensor Si may detect rm first known characteristic signal generated at rm known background geolocation by rm known background event at rm generation time. From a set of P third known characteristic signals where P= pi, p2... pP, sensor Si may detect pi third known characteristic signal generated at pi known background geolocation by pi known background event at pi known generation time. From a set of Q fourth known characteristic signals where Q = qi, q2...qq, sensor Si may detect qi fourth known characteristic signal generated at qi known background geolocation by qi known background event at qi generation time, where the ni, rm, pi, qi known background geolocations are different and where ni, rm, pi, qi known generation times are different. If there are multiple sensors positioned at respective sensing points of the fibre optic cable, each sensor may be configured to detect multiple different known characteristic signals. For example, a first sensor positioned at a first sensing point of the fibre optic cable and a second sensor positioned at a second sensing point of the fibre optic cable may both be configured to detect a first known characteristic signal at respective detection times and detect a second known characteristic signal at respective detection times, where the first known characteristic signal is generated at a first known background geolocation by a first known background event at a first known generation time, and where the second known characteristic signal is generated at a second known background geolocation by a second known background event at a second known generation time. For example, a first sensor positioned at a first sensing point of the fibre optic cable and a second sensor positioned at a second sensing point of the fibre optic cable may be configured to detect any first known characteristic signal from a first set of known characteristic signals and configured to detect any second known characteristic signal from a second set of known characteristic signal. For example, from a first set of N first known characteristic signals where N = ni, n2...nn, a first sensor Si may detect ni first known characteristic signal generated at ni known background geolocation by ni known background event at ni known generation time, and a second sensor S2 may detect n2 first known characteristic signal generated at n2 known background geolocation by n2 known background event at n2 known generation time. From a second set of M second known characteristic signals where M= rm, m2...mm, the first sensor Si may detect rm second known characteristic signal generated at rm known background geolocation by rm known background event at mi generation time, and the second sensor S2may detect m2 second known characteristic signal generated at m2 known background geolocation by m2 known background event at m2 generation time. From a third set of P third known characteristic signals where P= pi, P2... pP,the first sensor Si may detect pi third known characteristic signal generated at pi known background geolocation by pi known background event at pi known generation time, and the second sensor S2 may detect P2 third known characteristic signal generated at P2 known background geolocation by P2 known background event at P2 generation time. From a fourth set of Q fourth known characteristic signals where Q = qi, q2...qq, the first sensor Si may detect qi fourth known characteristic signal generated at qi known background geolocation by qi known background event at qi generation time, and the second sensor S2 may detect q2 fourth known characteristic signal generated at q2 known background geolocation by q2 known background event at q2 generation time where the ni, rm, pi, qi known background geolocations are different and where n2, m2, P2, q2 known background geolocations are different. The offset times for the sensor with respect to each different known characteristic signal may be determined as described above. The radial distances between each different known background geolocations and the sensor may be estimated using v=d / t as described above. When multiple different known characteristic signals are detected by the same sensor, the geolocation of the sensor (and thereby the sensing point on the fibre optic cable) may correspond to a geolocation determined by reverse multilateration of the different known background geolocations of the different known characteristic signals, and the radial distances between the different known background geolocations and the sensor. Alternatively, the geolocation of the sensor may correspond to multiple possible geolocations determined by reverse multilateration of the different known background geolocations of the different known characteristic signals, and the radial distances between the different known background geolocations and the sensor, wherein the sensor (and thereby the sensing point of the fibre optic cable) is geolocated at one of the multiple possible geolocations. When two different known characteristic signals are detected at the same sensor, embodiments of the method invention may comprise estimating, by the processor of the geolocating system, the geolocation of the sensor (and thereby the sensing point of the fibre optic cable) corresponds to two possible geolocations determined by reverse multilateration, wherein the sensor (and thereby the sensing point of the fibre optic cable) is geolocated at one of the two possible geolocations. In embodiments of the apparatus invention, the processor may be configured to estimate the two possible geolocations of the sensor by reverse multilateration of the known background geolocations of the two known characteristic signals and the two radial distances from each known background geolocation to the sensor. If the reverse multilateration technique uses two-dimensional geolocations, the first possible geolocation and the second possible geolocation estimated by the reverse multilateration technique are the two-dimensional geolocations where two circumferential geolocation regions for the sensor intersect at two points. Or, if the reverse multilateration technique uses three-dimensional geolocations, the first possible geolocation and the second possible geolocation are the three-dimensional geolocations where two surface geolocation regions for the sensor intersect. If there are multiple sensors positioned at sensing points along the fibre optic cable, this embodiment of the geolocating method may be repeated to estimate possible geolocations for each sensor. Figure 5 relates to a fourth geolocating example where a sensor positioned on a fibre optic cable (FOC) detects two different known characteristic signals generated at two different geolocations. In this example, the known characteristic signals are acoustic signals (AS1, AS2) generated by guns (G1, G2) at two different known background geolocations. The first known acoustic signal (AS1) is generated by the first gun (G1) at first known generation time (To) and at a first known background geolocation, and the first known acoustic signal (AS1) has a first known propagation speed. The second known acoustic signal (AS2) is generated by the second gun (G2) at a second known generation time (Ti) and at a second known background geolocation, and the second known acoustic signal (AS2) has a second known propagation speed. In this example, as shown in Figure 5, the processor of the geolocating system identifies the sensor detects both the first acoustic signal and the second acoustic signal. Using the method already described, the processor estimates the radial distance (R1, R2) for the sensor with respect to the different known background geolocations of each gun. The processor estimates the geolocation of the sensor (and thereby the sensing point of the fibre optic cable where the sensor is positioned) corresponds to two possible two-dimensional geolocations by using the reverse multilateration technique based on the known background geolocations of the guns and the respective radial distances between the known background geolocations of the guns and the sensor. The sensor (and thereby the sensing point of the fibre optic cable) is geolocated at one of the two possible two-dimensional geolocations. As shown in Figure 5, the reverse multilateration technique identifies the sensor (and thereby the sensing point of the fibre optic cable where the sensor is positioned) is geolocated at a first possible two-dimensional geolocation S’ or a second possible two-dimensional geolocation S”, and these possible geolocations relate to where the circumferential geolocation regions (CGR1, CGR2) for the sensor intersect. When the two different characteristic signals are detected by the same sensor, when the distance between the known background geolocations is the same as the sum of the two radial distances, embodiments of the invention may comprise estimating, by the processor of the geolocating system, the geolocation of the sensor (and thereby the sensing point of the fibre optic cable) corresponds to a geolocation of the sensor determined by reverse multilateration. In embodiments of the apparatus invention, the processor may be configured to estimate the geolocation of the sensor by reverse multilateration of the known background geolocations of the two known characteristic signals and the two radial distances from each known background geolocation to the sensor. If the reverse multilateration technique uses two-dimensional geolocations, the geolocation of the sensor estimated by the reverse multilateration technique relates to the two-dimensional geolocation where the two circumferential geolocation regions for the sensor meet at a single point of intersection. Or, if the reverse multilateration technique uses three-dimensional geolocations, the geolocation of the sensor relates to the three-dimensional geolocation where the two surface geolocation regions of the sensor meet at a single point of intersection. If there are multiple sensors positioned at sensing points along the fibre optic cable, this embodiment of the geolocating method may be repeated to estimate a geolocation for each sensor. When three different characteristic signals are detected at the same sensor, embodiments of the method invention may comprise estimating, by the processor of the geolocating system, the geolocation of the sensor (and thereby the sensing point on the fibre optic cable) corresponds to a two-dimensional geolocation of the sensor determined by reverse multilateration. In embodiments of the apparatus invention, the processor may be configured to estimate a two-dimensional geolocation of the sensor by reverse multilateration of the known background geolocations of the three known characteristic signals and the three radial distances from each known background geolocation to the sensor. The two-dimensional geolocation of the sensor estimated by the reverse multilateration technique relates to the two-dimensional geolocation where the three circumferential geolocation regions for the sensor intersect at a single point of intersection. If there are multiple sensors positioned at sensing points along the fibre optic cable, this embodiment of the geolocating method may be repeated to identify a two-dimensional geolocation for each sensor. Figures 6a-6c relates to a fifth geolocation example where sensors positioned on a positioned on a fibre optic cable (FOC) detect three different known acoustic signals (AS1, AS2, AS3) generated by three different known guns (G1, G2, G3) at three different known background geolocations. The first known acoustic signal (AS1) is generated by the first gun (G1) at a first known generation time (To) and at a first known background geolocation, and the first known acoustic signal has a first known propagation speed. The second known acoustic signal (AS2) is generated by the second gun (G2) at a second known generation time (Ti) and at a second known background geolocation, and the second known acoustic signal has a second known propagation speed. The third known acoustic signal (AS3) is generated by the third gun (G3) at a third known generation time (T2) and at a third known background geolocation, and the third known acoustic signals has a third known propagation speed. As shown in Figure 6a, the processor of the geolocating system identifies a sensor (S) detects the first known acoustic signal, the second known acoustic signal and the third known acoustic signal. As such, and using the method already described, the processor estimates the radial distances (R1, R2, R3) for the sensor with respect to each gun. As shown in Figure 6b, the processor estimates the two-dimensional geolocation of the sensor by using a reverse multilateration technique based on the known background geolocations of the guns and the respective radial distances. In the worked example shown in Figure 6c, the processor uses the multilateration technique based on the known background geolocations and radial distances, where the first known background geolocation is (0,0), first radial distance is 5, second known background geolocation is (4,0), second radial distance is 3, third known background geolocation is (0,3) and third radial distance is 4, to estimate the two-dimensional geolocation of the sensor (and thereby the geolocation of the sensing point of the fibre optic cable where the sensor is positioned) is (4,3). As shown in Figure 6a, the reverse multilateration technique based on the three known characteristic signals identifies the sensor (and thereby the sensing point of the fibre optic cable where the sensor is positioned) is geolocated where the circumferential geolocation regions (CGR1, CGR2, CGR3) for the sensor intersect. When three different known characteristic signals are detected at the same sensor, embodiments of the method invention may comprise estimating, by the processor of the geolocating system, the geolocation of the sensor (and thereby the sensing point of the fibre optic cable) corresponds to two possible three-dimensional geolocations determined by reverse multilateration, wherein the sensor (and thereby the sensing point of the fibre optic cable) is geolocated at one of the two possible geolocations. In embodiments of the apparatus invention, the processor may be configured to estimate the two possible three-dimensional geolocations of the sensor by reverse multilateration of the known background geolocations of the three known characteristic signals and the three radial distances from each known background geolocation to the sensor. The two possible three-dimensional geolocations of the sensor estimated by the reverse multilateration technique relate to a first possible three-dimensional geolocation and a second possible three-dimensional geolocation where the three surface geolocation regions for the sensor intersect at points of intersection. If there are multiple sensors positioned at respective sensing points along the fibre optic cable, this embodiment of the geolocating method may be repeated to estimate possible three-dimensional geolocations for each sensor (and thereby the respective sensing points along the fibre optic cable). Likewise, when four different known characteristic signals are detected at the same sensor, embodiments of method invention may comprise estimating, by the processor of the geolocating system, the geolocation of the sensor (and thereby the sensing point of the fibre optic cable) corresponds to a three-dimensional geolocation determined by reverse multilateration. In embodiments of the apparatus invention, the processor may be configured to estimate the three-dimensional geolocation of the sensor using reverse multilateration of the four different known background geolocations of the four known characteristic signals and the four radial distances from each of the four different known background geolocations to the sensor. The three-dimensional geolocation of sensor estimated by the reverse multilateration technique relates to the three-dimensional geolocation where the four surface geolocation regions of the sensor intersect at a single point of intersection. If there are multiple sensors positioned at respective sensing points along the fibre optic cable, this embodiment of the geolocating method may be repeated to estimate the three-dimensional geolocation for each sensor (and thereby the respective sensing points along the fibre optic cable). Figure 7a-c depicts examples of geolocating methods for a sensor positioned at a sensing point of a fibre optic cable as described above, when the sensor detects two, three or four different known characteristic signals. The different known characteristic signals are generated by a known background event at different known generation times and different known background geolocations. The known characteristic signals have known propagations speeds. In step 11 of Figure 7a, if the sensor detects two different known characteristic signals, a processor of a geolocating system estimates the radial distance of the sensor with respect to the known background geolocations for each respective known characteristic signal (distance between the sensor and each known background geolocation of the two characteristic signals). In step 12, the processor estimates two possible geolocations of the sensor by reverse multilateration of the two known background geolocations of the two known characteristic signals and the two respective radial distances from each known background geolocation to the sensor, and the processor further estimates the geolocation of the sensor (and thereby the sensing point of the fibre optic cable where the sensor is positioned) corresponds to the two estimated possible geolocations, wherein the sensor (and thereby the sensing point of the fibre optic cable) is geolocated at one of the two possible geolocations. Alternatively, in step 13, if the sensor detects two different known characteristic signals, and the distance between the known background geolocations is the same as the sum of the two radial distances, the processor of the geolocating system estimates a geolocation of the sensor by reverse multilateration of the two known background geolocations of the two known characteristic signals and the two respective radial distances from each known background geolocation to the sensor. The processor further estimates the geolocation of the sensor (and thereby the sensing point of the fibre optic cable) corresponds to the estimated geolocation. If there are multiple sensors positioned a respective sensing points of the fibre optic cable, the geolocating method may be repeated to estimate the geolocation for each sensor (and thereby each respective sensing point of the fibre optic cable). In step 14 of Figure 7b, if the sensor detects three different known characteristic signals, the processor of the geolocating system estimates the radial distance of the sensor with respect to the known background geolocations for each respective known characteristic signal. In step 15, the processor estimates a two-dimensional geolocation of the sensor by reverse multilateration of the three known background geolocations of the three known characteristic signals and the three respective radial distances from each known background geolocation to the sensor. The processor estimates the geolocation of the sensor (and thereby the sensing point of the fibre optic cable) corresponds to the estimated two-dimensional geolocation. Additionally or alternatively, in step 16, if the sensor detects three different known characteristic signals, the processor of the geolocating system estimates two possible three-dimensional geolocations of the sensor by reverse multilateration of the three known background geolocations of the three known characteristic signals and the three respective radial distances from each known background geolocation to the sensor, and the processor estimates the geolocation of the sensor (and thereby the sensing point of the fibre optic cable) corresponds to the two possible three-dimensional geolocations, wherein the sensor (and thereby the sensing point of the fibre optic cable) is geolocated at one of the two possible three-dimensional geolocations. If there are multiple sensors positioned a respective sensing points of the fibre optic cable, the geolocating method may be repeated to estimate the geolocation for each sensor (and thereby each respective sensing point of the fibre optic cable). In step 17 of Figure 7c, if the sensor detects four different known characteristic signals, the processor of the geolocating system estimates the radial distance of the sensor with respect to the known background geolocation for each respective known characteristic signal. In step 18, the processor estimates a three-dimensional geolocation of the sensor by reverse multilateration of the three known background geolocations of the four known characteristic signals and the four respective radial distances from each known background geolocation to the sensor, and the processor estimates the geolocation of the sensor (and thereby the sensing point of the fibre optic cable) corresponds to the estimated three-dimensional geolocation. If there are multiple sensors positioned at respective sensing points of the fibre optic cable, the geolocating method may be repeated to estimate the geolocation for each sensor (and thereby each respective sensing point of the fibre optic cable). Embodiments of the method invention may estimate, using the processor of the geolocating system, a geolocation of a further sensor positioned at a further sensing point of the fibre optic cable based on the estimated geolocation of the sensor and known geolocation of one or more known sensors positioned on the fibre optic cable. In embodiments of the apparatus invention, the processor may be configured to interpolate the estimated geolocation of the sensor and known geolocation of one or more known sensor to estimate the geolocation of the further sensor. For example, sensor Su may arranged between sensors S10 and S12, and the processor may be configured estimate the geolocation of sensor Su by interpolating the estimated geolocation of Swand known geolocations of sensor S12. As previously explained, when the geolocation of a sensor is known, the sensing point of the fibre optic cable where the sensor is positioned is thereby known. The geolocating method of the invention may be repeated until the geolocation of each sensor positioned at sensing points along the fibre optic cable is known. When the geolocation of all the sensors extending the full length of the fibre optic cable is known, the geolocation of the fibre optic cable along its full length is thereby known. Also, as explained previously, the fibre optic cable may be co-located with other hidden infrastructure. The fibre optic cable may be co-located by mounting or embedding the fibre optic cable with the other hidden infrastructure. Preferably, the co-located fibre optic cable is configured to extend substantially along the length of the hidden infrastructure. If the geolocation of the co-located fibre optic cable is known, the geolocation of the hidden infrastructure will thereby be known from the geolocation of the fibre optic cable. After a predetermined period of time, the geolocating method of the invention may be repeated to estimate a new geolocation of one or more sensors positioned on the fibre optic cable. Embodiments of the method invention may comprise comparing, by the processor of the geolocating system, the new geolocation of the sensor with respect to a previous geolocation of the sensor. If the new geolocation is different to a previous geolocation of the sensor, then it is understood that the fibre optic cable has moved. In embodiments of the apparatus invention, the processor of the geolocating system may be configured to compare the new geolocation and a previous geolocation of the sensor to identify if the sensor has moved. If there are multiple sensors positioned at sensing points along the fibre optic cable, the geolocating method may be repeated to identify if each sensor has moved. The geolocating invention may relate to a non-transitory computer readable medium (for example, a computer program product) that comprises instructions, which when executed by embodiments of the apparatus invention, cause the apparatus to carry out the geolocating method. For example, the non-transitory computer readable medium may comprise instructions, which when executed by embodiments of the geolocating system, cause the geolocating system to carry out the geolocating method. In particular, the non-transitory computer readable medium may cause the processor of the geolocating system to carry out the geolocating method. The computer readable medium may comprise a magnetic medium such as a hard disk, an electronic medium such as an SSD, flash memory or EEPROM, an optical medium such as an optical disc drive, or any other suitable computer readable medium. Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method for geolocating a fibre optic cable, wherein a first sensor of a fibre optic sensing system is positioned at a first sensing point of the fibre optic cable, wherein a geolocation of the first sensing point of the fibre optic cable corresponds to a geolocation of the first sensor, wherein the first sensor is configured to detect a first known characteristic signal at a first detecting time, wherein the first known characteristic signal is generated by a first known background event at a first known generation time and at a first known background geolocation, and wherein the first known characteristic signal has a first known propagating speed; the method comprising:determining, by a processor of a geolocating system, a first offset time of the first sensor, wherein the first offset time is the time difference between the first known generation time and the first detection time.

2. A method according to claim 1 comprising:identifying, by the processor, if the first offset time of the first sensor is substantially zero.

3. A method according to claim 2, when the first offset time of the first sensor is substantially zero, the method comprising:estimating, by the processor, a first geolocation of the first sensor corresponds to the first known background geolocation.

4. A method according to claim 2, when the first offset time of the first sensor is not substantially zero, comprising:identifying, by the processor, if the first offset time for the first sensor falls within a first resolution time range of the first sensor, wherein the first resolution time range is based on the first known propagation speed and spatial resolution of the first sensor.

5. The method of claim 4, when the first offset time falls within the first resolution time range, comprising:estimating, by the processor, the first geolocation of the first sensor corresponds to the first known background geolocation.

6. The method of claim 4, when the first offset time falls outside the first resolution time range, comprising:estimating, by the processor, a first radial distance of the first sensor, wherein the first radial distance is a direct distance between the first known background geolocation and the first sensor, and wherein the first radial distance is based on the first offset time and the known propagation speed.

7. The method of claim 6 comprising:identifying, by the processor, if the first radial distance corresponds to a first known depth of the first sensor; andwhen the first radial distance corresponds to the first known depth, estimating, by the processor, the first geolocation of the first sensor corresponds to the first known background geolocation.

8. The method of claim 6 comprising:estimating, by the processor, the first geolocation of the first sensor corresponds to a first circumferential geolocation region of multiple possible geolocations, wherein the first sensor is geolocated at one of the possible geolocations within the first circumferential geolocation region, wherein the first circumferential geolocation region is based on the first radial distance of first sensor and the first known background geolocation.

9. The method of claim 8 comprising:estimating, by the processor, the first geolocation of the first sensor corresponds to one or more first circumferential segment geolocation region of multiple possible geolocations, wherein the first sensor is geolocated at one of the possible geolocations within the one or more first circumferential segment geolocation region, wherein the one or more first circumferential segment geolocation region is based on the first circumferential geolocation region and segmenting geolocation data.

10. The method of claim 6 comprising:estimating, by the processor, the first geolocation of the first sensor corresponds to a first surface geolocation region of multiple possible geolocations, wherein the first sensor is geolocated at one of the possible geolocations within the first surface geolocation region, wherein the first surface geolocation region is based on the first radial distance of first sensor of the fibre optic cable and the first known background geolocation.

11. The method of claim 10 comprising:estimating, by the processor, the first geolocation of the first sensor corresponds to one or more first surface segment geolocation region of multiple possible geolocations, wherein the first sensor is geolocated at one of the possible geolocations within the one or more first surface segment geolocation region, wherein the first surface segment geolocation region is based on the first surface geolocation region and segmenting geolocation data.

12. The method of any of claims 1 to 11 comprising:receiving, at an input of the geolocating system, first geolocation data indicative of the first generation time and the first known background geolocation of the first known characteristic signal; andreceiving, at the input, first detecting time data indicative of the first detection time of the first known characteristic signal.

13. The method of claim 12, wherein the first geolocation data comprises first coincident generation time data corresponding to the first generation time of the first known characteristic signal, and first coincident known background geolocation data corresponding to the first known background geolocation of the first known characteristic signal.

14. The method of claim 12, wherein the first geolocation data comprises first intermittent time data and first intermittent geolocation data, the method comprising:deriving, at the processor, the first known generation time of the first known characteristic signal and the first known background geolocation of the first knowncharacteristic signal by interpolating the first intermittent time data, the first intermittent geolocation data and the first generation time.

15. The method of any of claims 6 to 14, wherein the first sensor is configured to detect a second known characteristic signal at a second detecting time, wherein the second known characteristic signal is generated by a second known background event at a second known generation time and at a second known background geolocation, and wherein the second known characteristic signal has a second known propagating speed, wherein the first known background geolocation and the second known background geolocation are different, the method comprising:determining, by the processor, a second offset time of the first sensor, wherein the second offset time is the time difference between the second known generation time and the second detection time;identifying, by the processor, if the second offset time for the first sensor falls within a second resolution time range of the first sensor, wherein the second resolution time range is based on the second known propagation speed and spatial resolution of the first sensor;when the second offset time falls outside the second resolution time range, estimating, by the processor, a second radial distance of the first sensor, wherein the second radial distance is a direct distance between the second known background geolocation and the first sensor, and wherein the second radial distance is based on the second offset time and the second known propagation speed.

16. The method of claim 15 comprising:estimating, by the processor, the first geolocation of the first sensor corresponds to one or more geolocation determined by reverse multilateration of the first known background geolocation, the second known background geolocation, the first radial distance and the second radial distance.

17. The method of claim 15 or 16, wherein the first sensor is configured to detect a third known characteristic signal at a third detecting time, wherein the third known characteristic signal is generated by a third known background event at a third known generation time and at a third known background geolocation, and whereinthe third known characteristic signal has a third known propagating speed, wherein the first known background geolocation, the second known background geolocation and the third known background geolocation are different, the method comprising:determining, by the processor, a third offset time of the first sensor, wherein the third offset time is the time difference between the third known generation time and the third detection time;identifying, by the processor, if the third offset time for the first sensor falls within a third resolution time range of the first sensor, wherein the third resolution time range is based on the third known propagation speed and spatial resolution of the first sensor;when the third offset time falls outside the third resolution time range, estimating, by the processor, a third radial distance of the first sensor, wherein the third radial distance is a direct distance between the third known background geolocation and the first sensor, and wherein the third radial distance is based on the third offset time and the third known propagation speed.

18. The method of claim 17 comprising:estimating, by the processor, the first geolocation of the first sensor corresponds to one or more geolocation determined by reverse multilateration of the first known background geolocation, the second known background geolocation, the third known background geolocation, the first radial distance, the second radial distance and the third radial distance.

19. The method of any of claims 17 or 18, wherein the first sensor is configured to detect a fourth known characteristic signal at a fourth detecting time, wherein the fourth known characteristic signal is generated by a fourth known background event at a fourth known generation time and at a fourth known background geolocation, and wherein the fourth known characteristic signal has a fourth known propagating speed, wherein the first known background geolocation, the second known background geolocation, the third known background geolocation and the fourth known background geolocation are different, the method comprising:determining, by the processor, a fourth offset time of the first sensor, wherein the fourth offset time is the time difference between the fourth known generation time and the fourth detection time;identifying, by the processor, if the fourth offset time for the first sensor falls within a fourth resolution time range of the first sensor, wherein the fourth resolution time range is based on the fourth known propagation speed and spatial resolution of the first sensor;when the fourth offset time falls outside the third resolution time range, estimating, by the processor, a fourth radial distance of the first sensor, wherein the fourth radial distance is a direct distance between the fourth known background geolocation and the first sensor, and wherein the fourth radial distance is based on the fourth offset time and the fourth known propagation speed.

20. The method of claim 19 comprising:estimating, by the processor, the first geolocation of the first sensor corresponds to a three-dimensional geolocation determined by reverse multilateration of the first known background geolocation, the second known background geolocation, the third known background geolocation, the fourth known background geolocation, the first radial distance, the second radial distance, the third radial distance and the fourth radial distance.

21. The method according to any of claims 1 to 20, a predetermined time period after estimating the first geolocation of the first sensor, comprising:repeating, by the processor, the method defined in any of claims 1 to 20 to estimate a second geolocation of the first sensor; andidentifying, by the processor, if the first sensor has moved during the predetermined time period based on the first geolocation and the second geolocation of the first sensor.

22. The method of any of claims 1 to 21 comprisingestimating, by the processor, a geolocation of a second sensor positioned at a second sensing point of the fibre optic cable by interpolating the first geolocationof the first sensor and the known geolocation of one or more known sensors positioned at a known sensing point of the fibre optic cable.

23. The method of any of claims 1 to 21, wherein the fibre optic cable is co-located with a hidden infrastructure, wherein the geolocation of the hidden infrastructure adjacent the first sensing point of the fibre optic cable corresponds to the first geolocation of the first sensor.

24. A geolocating system for geolocating a fibre optic cable, wherein a first sensor of a fibre optic sensing system is positioned at a first sensing point of the fibre optic cable, wherein a geolocation of the first sensing point of the fibre optic cable corresponds to a geolocation of the first sensor, wherein the first sensor is configured to detect a first known characteristic signal at a first detecting time, wherein the first known characteristic signal is generated by a first known background event at a first known generation time and at a first known background geolocation, and wherein the first known characteristic signal has a first known propagating speed, the geolocating system comprising:a processor configured to carry out the method as defined in any of claims 1 to 23.

25. A geolocating apparatus for geolocating the fibre optic cable comprising:a fibre optic sensing system comprising a first sensor positioned at a first sensing point of the fibre optic cable, wherein a geolocation of the first sensing point of the fibre optic cable corresponds to a geolocation of the first sensor, wherein the first sensor is configured to detect a first known characteristic signal at a first detecting time, wherein the first known characteristic signal is generated by a first known background event at a first known generation time and at a first known background geolocation, and wherein the first known characteristic signal has a first known propagating speed; anda geolocating system configured to carry out the method as defined in any of claims 1 to 23.

26. A non-transitory computer readable medium comprising instructions, which when executed by a computer, cause the computer to carry out the method steps defined in any of claims 1 to 23.

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