Abstract: This application relates to methods and apparatus for distributed fibre optic sensing. A sensing apparatus includes an optical arrangement which repeatedly interrogates a sensing optical fibre by launching coherent interrogating optical radiation into the sensing optical fibre. A detector detects Rayleigh backscatter from within the sensing fibre and a processor processes the detected backscatter to determine a measurement signal for each of a plurality of channels of the sensing optical fibre. The wavelength of the coherent interrogating optical radiation is modulated so as to vary between successive interrogations according to a modulation characteristic. The processor is operable to identify, for a given channel, an induced signal component corresponding to the modulation characteristic and determine an amplitude of the induced signal component so as to determine a compensation to be applied to the measurement signal for that channel to compensate for any variation in effective gauge length.

Abstract: A fibre optic sensing apparatus (400) is described which has an optical output path (403, 405, 404) configured to repeatedly interrogate a sensing optical fibre (402) by launching coherent optical radiation into the sensing optical fibre. A detector (407) is configured to receive optical radiation that is Rayleigh backscattered from the sensing optical fibre and output a detected backscatter signal in response to each interrogation and a processor (408) processes the detected backscatter signal. The apparatus is operable in a frequency swept mode, in which the sensing optical fibre is interrogated with a first set of interrogations of coherent optical radiation having different optical frequencies from one another to acquire, for at least one sensing portion of the sensing optical fibre, a backscatter spectral profile (201) of a measurement value of the detected backscatter signal with frequency across a first frequency range.

Abstract: This application relates to methods and apparatus for distributed fibre optic sensing. The apparatus includes an optical arrangement configured to generate a local oscillator signal (LO) and also to repeatedly interrogate a sensing optical fibre. Each interrogation comprises at least one pulse of coherent optical radiation at a launch frequency, which differs from that of the local oscillator signal by a carrier frequency. A mixer mixes backscatter signal from the sensing fibre with the local oscillator signal; and a detector arrangement provides at least one corresponding digital detector output signal (DX, DY). A processor processes each digital detector output signal to down-convert the digital detector output signal to a respective baseband signal based for each carrier frequency and convert each baseband signal to a phase signal. The processor is operable to apply time-domain low pass filtering to each baseband signal with a cut-off frequency, which is less than half the interrogation rate.

Abstract: This application relates to distributed fibre optic sensing. A sensor apparatus includes an interrogator for repeatedly interrogating a sensing optical fibre with interrogating optical radiation and detecting radiation which is Rayleigh backscattered from within the sensing fibre. In a first, polarisation sensing mode of operation, the interrogator is optically coupled to the sensing fibre via an optical system that includes a length of optical fibre upstream of a polariser and the interrogator is configured to output coherent optical radiation with an optical power such that the interrogating radiation experiences non-linear effects when propagating in the length of optical fibre, so as to reduce coherence of the interrogating radiation. In the first mode, the interrogator determines the extent of any variation in polarisation of the backscatter radiation. The apparatus may also be operable in a second, coherent sensing mode in which the interrogator outputs optical radiation below a non-linear threshold.

Abstract: This application describes methods and apparatus for fibre optic sensing. A receive unit includes an optical arrangement configured to receive coherent optical radiation that has propagated through a sensing optical fibre and to form first and second optical signals, wherein the second optical signal comprises optical radiation received from the sensing optical fibre later than the optical radiation of the first optical signal by a defined time period. A photodetector detects the first optical signal mixed with the second optical signal and a processor demodulates a derivative signal formed by interference of the first and second optical signals. First and second receive units may be located at opposite ends of a cable structure to provide respective measurement signals and a signal processor can process the two measurement signals to locate a disturbance.

Abstract: This application relates to methods and apparatus for distributed fibre optic sensing. A sensing apparatus (200) includes an optical arrangement (103, 201, 104) which repeatedly interrogates a sensing optical fibre (102) by launching coherent interrogating optical radiation into the sensing optical fibre. A detector (105) detects Rayleigh backscatter from within the sensing fibre and a processor (204) processes the detected backscatter to determine a measurement signal for each of a plurality of channels of the sensing optical fibre. The wavelength of the coherent interrogating optical radiation is modulated (201) so as to vary between successive interrogations according to a modulation characteristic.

Abstract: This application relates to methods and apparatus for distributed fibre optic sensing. The apparatus includes an optical arrangement configured to generate a local oscillator signal (LO) and also to repeatedly interrogate a sensing optical fibre. Each interrogation comprises at least one pulse of coherent optical radiation at a launch frequency, which differs from that of the local oscillator signal by a carrier frequency. A mixer mixes backscatter signal from the sensing fibre with the local oscillator signal; and a detector arrangement provides at least one corresponding digital detector output signal (DX, DY). A processor processes each digital detector output signal to down-convert the digital detector output signal to a respective baseband signal based for each carrier frequency and convert each baseband signal to a phase signal. The processor is operable to apply time-domain low pass filtering to each baseband signal with a cut-off frequency, which is less than half the interrogation rate.

Abstract: This application relates to distributed fibre optic sensing. A sensor apparatus includes an interrogator for repeatedly interrogating a sensing optical fibre with interrogating optical radiation and detecting radiation which is Rayleigh backscattered from within the sensing fibre. In a first, polarisation sensing mode of operation, the interrogator is optically coupled to the sensing fibre via an optical system that includes a length of optical fibre upstream of a polariser and the interrogator is configured to output coherent optical radiation with an optical power such that the interrogating radiation experiences non-linear effects when propagating in the length of optical fibre, so as to reduce coherence of the interrogating radiation. In the first mode, the interrogator determines the extent of any variation in polarisation of the backscatter radiation. The apparatus may also be operable in a second, coherent sensing mode in which the interrogator outputs optical radiation below a non-linear threshold.

Abstract: A method of distributed fibre optic sensing is described. In an example, a series of interrogations are launched into an optical fibre, each interrogation comprising interrogating radiation in at least one pulse pair, wherein the pulses of a pulse pair are introduced to the optical fibre with a time interval therebetween. Radiation backscattered therefrom is sampled, so as to obtain at least one sample from each interrogation. Phase modulation in the samples is determined and components of the phase modulation which are below a threshold frequency are isolated. Such a method of sensing could be used, for example, to monitor changes in temperature of the optical fibre.

Abstract: Methods and apparatus for obtaining an indication of fluid inflow in a section of a gas well having multiple perforations through which fluids comprising gas and/or liquid enter the well are described. In an example, the temperature within the section of the gas well is monitored at a plurality of locations. The locations comprising (a) a first set of locations, at or near a perforation; and (b) a second set of locations at which the temperature is substantially independent of the effects of the inflow of gas at a perforation. An indication of temperature excursions at the locations is obtained and used to obtaining an indication of the inflow of fluids to the well by relating the indications of temperature excursions from the second set of locations to liquid inflow and relating the indications of temperature excursions from the first set of locations to liquid and gas inflow.

Abstract: Methods and apparatus for determining fluid inflow to a well are described. In one example, a method comprise determining a first fluid flow velocity profile from slug tracking and determining a second fluid flow velocity profile from an estimate of fluid inflow at each of a plurality of perforations in a well. The first and second fluid flow velocity profiles are combined to provide a combined velocity profile. An indication of fluid inflow at least one perforation is derived from the combined velocity profile.

Abstract: This application describes methods and apparatus for distributed fiber optic sensing that reduce or compensate for the effects of laser phase noise. The method involves repeatedly interrogating an optical fiber (101), where each interrogation involves launching at least one interrogating pulse into the optical fiber; detecting optical radiation which is backscattered from within the fiber, and forming a measurement signal from a plurality of sensing portions of the fiber. The method involves forming a laser noise template based on the measurement signals within a frequency band of interest from a first set of said sensing portions; and applying a correction to the measurement signals from said plurality of sensing portions based on said laser noise template.

Abstract: The present application relates to road traffic monitoring to detect the number, speed and/or type of vehicles travelling on a road. Noise features (104) are deployed on, or formed in, the surface of a road (101). The noise feature (104) is arranged to generate a characteristic acoustic signature when traversed by the wheels of a vehicle (105) travelling within a lane of the road. A distributed acoustic sensor (102, 103) is deployed to detect occurrences of the characteristic acoustic signature. In some embodiments the noise element may comprise at least two distinct elements (104a, 104b), for instance rumble strips arranged transversely to the road, which are a known distance apart along the road. The acoustic signals from a wheel crossing both elements can be detected and used to determine the vehicle speed. The number of vehicle axles and axle separation can also be determined to categorize the type of vehicle.

Abstract: Fiber optic cable structures suitable for distributed acoustic sensing that are capable of discriminating between stimuli acting on the cable in different directions, the cable structure including a core structure (202, 203, 204) with an optical fiber wound around the periphery of the core structure, the core further including a mass (203) which is movable in a preferred direction within the cable such that movement of said mass in said preferred direction causes a change in length of the fiber wound around the periphery of the core.

Abstract: This application describes methods and apparatus for distributed fibre optic sensing that reduce or compensate for the effects of laser phase noise. The method involves repeatedly interrogating an optical fibre (101), where each interrogation involves launching at least one interrogating pulse into the optical fibre; detecting optical radiation which is backscattered from within the fibre, and forming a measurement signal from a plurality of sensing portions of the fibre. The method involves forming a laser noise template based on the measurement signals within a frequency band of interest from a first set of said sensing portions; and applying a correction to the measurement signals from said plurality of sensing portions based on said laser noise template.

Abstract: Methods and apparatus for determining fluid inflow to a well are described. In one example, a method comprise determining a first fluid flow velocity profile from slug tracking and determining a second fluid flow velocity profile from an estimate of fluid inflow at each of a plurality of perforations in a well. The first and second fluid flow velocity profiles are combined to provide a combined velocity profile. An indication of fluid inflow at least one perforation is derived from the combined velocity profile.

Abstract: Methods and apparatus for obtaining an indication of fluid in-flow in a section of a gas well having multiple perforations through which fluids comprising gas and/or liquid enter the well are described. In an example, the temperature within the section of the gas well is monitored at a plurality of locations. The locations comprising (a) a first set of locations, at or near a perforation; and (b) a second set of locations at which the temperature is substantially independent of the effects of the inflow of gas at a perforation. An indication of temperature excursions at the locations is obtained and used to obtaining an indication of the inflow of fluids to the well by relating the indications of temperature excursions from the second set of locations to liquid inflow and relating the indications of temperature excursions from the first set of locations to liquid and gas inflow.

Abstract: A method of distributed fibre optic sensing is described. In an example, a series of interrogations are launched into an optical fibre, each interrogation comprising interrogating radiation in at least one pulse pair, wherein the pulses of a pulse pair are introduced to the optical fibre with a time interval therebetween. Radiation backscattered therefrom is sampled, so as to obtain at least one sample from each interrogation. Phase modulation in the samples is determined and components of the phase modulation which are below a threshold frequency are isolated. Such a method of sensing could be used, for example, to monitor changes in temperature of the optical fibre.

Abstract: The present application relates to road traffic monitoring to detect the number, speed and/or type of vehicles travelling on a road. Noise features (104) are deployed on, or formed in, the surface of a road (101). The noise feature (104) is arranged to generate a characteristic acoustic signature when traversed by the wheels of a vehicle (105) travelling within a lane of the road. A distributed acoustic sensor (102, 103) is deployed to detect occurrences of the characteristic acoustic signature. In some embodiments the noise element may comprise at least two distinct elements (104a, 104b), for instance rumble strips arranged transversely to the road, which are a known distance apart along the road. The acoustic signals from a wheel crossing both elements can be detected and used to determine the vehicle speed. The number of vehicle axles and axle separation can also be determined to categorise the type of vehicle.

Abstract: This application relates to fibre optic cable structures that are suitable for distributed acoustic sensing, and which may usefully be used for seismic monitoring/detection. The cable structures provide good sensitivity may further exhibit a directional sensitivity and thus can be used to discriminate between stimuli acting on the cable in different directions. One such fibre optic cable comprises a core structure (202, 203, 204) with an optical fibre wound around the periphery of the core structure. The core comprises a mass(203)which is moveable in a preferred direction within the cable such that movement of said mass in said preferred direction causes a change in length of the fibre wound around the periphery of the core.