Abstract: Fiber optic cables with improved performance for use in distributed sensing, for instance in distributed acoustic sensors, are disclosed. In one embodiment a fiber optic cable (210) comprises a core (208) and cladding (206) disposed within a buffer material (202) surrounded by a jacket (204) and arranged so that the core is offset from the center of the cable. By offsetting the core from the center of the jacket any bending effects on the core can be maximized compared to the core being located at the center of the cable.

Abstract: This application describes method and apparatus for fiber optic distributed acoustic sensing (DAS) that allow for quantitative estimation of relatively large and continuous stimuli acting on the sensing fiber. An optical fiber (101) is interrogated with optical pulse and the Rayleigh backscatter detected to provide a DAS sensor. The method involves identifying a first stimulus acting on at least one sensing portion of the optical fiber, which results in an effective optical path length change within said sensing portion of at least the wavelength of the optical radiation. Such a path length change will result in signal wrapping leading to an observed variation (401) in backscatter intensity. The frequency of variation is detected and can be used to estimate the rate of change of path length. The method can be used to estimate strain rate and/or rate of change of temperature.

Abstract: Fiber optic cables with improved performance for use in distributed sensing, for instance in distributed acoustic sensors, are disclosed. In one embodiment a fiber optic cable (210) comprises a core (208) and cladding (206) disposed within a buffer material (202) surrounded by a jacket (204) and arranged so that the core is offset from the center of the cable. By offsetting the core from the center of the jacket any bending effects on the core can be maximized compared to the core being located at the center of the cable.

Abstract: This application describes methods and apparatus for monitoring flow of fluids within conduits using fiber optic Distributed Acoustic Sensing (DAS). The method can determine flow rate and/or flow regime within a conduit such as within a production well or a pipeline. Embodiments involve introducing an acoustic stimulus (802) into the fluid within the conduit (801); interrogating an optical fiber (104) deployed along the path of the conduit to provide a DAS sensor; and analyzing the acoustic signals detected by a plurality of channels of the DAS sensor to determine at least one flow characteristic. Analyzing the acoustic signal comprises identifying reflections (804) of said acoustic stimulus caused by the fluid within the conduit and analyzing said reflections to determine any Doppler shift (?f).

Abstract: Fiber optic cables with improved performance for use in distributed sensing, for instance in distributed acoustic sensors, are disclosed. In one embodiment a fiber optic cable (210) comprises a core (208) and cladding (206) disposed within a buffer material (202) and surrounded by a jacket (204) and arranged so that the core is offset from the center of the cable. By offsetting the core from the center of the jacket any bending effects on the core can be maximised compared with the core being located at the center of the cable.

Abstract: A fiber-optic sensor array (100) comprises a line array of fiber-optic sensor packages (AX, BX, CX, DX, AY, BY, CY, DY, AZ, BZ, CZ, DZ) each having a package input/output (i/o) fiber and each being arranged to output a finite output pulse series of optical output pulses via the package i/o fiber in response to input thereto of one or more interrogating optical pulses. The array further comprises a fiber-optic bus (104,106, 108, 110) extending along the length of the line array, each package i/o fiber being optically coupled to the fiber-optic bus at a respective positions along the line array. The array allows interrogation at a higher frequency than is the case for a serial array of the same number of fiber-optic sensing packages.

Abstract: This application describes method and apparatus for fibre optic distributed acoustic sensing (DAS) that allow for quantitative estimation of relatively large and continuous stimuli acting on the sensing fibre. An optical fibre (101) is interrogated with optical pulse and the Rayleigh backscatter detected to provide a DAS sensor. The method involves identifying a first stimulus acting on at least one sensing portion of the optical fibre, which results in an effective optical path length change within said sensing portion of at least the wavelength of the optical radiation. Such a path length change will result in signal wrapping leading to an observed variation (401) in backscatter intensity. The frequency of variation is detected and can be used to estimate the rate of change of path length. The method can be used to estimate strain rate and/or rate of change of temperature.

Abstract: This application describes methods and apparatus for fiber optic distributed acoustic sensing (DAS) where microstructured fiber (202), such as holey fiber or photonic crystal fiber is used as the sensing fiber (104). The microstructured fiber is configured so to provide at least one of enhanced sensitivity to a given incident acoustic signal; an enhanced non-linear optical power threshold and directional sensitivity. By configuring the microstructured fiber to be more compliant than an equivalent solid fiber and/or provide a large refractive index variation in response to applied strain, the response to a given acoustic stimulus may be larger than for the equivalent fiber, Providing a hollow core may allow higher optical powers and by providing a directionality to microstructured (304) allows the fiber to be used in a DAS system with a directional response.

Abstract: Interrogation of a phase based transducer is performed by temporally overlapping and interfering a single pulse output from the transducer to determine the rate of change with time of the measurand represented as a phase change. The rate of change, or derivative of the phase change typically has a much smaller amplitude than the signal itself, and the derivative measurement therefore has reduced sensitivity. In this way, large amplitude signals which might otherwise be subject to overscaling effects can be measured more effectively.

Abstract: This application describes methods and apparatus for monitoring flow of fluids within conduits using fibre optic Distributed Acoustic Sensing (DAS). The method can determine flow rate and/or flow regime within a conduit such as within a production well or a pipeline. Embodiments involve introducing an acoustic stimulus (802) into the fluid within the conduit (801); interrogating an optical fibre (104) deployed along the path of the conduit to provide a DAS sensor; and analysing the acoustic signals detected by a plurality of channels of the DAS sensor to determine at least one flow characteristic. Analysing the acoustic signal comprises identifying reflections (804) of said acoustic stimulus caused by the fluid within the conduit and analysing said reflections to determine any Doppler shift (?f).

Abstract: A method of sensing using a phase based transducer in which a transducer response is provided at multiple different levels of sensitivity. The different levels of sensitivity are used to produce an output which does not overscale across a wide range of signal levels. This is particularly useful for use in conjunction with multiplexed arrays of fiber optic sensors for seismic surveying. Methods of sensor calibration and noise reduction are also described.

Abstract: This application describes methods and apparatus for fibre optic distributed acoustic sensing (DAS) where microstructured fibre (202), such as holey fibre or photonic crystal fibre is used as the sensing fibre (104). The microstructured fibre is configured so to provide at least one of enhanced sensitivity to a given incident acoustic signal; an enhanced non-linear optical power threshold and directional sensitivity. By configuring the microstructured fibre to be more compliant than an equivalent solid fibre and/or provide a large refractive index variation in response to applied strain, the response to a given acoustic stimulus may be larger than for the equivalent fibre, Providing a hollow core may allow higher optical powers and by providing a directionality to microstructured (304) allows the fibre to be used in a DAS system with a directional response.

Abstract: Interrogation of a phase based transducer is performed by comparing the state of the transducer at two points in time to determine the rate of change with time of the measurand represented as a phase change. The rate of change, or derivative of the phase change typically has a much smaller amplitude than the signal itself, and the derivative measurement can therefore be thought of as a low sensitivity measurement to be used instead of or in combination with the normal signal measurement having higher sensitivity. In this way, large amplitude signals which might otherwise be subject to overscaling effects can be measured more effectively. For a signal with the majority of its energy centered at approximately 800 Hz, for example, the derivative of that signal will typically be attenuated by 60 dB with a period between the two measurement times of 200 ns.

Abstract: A method of distributed acoustic sensing (DAS) whereby the derivative or rate of change of a signal backscatted from a fiber is measured. The change, or derivative of the phase measured in this way has a much smaller amplitude than the signal itself if the difference between the two times at which the signal is measured is much less than the period of the signal being measured, resulting in lower sensitivity. Frequency shifts can be applied to temporally displaced return signals to compare the rate of change, for example by employing an output interferometer arranged to modulate the signal in each arm by a different frequency shift.

Abstract: Fibre optic cables with improved performance for use in distributed sensing, for instance in distributed acoustic sensors, are disclosed. In one embodiment a fibre optic cable (210) comprises a core (208) and cladding (206) disposed within a buffer material (202) and surrounded by a jacket (204) and arranged so that the core is offset from the centre of the cable. By offsetting the core from the centre of the jacket any bending effects on the core can be maximised compared with the core being located at the centre of the cable.

Abstract: A method of interrogating a phase based transducer by providing a pulsed input including two different wavelengths in which the different wavelength components can be used to derive a phase change experienced by a synthetic wavelength, and by arranging for the synthetic wavelength to be significantly greater that the component wavelengths, the phase so detected has a reduced sensitivity, and is less susceptible to overscaling effects.

Abstract: A method of distributed acoustic sensing (DAS) whereby the derivative or rate of change of a signal backscatted from a fibre is measured. The change, or derivative of the phase measured in this way has a much smaller amplitude than the signal itself if the difference between the two times at which the signal is measured is much less than the period of the signal being measured, resulting in lower sensitivity. Frequency shifts can be applied to temporally displaced return signals to compare the rate of change, for example by employing an output interferometer arranged to modulate the signal in each arm by a different frequency shift.

Abstract: A fibre optic accelerometer particularly intended for use with an interferometer using the compliant cylinder approach but further providing a seismic mass at the core of the cylinder resulting in improved sensitivity and rejection of out-of-axis inputs.

Abstract: Interrogation of a phase based transducer is performed by temporally overlapping and interfering a single pulse output from the transducer to determine the rate of change with time of the measurand represented as a phase change. The rate of change, or derivative of the phase change typically has a much smaller amplitude than the signal itself, and the derivative measurement therefore has reduced sensitivity. In this way, large amplitude signals which might otherwise be subject to overscaling effects can be measured more effectively.

Abstract: A method of sensing using a phase based transducer in which a transducer response is provided at multiple different levels of sensitivity. The different levels of sensitivity are used to produce an output which does not overscale across a wide range of signal levels. This is particularly useful for use in conjunction with multiplexed arrays of fibre optic sensors for seismic surveying. Methods of sensor calibration and noise reduction are also described.