Optasense Holdings Limited

Alastair Godfrey

Roger Ian Crickmore

David John Hill

Magnus McEwen-King

Robert Ellwood

Andrew Biggerstaff Lewis

Chris Esprey

William Bartling

Philip Newton Winder

Christopher Minto

Magnus Mcewen-King

This application describes a fibre optic cable structure which is advantageous for distributed fibre optic sensing, for example distributed acoustic sensing (DAS). The fibre optic cable structure includes an optical fibre for distributed fibre optic sensing and is configured to comprise at least one longitudinal section of a first type, which exhibits a change in effective optical path length of the optical fibre of one polarity in response to a given applied force, and which is adjacent to at least one longitudinal section of a second type, which exhibits a change in effective optical path length of the optical fibre of the opposite polarity in response to an equivalent applied force. When used for DAS, the response of a sensing portion that includes sections of both the first and second types, will include or exclude certain wavenumber by summation, which provides a directional sensitivity to incident waves.

Fibre optic cable sensing apparatus

A fibre optic cable structure () suitable for fibre optic sensing with an improved sensitivity to an environmental parameter is described. The structure () includes an optical fibre () and a bend inducer () responsive to the environmental parameter to control bending of the optical fibre. The bend inducer () is configured to adopt a first configuration, that induces a first curvature of the optical fibre, at a first value of the environmental parameter and to adopt a second configuration at a second, different, value of the environmental parameter that induces a second, different, curvature of the optical fibre. By action of the bend inducer () a change in value of the environmental parameter imparts a bending force on the optical fibre.

Fibre optic cables

This application relates to a fibre optic cable () suitable for use with a distributed fibre optic sensor apparatus (). The fibre optic cable includes at least one optical fibre () and a force transformer () mechanically coupled to the at least one optical fibre. The cable may also include at least one cladding later () and/or a compliant material (). The cable may be surrounded by a jacket layer (). The force transformer () is configured to transform transverse forces due to dimension changes of the cable arising from a temperature variation of the cable into longitudinal forces to counteract the longitudinal component of said dimension change over a tuned temperature range. In this way optical path length changes due to a change of temperature can be reduced or eliminated providing a cable which is insensitive to temperature.

Cable for distributed sensing

A temperature sensor for measuring temperature is described. The sensor comprises a fibre optic cable () comprising optical fibre () and an interrogator unit () configured to interrogate the optical fibre with electromagnetic radiation, detect any radiation that is Rayleigh backscattered within the optical fibre and determine a measurement signal indicative of temperature changes for at least one longitudinal sensing portion of the optical fibre. A controllable thermal element, which may be a heating element, such as an electrically conducting element (), is arranged along the length of the fibre optic cable () and in thermal communication with the fibre optic cable (). A controller () is configured to generate a thermal variation in the controllable thermal element, e.g. by generating a time varying electric current in the electrically conducting element (). An analyser () is configured to analyse the measurement signal, extract a thermal response signal corresponding to the thermal variation and compare the thermal response to a predetermined characteristic to determine the temperature of the fibre optic cable at said longitudinal sensing portion.

Fibre optic temperature measurement

This applications relates to methods and apparatus monitoring of subsea cables (), for example subsea power or telecommunication cables, and in particular to determining a burial status for such cables. The method involves interrogating a sensing optical fibre () with an interrogator unit () to perform distributed fibre optic sensing. The sensing optical fibre () is deployed along the path of the subsea cable () and may, in some instances, form part of the subsea cable. A frequency spectrum of the respective measurement signal from at least one sensing portion of the sensing fibre (), which corresponds to pressure variations (&#x394;P(t)) due to surface water waves () to determine a burial status. High frequency components of the pressure variation that are detected in any unburied section () are attenuated in buried sections ().

Monitoring subsea cables

This application relates to methods and apparatus for monitoring data obtained as a train () travels on along a rail track () to detect the occurrence and/or severity of any significant in-train forces, such as may be caused by heavy braking or excessive acceleration. The method involves taking a first data set corresponding to measurement signals from a plurality of channels of at least one fibre optic distributed acoustic sensor () having a sensing fibre () deployed to monitor at least part of the rail track. The first data set corresponds to measurement signals acquired as the train passes along the rail track. The method involves analysing the measurement signals to detect a first characteristic signature () which consists of a sequence of acoustic transients that appear to propagate rearwards along the train.

Distributed fibre optic sensing for in-train forces monitoring

This application relates to methods and apparatus for distributed fibre optic sensing that can provide an indication of the absolute value of pressure acting on a sensing portion of a fibre optic cable. A sensor apparatus () has a first fibre optic cable structure () comprising a first optical fibre () and an interrogator () configured to perform distributed acoustic sensing on the first optical fibre () to provide a measurement signal from at least one sensing portion of the first optical fibre. The first fibre optic cable structure () is configured such that a sensitivity of a sensing portion () to an incident pressure stimulus (&#x394;P, &#x394;P) depends on the ambient pressure (AP, AP) at the location of the respective sensing portion. A processor () is configured to process the measurement signal in response to an incident pressure stimulus (&#x394;P, &#x394;P) based on a predetermined sensitivity profile () to determine an indication of the ambient pressure at the respective sensing portion.

Distributed pressure sensing

This application relates to a fibre optic cable structure suitable for use as a sensing fibre optic for distributed acoustic sensing and having an improved sensitivity to transverse pressure waves. The application describes a fibre optic cable () having a longitudinal cable axis and comprising at least one optical fibre (). The cable also comprises a compliant core material () mechanically coupled to the optical fibre(s), possible via a buffer () such that a longitudinal force acting on the compliant core material induces a longitudinal strain in the optical fibre(s). At least one deformable strain transformer () is coupled to the compliant core material and configured such that a force acting on the strain transformer in a direction transverse to the cable axis results in a deformation of the strain transformer thereby applying a longitudinal force to the compliant core material.

Fibre optic cable with tuned transverse sensitivity

This application relates to methods and apparatus for monitoring power cables () carrying multiple AC phases to detect deformation of the power cable. A distributed fibre optic interrogator unit () is used to interrogate a sensing optical fibre () coupled to the power cable to provide a measurement signal from each of a plurality of longitudinal sensing portions of the sensing optical fibre. An analyser () is configured to analyse the measurements signals to detect a characteristic of an imbalance in magnetic fields. The characteristic may be a signal component with a characteristic frequency related to the power frequency and number of AC phases, the sensing optical fibre may be sensitised to magnetic fields and the characteristic frequency may be 2n times the power frequency where n is the number of phases, e.g. six times the power frequency for three phase AC.

Monitoring of power cables with distributed fibre optic sensing

This application relates to methods and apparatus for determining the location of a fault in a power cable of a power distribution network. The method involves monitoring at least one electrical parameter of the power distribution network to detect the occurrence of a fault and determine at least one time window for the occurrence of the fault The method involves analysing a first set of measurement signals is obtained by a distributed fibre optic sensor having a sensing optical fibre deployed along the path of the power cable. The first set of measurement signals include signals indicative of the extent of any disturbances along the sensing fibre over a known time period. The measurement shallots are analysed based on the at least one time window for measurement signals and the location of the fault in the power cable is identified based on the location of one or more longitudinal sensing portions of the sensing fibre.

Detecting failure locations in power cables

This application relates to methods and apparatus for monitoring a geological formation that is or has been subjected to steam injection using a distributed fiber optic sensor apparatus to perform distributed fiber optic sensing on an optical fiber deployed in a borehole running through the geological formation to acquire a first set of measurement signals being indicative of disturbances acting on each of a plurality of longitudinal sensing portions of the first optical fiber where at least part of the optical fiber is disposed adjacent to at least a first electrode spaced apart from a second electrode and the first and second electrode are separated by a void that is filled with a gas and an electrical source applies a potential difference to the first and second electrodes such that an ionization event within the gas results in an avalanche multiplication.

Monitoring a geological formation

A system for moving object () detection is provided, the system comprising a fiber optic interrogator () adapted to provide distributed acoustic sensing on a optic fiber (), for example arranged along a border. The measurement signals from each of a plurality of sensing portions () of said fiber are analyzed to determined a characteristic of a Doppler shift. The characteristic of a Doppler shift may be a generally continuous decrease in detected frequency. By detecting the time at which the rate of change of frequency is at a maximum for each of the sensing portions the time of closest approach (ti, t, tt, t) of the object to those sensing portions can be determined with the sensing portion the object approaches closest to showing the greatest value of maximum rate of change of frequency. The distance of closest approach and velocity can be determined.

Detection of moving objects

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 () 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 () 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.

Fibre optic distributed sensing

A method and apparatus for determining the relative orientation of objects downwell, and especially to determining perforator orientation, involves varying the orientation of an object, such as a perforator gun () in the wellbore () and activating at least one directional acoustic source (-). Each directional acoustic source is fixed in a predetermined location to the object and transmits an acoustic signal preferentially in a known direction. The directional acoustic sources are activated so as to generate sound in a plurality of different orientations of said object. An optical fiber () is interrogated to provide distributed acoustic sensing in the vicinity of the object; and the acoustic signals detected by the optical fiber are analyzed to determine the orientation of the at least one directional acoustic source relative to the optical fiber, for instance by looking at the relative intensity in the different orientations.

Determining perforation orientation

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 () into the fluid within the conduit (); interrogating an optical fiber () 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 () of said acoustic stimulus caused by the fluid within the conduit and analyzing said reflections to determine any Doppler shift (&#x394;f).

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Farnborough, United Kingdom

Alastair Godfrey