Abstract: The application describes methods and apparatus for distributed fiber sensing, especially distributed acoustic/strain sensing. The method involves launching interrogating radiation in to an optical fiber and sampling radiation backscattered from within said fiber at a rate so as to acquire a plurality of samples corresponding to each sensing portion of interest. The plurality of samples are divided into separate processing channels and processed to determine a phase value for that channel. A quality metric is then applied to the processed phase data and the data combined to provide an overall phase value for the sensing portion based on the quality metric. The quality metric may be a measure of the degree of similarity of the processed data from the channels. The interrogating radiation may comprise two relatively narrow pulses separated by a relatively wide gap and the sampling rate may be set such that a plurality of substantially independent diversity samples are acquired.

Abstract: The application describes methods and apparatus for distributed fiber sensing, especially distributed acoustic/strain sensing. The method involves launching at least first and second pulse pairs into an optical fiber, the first and second pulse pairs having the same frequency configuration as one another and being generated such that the phase relationship of the pulses of the first pulse pair has a predetermined relative phase difference to the phase relationship of the pulses of the second pulse pair. In one embodiment there is a frequency difference between the pulses in a pulse pair which is related to the launch rate of the pulse pairs. In another embodiment the phase difference between the pulses in a pair is varied between successive launches. In this way an analytic version of the backscatter interference signal can be generated within the baseband of the sensor.

Abstract: This invention relates to the fiber optic distributed acoustic sensing to detect P and S waves in a solid medium. Distributed acoustic sensing can be achieved using an unmodified fiber optic by launching optical pulses into the fiber and detecting radiation which is Rayleigh backscattered there from. By analyzing the returns in analysis bins, acoustic disturbances can be detected in a plurality of discrete longitudinal sections of the fiber. The present invention extends such fiber distributed acoustic sensing to detection of S and P waves.

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: A method for monitoring for seismic events by interrogating an optic fiber which forms part of an existing communications infrastructure to provide distributed acoustic sensing (DAS). The signals provided by the distributed sensing provide measurements at each of a plurality of discrete portions along the fiber, which may be many tens of kilometers in length. Warning or measurement and consequently prediction of seismic activity can be provided by collecting data over a wide area, without the need to deploy a correspondingly large fiber network.

Abstract: The application describes methods and apparatus for distributed fiber sensing, especially distributed acoustic/strain sensing. The method involves launching interrogating radiation in to an optical fiber and sampling radiation backscattered from within said fiber at a rate so as to acquire a plurality of samples corresponding to each sensing portion of interest. The plurality of samples are divided into separate processing channels and processed to determine a phase value for that channel. A quality metric is then applied to the processed phase data and the data combined to provide an overall phase value for the sensing portion based on the quality metric. The quality metric may be a measure of the degree of similarity of the processed data from the channels. The interrogating radiation may comprise two relatively narrow pulses separated by a relatively wide gap and the sampling rate may be set such that a plurality of substantially independent diversity samples are acquired.

Abstract: Methods and apparatus for tracking objects in conduits, especially for tracking the movement of a pig in a pipeline, are disclosed. Distributed acoustic sensing is used to obtain signal returns from a plurality of discrete longitudinal sensing portions of a fibre optic cable which is laid along the length of the conduit. The method involves detecting an acoustic signature corresponding to the object moving in the conduit. The acoustic signature may comprise a series of pressure pulses caused by the object traversing joints etc. in the conduit. Preferably the method involves detecting pressure waves caused by the object as it moves through the conduit. Detecting the characteristic pressure wave signal provides discrimination of the object from other acoustic disturbances. In pipeline applications a pig can be tracked in real time, if the pig stops moving an alert can be generated and the location of the pig will be known.

Abstract: This application relates to methods and apparatus for monitoring hydraulic fracturing during oil/gas well formation. A fiber optic cable (102) deployed down a well bore (106), which may be the well bore in which fracturing is performed, is interrogated to provide a distributed acoustic sensor. Data is sampled from at least one longitudinal sensing portion of the fiber and processed to provide at least fracturing characteristic. The fracturing characteristic may comprise the characteristics of high frequency transients indicative of fracturing events (606). The intensity, frequency, duration and signal evolution of the transients may be monitored to provide the fracturing characteristic. Additionally or alternatively the fracturing characteristic may comprise the longer term acoustic noise generated by fracture fluid flow to the fracture sites. The intensity and frequency of the noise may be analyzed to determine the fracturing characteristic. The method allows real-time control of the fracturing process.

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: A fiber-optic sensor package (10) comprises a serial array of fiber-optic sensing coils (14, 16, 18, 20, 22) each of which is comprised in a respective fiber-optic sensor. Four (14, 16, 18, 22) of the coils are housed in a protective casing (24). One of the coils (22) is comprised in a fiber-optic electromagnetic field sensor. The other coils are comprised in respective fiber-optic geophones and/or hydrophones. A single package of the invention allows detection of both seismic and electromagnetic signals. An array of packages of the invention provides detection of both seismic and electromagnetic signal at a series of positions over a long distance or wide area, thus avoiding the need for two conventional arrays. A packages of the invention, and an array of such packages, require little or no electrical power input.

Abstract: A plurality of sensors output information into a distributed acoustic sensing (DAS) system via acousto-mechanical signals. The sensors are coupled to the optic fibre at the centre of the DAS system indirectly, the acousto-mechanical signal being transmitted via an intermediary body, such as the ground or a conduit.

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.