Abstract: An optical sensing system, comprising a plurality of optical sensors (206 . . . 209), first (203 . . . 205) and second (215, 216) input trees, and first (210 . . . 212) and second (219, 220) output trees. The first and second input trees are configured to direct respective optical signals from a first (202) and a second (214) input to each of the sensors, and the first and second output trees are configured to direct optical signals from each of the sensors (206 . . . 209) to a first (213) and a second (221) optical output. The first and second input and/or output trees are configured to direct signals of particular wavelengths (A1 . . . D-1, A2 . . . D2) from particular sensors to the first and second input and/or output, respectively. A redundant connection system thus provided allows continued operation in the event of a fibre or cable break (300).

Abstract: There is described a method and apparatus for processing light pulses returned from an optical sensor, wherein the light pulses are applied to two interferometer arrangements, a first interferometer arranged simply to superimpose two pulses and detect a first resulting value, and the other interferometer being arranged to apply a relative phase shift of about ?/2 before super-imposing the two pulses to detect a second resulting value. The relative phase shift is applied by shifting the phase of one or both of the pulses. The first and second resulting values are divided to give a third value, representative of the sensor state. A seismic sensor array using such an apparatus to process returning pulses is also described.

Abstract: There is described an apparatus and method for deploying a sensor array comprising a plurality of sensors (5) joined together by connection cables (6) in one or more “chains”, and connected to an input/output device (8). An input/output connection unit (7, 15) is provided on a carrier (10), and the sensors are held on or in deployment devices (14a-14e) mounted to the carrier with their connection cables (6) connected to the input/output connection unit (7, 15). The input/output device (8) may also be mounted to the carrier, and connected to the connection unit (7, 15). The carrier (10), sensors, and input/output device (8) may be delivered as a single package to the area where the sensor array is to be deployed. The sensors are then moved from the carrier to their final positions. The deployment devices (14a-14e) may be detached from the carrier and moved sequentially to the sensor positions, and a sensor may be removed from the deployment device at each sensor position.

Abstract: There is described an optical seismic cable (2) comprising a number of sensor units (5) spaced along the length of the cable and a number of connection units (4) spaced along the length of the cable, the cable further comprising a number of first optical fibres extending substantially continuously along the cable from one connection unit to the next, and a number of second optical fibres which each extend part-way along the cable between adjacent connection units, and wherein at each connection unit at least one first optical fibre is joined to a second optical fibre, and wherein at the sensor units sensors are joined to said second optical fibres.