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 n/2 before superimposing 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.