Fibre Optic Sensing

A distributed fibre optic sensing apparatus, comprising:

• an optical arrangement configured to repeatedly interrogate a sensing optical fibre, wherein each interrogation comprises launching coherent interrogating optical radiation into said sensing optical fibre;
• a detector configured to detect a backscatter signal comprising optical radiation which is Rayleigh backscattered from within the sensing fibre in response to an interrogation;
• and
• a processor configured to process the detected backscatter signal to determine, for each of a plurality of channels of the sensing optical fibre, a measurement signal indicative of a phase modulation of the detected backscatter signal;
• wherein the apparatus is operable to controllably modulate a wavelength of the coherent interrogating optical radiation to vary between successive interrogations according to a modulation characteristic, and
• wherein the processor is configured, for at least one channel, to:
• identify an induced signal component corresponding to said modulation characteristic,
• determine an amplitude of the induced signal component;
• and
• determine a compensation to be applied to the measurement signal for that channel based on the amplitude of the induced signal component to compensate for any variation in effective gauge length.