Method of assessing attenuation in a distributed optical fiber sensing system

Attenuation in the deployed fibre of a distributed optical fibre sensing system, such as a distributed temperature sensing system, is assessed by calculating a signal impairment function SIF based on a function calculated from measured Stokes and anti-Stokes backscatter signals and a constant B that gives a ratio of the Stokes and anti-Stokes signal a minimum sensitivity to temperature of the deployed fibre. This function is adjusted for the characteristics of a pristine optical fibre of the same type as the deployed fibre to give the SIF, which for a non-degraded fibre has a constant value of zero. A plot of SIF along the length of the fibre can then be analyzed for any deviations from zero, which indicate a change in the fibre attenuation. The resulting data can be used for qualitative and quantitative assessment of the fibre attenuation, and for correcting temperature profiles obtained from measured anti-Stokes signals.