On the Impact of the Gauge Length Value on Distributed Acoustic Sensing Data Quality

Summary Distributed Acoustic Sensing (DAS) is an innovative technique which has been recently employed for near-surface geophysics purposes. It involves the analysis of a fibre-optic cable by sending a laser pulse from an interrogator unit. The phase of the backscattered signal contains the information of the strain on the cable, enabling the detection of a passing seismic wave. Allowing the interrogation of long profiles and the generation of a dense spatial sampling, uneasy to obtain with classic techniques, DAS instrumentation proved its relevance for seismic applications. DAS measurements rely on the estimation of the difference in the phase of the studied backscattered light between two positions on the fibre. The distance separating those two positions is called gauge length. Because the signal to noise ratio values obtained with DAS appear generally low when compared with values acquired with geophones, it is required to know the optimal gauge length value to use in order to obtain the best data quality. In this paper, the impact of gauge length variations on DAS data quality is explored on the same set of data acquired during a well-to-well seismic survey along a fibre-optic cable running in four different monitoring wells.