A Guide to the Seismic Wavefield as seen by DAS

Abstract

Distributed acoustic sensing (DAS) exploiting fibre optic cables provides high-density sampling of the seismic wavefield. Scattered returns from multiple laser pulses provide local averages of strain rate over a finite gauge length. The nature of DAS signal depends on the orientation of the cable with respect to the passing seismic waves, and so differs from conventional seismometer response. For local events the dominant part of the strain rate can be extracted from the difference of ground velocity resolved along the fibre at the ends of the gauge interval. For more distant events the response at seismic frequencies can be represented as the acceleration along the fibre modulated by the wave slowness resolved in the same direction, which means there is a strong dependence on cable orientation. The work concentrates on direct interpretation of DAS signals. The nature of the wavefield can be elucidated with the aid of frequency--slowness domain representations for body waves and surface waves that are also suitable for simulation of DAS response. The DAS wavefield is examined at local, regional and global scales. In each case examples of DAS recordings are presented, together with a discussion of suitable ways of simulating the DAS response for the particular regime. There is an emphasis on the specific nature of DAS records and the way in which sensitivity to cable orientation relative to passing wavefronts can be utilised to advantage. The exploitation of ambient noise via cross-correlation of DAS channels is also considered using a simple visual approach to examine the effect of cable geometry on the extraction of Rayleigh and Love wave signals. The slowness-domain representation of the DAS signal allows analysis of the array response of cable configurations under stacking indicating a bias due to the slowness weighting associated with the effect of gauge length. Unlike seismometer arrays the response is not described by a single generic stacking function. Suitable designs for DAS layouts are presented for both stacking of distant events and cross-correlation studies. Show more