The Effect of Sharp Bends and Cable Coupling on DAS Recording in an Urban Environment

Distributed acoustic sensing (DAS) records represent the strain (rate) in an optical fiber averaged over a gauge length about a specified channel. In the presence of bends and variable coupling to the surroundings, the effect of gauge-length averaging is more complex than for a straight cable. Analysis for an incident plane wavefront shows how both changes in cable orientation and coupling can produce significant changes in signal amplitude in the immediate neighborhood of the change. Coupling effects can be distinguished by the strong correlation of P and S behavior because orientation factors differ for the two wavetypes. Such effects are illustrated with DAS records from an urban cable in Perth, Western Australia, with cable segments along nearly orthogonal streets in a grid pattern and also long cable loops with poor ground coupling.