Distributed Acoustic Sensing (DAS) Waveforms: Effects of Source Signature, Geometry, and Attenuation

Abstract

Distributed acoustic sensing (DAS) records strain integrated over a finite gauge length depending on incidence angle, frequency, material velocities, fiber curvature, and gauge length. The DAS responses to a monofrequency plane wave of these are well understood. However, neither the full response of a DAS waveform to a source signature pulse over a band of frequencies nor this signature's evolution over the length of a single DAS channel due to attenuation have been examined. We use analytic expressions and numerical simulations to quantify how realistic source wavelets and in gauge attenuation reshape DAS strain/strain rate, including broadening, lobe splitting, and in some cases near, vanishing of the recorded signal. Full elastic wavefield modeling shows that fiber curvature and non-planar wavefronts can further shift apparent transit times relative to co-located point sensors. We illustrate these effects with a field example in which 1 m spaced downhole geophones are used to reconstruct a simultaneously acquired DAS record. The results delineate when DAS behaves like a scaled geophone and when it does not, and they motivate forward modeling for studies that demand high fidelity in timing and spectral content. A series of analytic and numerical tools are provided to assist with such evaluations.