Modelling the Response of Helically Wound DAS Cables to Microseismic Arrivals
Alan F. Baird
Abstract
Summary Helically wound cables (HWCs) have been introduced to address the problem of broadside insensitivity of DAS to P-waves. They are well suited for applications that mainly involve P-waves like surface seismic, but downhole applications like microseismic monitoring involve a wider range of propagation directions and the need to also record S-waves, requiring a broader sensitivity analysis to determine their suitability. Here we model the response of HWCs by projecting the strain tensors of P- and S-waves onto helical fibre geometries. We show that although HWCs boost broadside sensitivity of P-waves they have a destructive effect on S sensitivity, due to the deviatoric nature of the S-wave strain tensor. Illustrating this with a synthetic microseismic waveform, we find that S amplitudes are highest for straight fibres, and decrease as the wrapping angle in lowered, eventually vanishing at ∼35°. For smaller wrapping angles S amplitudes increase again, but with opposite polarity. We conclude that HWCs are ill-suited for microseismic applications, but may provide valuable supplementary information. We suggest a potential application for a cable with multiple helixes with differing wrapping angles, which could exploit the differences in response of P- and S-waves to provide an automated method of classifying arrivals by phase.