Modelling the Response of Helically Wound DAS Cables to Microseismic Arrivals

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.