Comparative earthquake sensitivity of straight and triply helical fibers in surface distributed acoustic sensing

Although distributed acoustic sensing (DAS) using optical fiber cables offers superior spatial and temporal resolution, surface-deployed DAS still faces a significant challenge: straight fiber (SF) cables predominantly record vibrations parallel to cable orientation, limiting the accurate vertical components. Helically wound fiber (HWF) configurations have been proposed to address this limitation of SF and capture multi-directional seismic waves. However, because we cannot perfectly distinguish the seismic signals from the noises in field data, evaluating the comparative sensitivity to large-scale vibrations, such as seismic waves, acquired by SF and HWF is challenging. This study aims to precisely access the sensitivity of HWFs and SFs in surface-deployed DAS measurements for seismic waves excited by earthquake events, which likely propagate vertically. We present a novel approach to analyze the sensitivity of SFs and HWFs by deploying three SFs and triply helically wound fibers, which comprise three HWFs with identical specifications within a single cable. Our results show that the sensitivity ratio of HWF to SF consistently maintains a value of about 0.6 in the conditions of this study. This finding indicates a discrepancy with existing theories on HWF sensitivity that depend on its wrapping angle.