Thomas Samuel Hudson , Anna Stork, Jack Broderick Muir , Andreas Fichtner

Distributed Acoustic Sensing (DAS) allows one to measure strain at metre-resolution along a fibreoptic cable, increasing spatial sampling of a seismic wavefield compared to conventional instrumentation. However, the challenge of measuring DAS-derived strain amplitude currently limits applications of this technology. Absolute amplitude measurements are required for estimating earthquake magnitudes, moment tensor inversion and attenuation tomography, for example. Central to this challenge is quantifying how well the fibre is coupled to the subsurface. Here, we present a method using coherency to pragmatically estimate coupling of fibre to the medium. We first introduce a theoretical justification relating coherency to relative coupling between channels and calibrating this to obtain absolute coupling coefficients, before evidencing the performance of the method using various examples from glaciers to downhole geothermal deployments. We apply the method to estimate earthquake magnitudes, comparing values to independent geophone estimates. The results allow us to explore whether quantifying coupling is possible or indeed necessary to account for in certain instances. We find that although coupling of fibre to the medium is important, results suggest that practically in many cases, it may be appropriate to simply make the binary first-order assumption that fibre is either approximately perfectly coupled or too poorly coupled for any amplitude analysis. While our findings do not comprehensively solve the fibre-optic coupling problem, the theory and results provide a practical foundation with which to start using DAS-derived amplitude information in earnest.