Distributed acoustic sensing, transfer functions and moment magnitude determination: application to high-frequency downhole data

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Distributed Acoustic Sensing (DAS) enables sampling seismic wavefields along optical fibers at a spatial resolution of less than one meter, over distances beyond several tens of kilometers. This makes DAS a powerful tool to record seismic events densely along 2D directions, whether horizontally along the Earth’s surface or vertically in boreholes. Compared to traditional seismic sensors measuring ground motion units, DAS provides uniaxial strain measurements along the fiber with often imperfectly known transfer functions between the measurements and true ground motion. This can generate uncertainties in the derivation of seismic source parameters, such as the magnitude, that require an absolute measurement of the ground motion and a known instrument response. In this study, we examine the DAS transfer function, mapping DAS data to reference velocity records obtained from multiple co-located accelerometers. Our investigation makes use of downhole recordings from the FORGE (Frontier Observatory for Research in Geothermal Energy) field site situated in Utah, USA. Overall, we find that the DAS response estimated at different depth positions follows a consistent trend and deviates significantly from a flat response only below 80 Hz. An average site-specific DAS system response is then used to convert microseismic event recordings into calibrated velocity records with improved amplitude accuracies. Subsequently moment magnitudes Mw are derived from the P-wave records with results matching the independent accelerometer-based estimations with high fidelity for events with Mw > -1.0.