Sediment Corrections for Distributed Acoustic Sensing

On continental margins, sediments cause significant and spatially variable delays in seismic phase arrival times. The strong impedance contrast of the sediment-bedrock interface causes P-wave splitting that is clearly seen on Distributed Acoustic Sensing recordings of earthquakes, resulting in additional phase arrivals that must be picked separately. We introduce sediment corrections to correctly account for those additional phases in the hypocenter localization procedure. Conceptually, the sediment correction method differs from the commonly-used station corrections; instead of introducing a mathematically optimal constant time delay for each station and each phase, the corrections are derived from a physical, first-order modeling of the wave propagation in the sediments. To calibrate the sediment corrections, a two-step procedure is adopted: (i) the delay between the P-phase and the converted Ps-phase is taken as a proxy of the sediment thickness; (ii) the P- and S-wave speeds are determined through inversion. We show that sediment corrections are able to account for most of the observed bias while considerably reducing the number of free parameters compared to classical station correction. Moreover, the observed local delays are almost fully explained by the presence of the sedimentary layer, rather than by the 3D velocity variations of the bedrock. We retrieve v_P and v_S values that are compatible with values commonly found for sediments. Given the simplicity and physical foundation of the proposed method, we recommend the use of sediment corrections over station corrections whenever significant P-wave splitting can be observed.