Evaluating subsurface imaging performance with ocean-bottom distributed acoustic sensing and double beamforming

SUMMARY The high cost of active surveys and the scarcity of underwater instruments hinder the availability of seismic imaging in oceanic environments. Ocean-bottom Distributed Acoustic Sensing (OBDAS) utilizing existing telecommunicational infrastructure is an alternative and economical approach to illuminate the subsurface at unprecedented resolution and over distances of tens of kilometers. In this study, we utilize OBDAS data along a 60-km cable perpendicular to the coast of Oregon to image the continental shelf subsurface. We extract landward and seaward surface waves via cross-correlation and coherent stacking of the ambient seismic field. To stably measure dispersions of both the fundamental modes and higher overtones, we apply a double-beamforming (DBF) workflow across different array subsections in the 0.2–3 Hz band with a spatial averaging technique. We perform a perturbational-based inversion scheme to reliably invert for S-wave velocities over the first 2000 m of the subsurface underlying the fiber-optic cable. By comparing our results with the 1-D slant-stack approach, we demonstrate the applicability of the DBF method on the OBDAS data set and the enhanced spatial resolution of this approach. From our single-mode DBF results, we observe a coherent layering feature in the Florence shelf-sea region, while multimode DBF results reveal possibly smaller-scale heterogeneity in the study region.