Marine Sediment Characterized by Ocean-Bottom Fiber-Optic Seismology

The Sanriku ocean-bottom seismometer system uses an optical fiber cable to guarantee real-time observations at the seafloor. A dark fiber connected to a Distributed Acoustic Sensing (DAS) interrogator converted the cable in an array of 19,000 seismic sensors. We use these measurements to constrain the velocity structure under a section of the cable. Our analysis relies on 24 hr of ambient seismic field recordings. We obtain a high-resolution 2-D shear-wave velocity profile by inverting multimode dispersion curves extracted from frequency-wave number analysis. We also produce a reflection image from autocorrelations of ambient seismic field, highlighting strong impedance contrasts at the interface between the sedimentary layers and the basement. In addition, earthquake wavefield analysis and modeling help to further constrain the sediment properties under the cable. Our results show for the first time that ocean-bottom DAS can produce detailed images of the subsurface, opening new opportunities for cost-effective ocean-bottom imaging in the future.