Marine Sediment Characterized by Ocean‐Bottom Fiber‐Optic Seismology

Abstract

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.

Utilizing distributed acoustic sensing and ocean bottom fiber optic cables for submarine structural characterization

“…Coherent Scholte wavefields. Observations of ocean surface gravity waves and Scholte (P-SV solid-liquid interface) waves from marine DAS records have been recently reported by 34,35,41,42 . However, the raw strainrate records of DAS ( Fig.…”

“…At present, there are over 350 active submarine cables spanning 1.2 million kilometers connecting very close to 100 countries (TeleGeography 33 ). As shown by two recent studies 34,35 , DAS offers the capacity to turn these global cables into a powerful sensing resource if appropriate analysis tools are utilized, providing a path towards characterizing previously hidden offshore structures.…”

Utilizing Distributed Acoustic Sensing and Ocean Bottom Fiber Optic Cables for Submarine Structural Characterization

“…The resulting autocorrelation profile (Fig.4a) indicate a distinct lateral variation along the 20 km cable with high spatial resolution (20 meters). At this point, we are not confident that the autocorrelation horizons should be interpreted as specular reflections as suggested in past studies [35]. However, we can identify several boundaries as indicated by the dashed line on the the profile.…”

“…Observations of ocean surface gravity waves and Scholte (P-SV solid-liquid interface) waves from marine DAS records have been recently reported by Sladen et al [41], Williams et al [42], Lindsey et al [34], Spica et al [35]. However, the raw strain-rate records of DAS ( Fig.2) are complicated by the superposition of a variety of coherent signals dominated by different frequency components, as well as incoherent and optical noise effects, e.g., temperature drift, interrogator unit shake, coupling issues.…”

“…Ambient noise autocorrelation technique has been successfully applied to image subsurface structure on Earth and Mars [45,46], and has recently been used with DAS data offshore the Sanriku coast of Japan to image marine sediment thickness and velocity properties [35]. We obtain autocorrelation (zero offset cross-correlation) functions along the densely sampled DAS array ( Fig.4a), as by-products of ambient noise cross-correlation.…”