Seismic interferometry analysis using ambient noise records observed by distributed acoustic sensing with Tokai-Oki seafloor cable

Abstract

Distributed acoustic sensing (DAS) is a new technology, which can turn a fiber optic cable into array of acoustic and seismic receivers, and is now becoming a new approach to observe seismic signal with ultra-high spatial density. DAS technology enables us to measure seismic signals with high spatial resolution at harsh environment including volcano area, the polar regions and ocean bottom, etc. because DAS observation needs only fiber optic cable and DAS acquisition unit. Also, DAS technology can be easily applied to monitoring and repeated survey. In this study we applied seismic interferometry technique to ambient noise records observed by DAS acquisition unit connected to the Tokai-Oki seafloor cable. We observed short-term data on September 2017. Seismic interferometry technique can turn ambient noise records into pulsive pseudo shot records by calculating cross-correlation between each trace. Therefore, the analysis provides high-density shot gathers along the cable. We applied band-pass filter with frequency range from 1.0 to 3.0 Hz to the ambient noise records. Obtained pseudo shot gathers include clear signals. The apparent velocities of signals shown in the shot gathers are approximately 300-800 m/s. Direct and reflected S-wave, and surface wave may be included in the shot gather. Our results suggest that only one hour continuous data are needed to obtain the clear shot gathers and the pseudo shot gather can be used for subseafloor structural monitoring.