Acquisition & Sensor technologies

[AS-02] Turning the Muroto seafloor cable into a long DAS sensing array

*Martin Karrenbach¹, Robert Ellwood¹, Victor Yartsev¹, Steve Cole¹, Eiichiro Araki², Toshinori Kimura², Hiroyuki Matsumoto² (1. OptaSense (United States of America), 2. JAMSTEC (Japan))

Unused existing submarine cables containing standard telecom fiber for data transmission can be turned into seismic sensing arrays efficiently. We demonstrate a case where a 120km long submarine cable provided a 50km long active distributed fiber-optic sensing array. The cable (Muroto Cable) originates in the landing station near Cape Muroto, Japan, and extends offshore from there. It provides power and data transmission to distant sea floor observatories. An active source seismic data set was acquired with fiber-optic DAS sensing technology. Clear signals from an airgun source were recorded over the entire sensing array for source-receiver offsets up to 100 kilometers. Passive seismic data (earthquakes) were also recorded. Location consistent variations in strain sensing amplitudes due to seafloor coupling were observed, indicating variability of ground/seafloor coupling. Utilizing direct wave field arrivals from a controlled airgun source we estimate the cable’s sensing directionality pattern. We observe direct, reflected, refracted and mode-converted arrivals allowing analysis on various spatial scales. This proof of concept survey demonstrates that legacy power and data transmission cables for offshore oil and gas infrastructure, trans-oceanic data transmission, or connection to other offshore infrastructure can be used to acquire high-quality seismic data. The study also provides insight into the design and installation of new dedicated seafloor cable sensing arrays.