Distributed Optical Fiber Sensing in Scientific Deep Seafloor Borehole Observatory in the Nankai Trough

The Nankai Trough hosts diverse seismic phenomena across various timescales, requiring deep borehole observatories for comprehensive monitoring. In addition to the seafloor borehole observatories constructed in IODP NantroSEIZE project, we developed a new optical fiber-based borehole observation system, integrated with the DONET submarine cable network. In November 2023, a 500 m deep borehole observatory (C9038B) was installed off the Kii Channel, enabling real-time monitoring of strain and pore-fluid pressure, for the first implementation of such seafloor borehole observation system. In October 2024, we successfully conducted distributed optical fiber sensing (DOFS) in the borehole for the first time by compensating optical losses with amplifier systems. Both Distributed Acoustic Sensing (DAS) and Rayleigh Frequency Acoustic and Strain (RFAS) techniques provided detailed insights into strain variations induced by microseisms and tidal fluctuations at the borehole situated in the front of Nankai Trough subduction.