Distributed Sensing of Ocean-Earth Interface on Hadal-station deployed fiber-optic cable

Fiber-optic distributed acoustic sensing (DAS) has exhibited potential in ocean monitoring. However, its monitoring region depends upon preexisting submarine telecommunication cables. In this study, we used a fine-scale optical fiber array (FOFA) DAS to sense the deep-ocean environment and illuminate underwater shear velocity structure. On the R/V Tansuoyihao TS-37-2 expedition in 2023, we carried out a DAS experiment to monitor deep-sea microseism in the central rift zone of the Philippine Sea basin. By utilizing the Hadal station（H-Station）and remotely operated vehicles (ROV) for collaborative operations, we successfully deployed a 560-meter long, 3-mm diameter optic-fiber array on the seabed at a depth of 5,569 meters. We obtained continuous data of seafloor strain field with a spatial 400 channels and a resolution of 1.6 meters. Through the processing of DAS data from UTC November 11, 2023, between 19:00 PM and November 12, 2023, 02:30 AM, combined with previous observations from hydrophones and ocean bottom seismometer (OBS), the results indicate: 1) The submarine in the study area is quiet, with a less than 20×10^-9 strain variation. 2) DAS can perceive microseism from solid media or water disturbances from fluid media within a wide frequency band (10,000 s ~ 250 Hz). In power spectra density (PSD) profile, the 0.32 Hz contains most of the energy, mainly originating from water disturbances. The second at 0.78 ~ 2.5 Hz exhibits multiple dispersion in the F-K domain, representing Scholte waves. The third is reaching 19.5 Hz, is a noise likely from rotating ship propellers near the water surface. Additionally, narrowband signals at 12.4 Hz, 24.8 Hz, 37.5 Hz, 56.1 Hz, and 62.4 Hz, which exhibit harmonic properties, may be related to local oscillator noise from H-station. 3) Based on Scholte waves and its 7th-order modes dispersion curves collected by DAS, we got deep-sea 1D sub-bottom shear-wave velocity structure. This study reveals deep-sea distributed strain fields at the meter-scale resolution for the first time, confirming the effectiveness of DAS in deep-sea environmental monitoring. It will provide new technical support for study on ocean-solid interactions, turbidity currents, earthquakes, and biological acoustic events, as well as Scholte wave velocity inversion.