[S-TT42] Advancing Earth Science through Optic Fiber Sensing Techniques and Integrated Analysis

[STT42-10] Subsurface Reflection Imaging from offshore DAS Observations: Autocorrelation Analysis of Natural Earthquakes

Yuji Sakagami¹, Tatsunori Ikeda¹, *Takeshi Tsuji^1,2 (1.Faculty of Engineering, Kyushu University, 2.School of Engineering, The University of Tokyo)

Keywords:DAS, Seismic Interferometry, Seismic reflection profile, Natural earthquake

Compared to conventional seismometers, the distributed acoustic sensing (DAS) enables high spatial resolution observations over large areas, and once optical fiber cables are installed, continuous seismic monitoring becomes feasible. In this study, we applied autocorrelation analysis with spectral whitening to natural seismic data obtained from DAS observations using optical fibers installed off the Sanriku coast and estimated the subsurface structure beneath the seafloor by producing a seismic reflection profile. To enhance the signal-to-noise ratio, we stacked autocorrelation functions from 23 earthquakes. Extracting shallow reflection structures through autocorrelation analysis is challenging due to high-amplitude signals near zero travel time. To address this, we computed a moving average over 5 km along the fiber and suppressed apparent shallow reflections by subtracting this average from the original autocorrelation function. The processed autocorrelations revealed subseafloor basement structures. Additionally, our results captured disturbances in reflected waves associated with faults and shallow reflective surfaces that were not detected in previous ambient noise studies for same fiber optic cable. This is likely because natural earthquakes generate strong energy across a broad frequency range, and S-wave reflections from faults can propagate to the surface, where they are recorded by DAS. These findings demonstrate that DAS observations can provide high-resolution imaging of subsurface structures.