[S-VC28] Hydrothermal systems of volcanoes

[SVC28-02] Geothermal exploration using distributed acoustic sensing in a borehole in Otake-Hatchobaru geothermal field in Japan

*Junzo Kasahara^1,2, Yoko Hasada^1,5, Hitoshi Mikada^1,4, Hiroshi Ohnuma¹, Yoshihiro Fujise³ (1.ENAA, 2.Shizuoka University, 3.WELMA, 4.Kyoto Univ., 5.Daiwa Explopration and Consulting)

Keywords:Geothermal exploration, supercritical water, DAS, Hatchobaru geothermal field

In 2023, as a part of NEDO supercritical geothermal project, we conducted a geophysical exploration in the Otake-Hatchobaru geothermal field in Kyushu, Japan. The Kyushu Electric Power Co (KEP) owns the Hatochobaru and the Otake Geothermal Power Plants. We used the optical fiber installed in the H-26 geothermal borehole in the Hatchobaru Geothermal Power Plant down to a depth of 1,600 m. We operated a medium size seismic vibrator at twelve locations surrounding the H-26 borehole. We also used 26 surface seismometers near the borehole. We obtained the Vp structure using the first arrivals of the distributed acoustic sensing (DAS) and surface seismometer records and computed the twelve synthetic DAS records. We migrated the reflection arrivals in 3D space using the Vp structure obtained in this study. The reflections focused around 2.0–2.5 km, probably the basement rock zone filled with fluid. More than thirty microearthquakes whose maximum magnitude was M=1.2 were observed during two weeks. The grid search minimizing O-C was applied to determine the hypocenters using the Vp structure and P and S first arrivals of DAS and surface seismometers. The hypocenters were distributed between the Hatchobaru and the Otake Geothermal Power Plants at a 2.2–4.0 km depth, and the depth and spatial distributions suggest the microearthquake were the ordinary volcano-seismic activity in the Kuju volcanic area.