[S-VC30] Hydrothermal systems of volcanoes

[SVC30-06] Geothermal exploration of Takigami geothermal field using optical fiber DAS and DTS methods

*Junzo Kasahara^1,2, Yoko Hasada^1,4, Shigetsugu Furuya³, Masahiro Shimazaki³, Hitoshi Mikada⁵, Hiroshi Ohnuma¹, Yoshihiro Fujise⁶ (1.ENAA, 2.Shizuoka University, 3.Idemitsu Kosan Co. Ltd., 4.Daiwa Exploration and Consulting Co. Ltd., 5.Kyoto Univ. , 6.WELMA Co. Ltd.)

Keywords:Geothermal exploration, supercritical water, DAS, Takigami geothermal field, VSP, S-to-P conversion

In 2022, we carried out a geothermal exploration in the Takigami geothermal field using DAS and DTS measurements by the optical fiber system. We installed the optical fiber to 2,000 m depth in the TP-2 geothermal well. We operated twelve seismic sources surrounding Takigami Geothermal Power Plant. The temperature was 226°C at 2,000 m depth. Using the first P arrivals obtained by DAS and surface seismometers, we determined the Vp structure along the E-W section. We modeled the vertical fault with N-S strike, which was estimated by gravity and well information (Takenaka et al. 1985), at 600 m east of the TP-2 wellhead. The estimated vertical displacement is 1,000 m. We separated the DAS waveforms into upgoing (reflected) and downgoing arrivals. Using reflected arrivals, we made a 3D migration image. Two intense reflections are identified at around 1,000 m and between 2,000 m and 3,000 m, respectively. We observed several natural earthquakes which occurred in northern Kyushu during the survey period. DAS recorded S-to-P conversion phases, which correspond to 1,000 m and 2,500 m depth, at some of these earthquakes. The S-to-P conversion depths coincide with the intense reflection depths described above.

This study is based on results from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO JPNS21991). We thank the officers in NEDO for their aggressive support during our studies. We greatly thank Idemitsu Oita Geothermal Co., Ltd. and Idemitsu Kosan Co., Ltd. for permission to use their geothermal wells and for providing their informal database of geothermal fields. We also thank WELMA Co., Ltd., Oyo Co., Ltd., Hanshin Consultants Co., Ltd., and Daiwa Exploration and Consulting Co., Ltd. for helping with fieldwork.