To know the precise geothermal characteristics of test field, we conducted a seismic observation by a fiber-optic distributed acoustic sensor (DAS) and ground surface seismometers in November 2018. We installed 20 sets of 3C seismometers and deployed a fiber-optic cable for the DAS down to 977 m-depth. The first arrivals of the M5.2 earthquake (EQ5) measured by DAS are traveling from the 977 m-depth to the wellhead with the velocity of ~3 km/s. Surface seismometers show unusual waveform characteristics. Although the amplitudes of the P-wave first arrivals of EQ5 on vertical seismometers are small, the EW and the NS seismometers show weak P first arrivals and succeeding intense phase with a second delay. We thought that this phase is possibly P-to-S converted phase and examined the cause of estimated P-to-S conversion. We assumed a high Vp/Vs zone at the 4 km-depth with 500 m-thick. The models can explain the observed characteristics.　We also examined the effectiveness to detect any high Vp/Vs reservoir using FWI (Full-waveform Inversion) method. The FWI suggests that we can retrieve the location and shape of high Vp/Vs zone if we use the combination of active seismic source(s), DAS in the borehole and surface geophones. However, the physical properties of the assumed reservoir are not fully retrieved.

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