SEGJ Technical Conference

Possibility of high Vp/Vs zone in the geothermal filed suggested by the P-to-S conversion

Abstract To know the precise geothermal characteristics of the test field, we conducted a seismic observation by a fiber-optic distributed acoustic sensor (DAS) and ground surface seismometers in 2018. We deployed a fiber-optic cable for the DAS down to 977 m-depth. 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 effectivity 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.