Geophysical exploration of subsurface structure in six geothermal fields using distributed acoustic and temperature sensing methods

Abstract

We have conducted geophysical studies using the optical fiber sensing method in Japan's six hot geothermal wells to develop a new geophysical method to explore geothermal subsurface structures. The temperature profiles were obtained from six surveys using the optical five-distributed temperature sensing (DTS) method. The hottest temperature was 288°C at 2,000 m depth. The obtained temperature profiles differ from one to another, suggesting differences in the local geological circumstances. We obtained seismic records from active sources with distributed acoustic sensing (DAS) method. Using seismic rerecords, we obtained the subsurface velocity structure of the survey region. The DAS records of all sites showed numerous seismic reflections to ca. 4 km depth, which could be caused by fractures filled with fluid. Our study proved to be very useful for the geophysical characterization of geothermal fields. The reflective zones imaged by reflected arrivals can be used for the subsequent investigation of geothermal development.