2:42 PM - 3:00 PM
[SVC30-04] Deep underground structure and geothermal system conceptual model inferred from integrated geophysical surveys
Keywords:Kuju volcano, Otake-Hatchobaru geothermal field, Supercritical geothermal resource, Geothermal system conceptual model
Kuju volcano, located in the central Oita prefecture, consists of about 20 volcanic domes in an area of about 13 km from east to west and about 10 km from north to south. Kuju-Iwoyama is one of the active volcanoes, which is caused a phreatic eruption in 1995, contains many fumaroles and hot springs. The project “Potential survey and estimation of power generation of supercritical geothermal resources in Kuju area”, which is funded by New Energy and Industrial Technology Development Organization (NEDO) is progressing in this area since 2021. This project makes a hydrothermal conceptual model based on the geophysical, geological and geochemical properties. In order to get these properties, some surveys such as MT, seismic, gravity, fumarole gas analysis were conducted.
Aizawa et al. (2022) indicates the existence of the low resistivity body of less than 20 Ωm at depth of about -3 km a. s. l. with 5 km in diameter below the Chojabaru. The western and eastern margins of the low resistivity body are located at Otake-Hatchobaru geothermal area and Kuju-Iwoyama, respectively. This low resistivity body coincides with the low-density area estimated from the gravity anomaly. In addition, the high-density area, which reflects the old volcanic body and the basement rocks (granite and metamorphic rock), exists along the east, west and south margins of the low resistivity body.
Many faults with trending east- west direction caused by the regional extension stress field are located in this area. However, the faults in the Otake-Hatchobaru geothermal field trends NW-SE or NE-SW normal faults. The geothermal reservoir of this geothermal field is located in these faults. According to Ishibashi et al. (2022), the isotope of H2 and He in the fumarolic gas indicates the contribution of the magmatic fluid. This result implies that some NW-SE or NE-SW faults connect to the deeper magmatic body through the basement rock and play the role of the path of magmatic fluid. Moreover, almost of the hypocenters observed by the seismic monitoring concentrates along western margin of the low resistivity body below the Otake-Hatchobaru geothermal field (Tsuji et al, 2022). The concentration of hypocenters located at the western part of Mt. Kuroiwa coincides with the high temperature area estimated from the temperature in the boreholes. The previous hydrothermal conceptual model constructed by the previous NEDO project (Kitamura et al., 2023) is improved by the integration of the above results. We will report the detail of the survey results and the conceptual model.
Aizawa et al. (2022) indicates the existence of the low resistivity body of less than 20 Ωm at depth of about -3 km a. s. l. with 5 km in diameter below the Chojabaru. The western and eastern margins of the low resistivity body are located at Otake-Hatchobaru geothermal area and Kuju-Iwoyama, respectively. This low resistivity body coincides with the low-density area estimated from the gravity anomaly. In addition, the high-density area, which reflects the old volcanic body and the basement rocks (granite and metamorphic rock), exists along the east, west and south margins of the low resistivity body.
Many faults with trending east- west direction caused by the regional extension stress field are located in this area. However, the faults in the Otake-Hatchobaru geothermal field trends NW-SE or NE-SW normal faults. The geothermal reservoir of this geothermal field is located in these faults. According to Ishibashi et al. (2022), the isotope of H2 and He in the fumarolic gas indicates the contribution of the magmatic fluid. This result implies that some NW-SE or NE-SW faults connect to the deeper magmatic body through the basement rock and play the role of the path of magmatic fluid. Moreover, almost of the hypocenters observed by the seismic monitoring concentrates along western margin of the low resistivity body below the Otake-Hatchobaru geothermal field (Tsuji et al, 2022). The concentration of hypocenters located at the western part of Mt. Kuroiwa coincides with the high temperature area estimated from the temperature in the boreholes. The previous hydrothermal conceptual model constructed by the previous NEDO project (Kitamura et al., 2023) is improved by the integration of the above results. We will report the detail of the survey results and the conceptual model.