Deep underground structure and hydrothermal modeling inferred from integrated geophysical surveys

Abstract

A detailed study of the deep geothermal system in Kuju volcano, central Kyushu, Japan. This study was part of a project funded by the New Energy and Industrial Technology Development Organization (NEDO) to explore supercritical geothermal resources. This study involves the development of a hydrothermal conceptual model that incorporates geophysical, geological, and geochemical data. Geophysical surveys, such as magnetotellurics (MT), seismic, gravity, and fumarole gas analyses, were conducted. The conceptual model indicates that faults in the Hatchobaru area may act as pathways for magmatic fluids, contributing to the hightemperature zone identified on the western side of Mt. Kuroiwa. The model also suggests the existence of an upflowing path for volcanic fluids. The geological setting of the Kuju volcanic complex is characterized by faults aligned in an east-west direction owing to north-south extensional stress. The study also discusses the resistivity structure, which reveals a low-resistivity zone associated with faults near the Hatchobaru-Otake power plant. Additionally, the model inferred the presence of a magma chamber and identified pathways for magmatic fluid flow. A numerical simulation using the TOUGH2 was conducted to validate the model. The simulation results closely matched the observed temperature and pressure data, although further refinement of the model is recommended for greater accuracy.