Electromagnetic Exploration of Supercritical/Super-Hot Geothermal Systems

Recent geothermal research has focused on technologies for harnessing the significant energy of high-temperature and high-pressure fluids within the deep subsurface for next-generation geothermal power generation. In particular, many countries have explored the use of supercritical geothermal fluids, i.e., fluids with temperature and pressure conditions exceeding the critical point of pure water (374 °C, 22.1 MPa), for power generation. The Iceland Deep Drilling Project confirmed the presence of supercritical geothermal fluids in two deep wells in Iceland. Note that supercritical (also called super-hot and ultra-hot) geothermal fluids are not necessarily in a supercritical state because they contain dissolved components. Practical supercritical geothermal generation requires efficient exploration techniques to identify supercritical geothermal systems; electromagnetic exploration, typified by the magnetotelluric method, is a promising example. Conventional shallow geothermal systems are characterized by a low-resistivity clay cap layer and an underlying geothermal reservoir with relatively high resistivity. In contrast, supercritical geothermal fluids are remarkably conductive because they include saline fluids originating from magma or seawater. Therefore, resistivity explorations of supercritical geothermal systems should focus on low-resistivity bodies that indicate reservoirs, ensuring careful investigation of the properties of supercritical geothermal fluids. This review summarizes existing research on the resistivity of supercritical geothermal fluids and surrounding rocks, as well as previous explorations of supercritical geothermal systems conducted using the magnetotelluric method and their implications. Finally, we discuss the scope for future research aimed at exploiting the potential of supercritical geothermal power generation and moving toward carbon neutrality.