Ability of the magnetotelluric method to image a deep conductor: Exploration of a supercritical geothermal system

Abstract

Fluids trapped under supercritical conditions are potential geothermal resources yielding high well-productivity. These supercritical fluids may be imaged as deep conductors (> 2km depth) using the magnetotelluric (MT) method. However, MT imaging of a deep conductor is strongly dependent on the conductor geometries and the overlying clay layers. The ability of the MT method to image a deep conductor needs clarification to understand the usefulness of MT for exploring supercritical fluids and accurately relate the imaged conductor to supercritical fluids. In this study, we investigated MT imaging of a deep conductor of supercritical fluids using numerical tests, based on a conceptual resistivity model for a supercritical geothermal system in the Kakkonda area, northeast Japan. The test result showed that the MT method was able to image the deep conductor of supercritical fluids. However, we found that the shape and resistivity value of the imaged deep conductor might significantly differ from those of the true conductor. Specifically, if a deep vertically elongated conductor is imaged by MT inversion, it is necessary to be aware that the actual bottom part may be much wider in the horizontal directions than the imaged size.