Characteristics of vapor-dominated geothermal systems from the viewpoint of resistivity structure

Vapor-dominated geothermal areas are regions that only produce vapor from a geothermal reservoirs. Vapor-dominated geothermal areas have been reported worldwide (e.g., Larderello (Italy), Geysers (USA)) and have been studied from a range of perspectives. In this study, we investigated the characteristic resistivity structure in a vapor-dominated geothermal reservoir in Sengan, Iwate, Japan; an area where the White et al. (1971) model has been accepted.

We performed a 3D inversion with ModEM (Egbert and Kelbert, 2012) using magnetotelluric (MT) data from 68 stations. We determined an optimized solution according to the obtained results with the smallest root mean square value by considering the calculation conditions, including the initial resistivity, a smoothing parameter, and a Lagrange multiplier. The 3D inversion results are used to confirm zones of clay alteration, faulting, and low-permeability shielding, which control the geothermal system in this area. The 3D resistivity structure matches well with electrical logging data obtained from as deep as 1 km, and is therefore more reliable than estimates based on logging data from sea level and above. Our aim is to evaluate the temperature and hydraulic permeability in this area using artificial intelligence technology based on the 3D resistivity structure and geological data.

The MT data used in this study were provided by the Tohoku Sustainable & Renewable Energy Co. Inc. under the research project "Technology for estimation of deep geothermal structures and drilling bit life using artificial intelligence" organized by the New Energy and Industrial Technology Development Organization.