2005 Volume 27 Issue 2 Pages 131-148
Accurate understanding of deep geologic structures is very important for characterizing and evaluating geothermal reservoirs. This study aims at three-dimensional interpretation of geothermal reservoirs by a magnetotelluric (MT) survey, inversion of apparent resistivity, and spatial interpolation of one-dimensional (1D) resistivity models. The western side of Mt. Aso crater, central Kyushu, southwestern Japan, was chosen as a study area. The MT measurement was carried out at 26 sites and the data were processed by the remote reference method to reduce local noises. Based on generally low values of skewness of the impedance tensor, local geologic structure at each site was approximated as one-dimensional. Therefore, 1D inversion was applied to the MT data at each site. Resultant 1D resistivity models of all sites were then interpolated by the three-dimensional optimization principle method. The resistivity distribution revealed some continuous conductors of less than 10 Ω·m near hot springs. These conductors may correspond with cap rocks, because the resistivity decreases largely with impermeable clay minerals that are common in cap rocks of geothermal reservoir systems. Thus, two geothermal reservoirs, whose shapes were estimated to be pillars, were detected under the cap rocks at an elevation range from -1000 to -3000 m. By comparing the resistivity model with the temperature distribution computed by a fluid-flow simulation at the steady state, the location and dimension of the estimated reservoirs were validated.
