Three-Dimensional Magnetotelluric Investigation in Geothermal Fields in Japan and Indonesia

Abstract

A three-dimensional (3D) inversion code for magnetotelluric data has been developed and tested with field datasets obtained in several geothermal fields in Japan and Indonesia. The inversion code utilizes the least-squares inversion method with smoothness regularization. The forward modeling is by the finite difference method. Static shift, which is often an obstacle in the interpretation of MT data, is solved together with the underground resistivity structure in the inversion. A Bayesian criterion ABIC is applied to searching for the optimum trade-off among the minimization of the data misfit, model roughness and static shifts. The code has been successfully applied to the field data, showing sufficient stability, reliability and robustness in the computation. In this paper, 3D inversion results of the MT data obtained in the Ogiri geothermal field, southwestern Japan, and in the Mataloko geothermal field, eastern Indonesia, are presented. Recovered 3D resistivity models are generally similar to the two-dimensional (2D) inversion models, however, the deeper portion of the 3D model seems to be more realistic than that of the 2D model. The 3D models are also in a good agreement with the geological model of the geothermal reservoirs. These results indicate the necessity of the 3D interpretation for geothermal exploration and other application in complicated geological environments.