Stable 3-D inversion of MT data and its application to geothermal exploration

Abstract

A stable inversion technique has been developed for three-dimensional (3-D) interpretation of magnetotelluric (MT) data. The inversion method was based on the Gauss-Newton (linearized least-squares) method with smoothness regularization. Static shifts were also treated as unknown parameters in the inversion. The forward modeling was done by using the staggered-grid finite difference method. The Akaike Bayesian information criterion (ABIC) was applied to searching for the optimum trade-off parameters among the minimization of the data misfit, model roughness and static shifts. The inversion method was then applied to a large volume MT dataset obtained in the Ogiri geothermal area, southwestern Japan. The inversion was performed stably, resulting in a good fitting between the observed and computed apparent resistivities and phases. The recovered 3-D resistivity structure was generally similar to that obtained by a two-dimensional (2-D) inversion in a previous work, although the deeper portion of the 3-D model seemed to be more realistic than that of the 2-D one. The 3-D model was also in a good agreement with the geological interpretation of the geothermal reservoirs. These results indicated the necessity of the 3-D interpretation for geothermal exploration and other applications in complicated geological environments.