2023 Volume 45 Issue 3 Pages 175-194
In order to examine the performance of three-dimensional (3D) inversion of magnetotelluric (MT) data for geothermal exploration, where accurate numerical modeling is necessary against rough topography variation, we have utilized two inversion codes, FEMTIC and WSINV3DMT, for 3D inversion of MT data obtained in a geothermal area near the Hakkoda volcano, northern Japan. FEMTIC, a finite-element modeling (FEM) code, can incorporate either tetrahedral elements or deformed non-conforming hexahedral elements in the 3D mesh, while WSINV3DMT, a finite-difference modeling (FDM) code, uses rectangular cells. We used the same subset of the MT data (all components of the impedance and tipper at 15 frequencies at 34 stations) for both inversions, with the same noise-floor setting. As a result, we have recognized that the resistivity model by the WSINV3DMT code produce small but extremely high- or low-resistivity anomalies at the ground surface and resistivity distribution becomes rough. In addition, several thin horizontal anomalies of high- and low-resistivities alternately appear in shallow parts. These anomalies seem to be due to numerical errors by the rectangular approximation of the boundary between the air and the ground. In contrast, the FEMTIC code does not have significant numerical errors by the approximation of topographic changes, and no such anomalous structures appear in shallow parts and the resistivity distribution at the surface is smooth. For deeper parts, models by FEMTIC and WSINV3DMT are generally similar, however, the shapes of the main low-resistivity layer, which corresponds to the clay cap of geothermal reservoirs, are different between them. The WSINV3DMT code tends to generate extreme resistivity anomalies in the deeper parts. Comparison with resistivity logging data in geothermal exploration wells shows better matching for the FEMTIC models than the WSINV3DMT models. These investigations suggest that the FEM inversion is more reliable than the FDM when we include the topography in the 3D MT inversion.
