2022 Volume 44 Issue 1 Pages 7-21
The magnetotelluric (MT) method is an essential tool for the exploration of geothermal resources, and threedimensional (3D) inversion is now routinely applied to MT data in geothermal fields in the world. One of the challenges in the 3D MT inversion is the treatment of the topography because the finite-difference method, which is commonly used for 3D inversion codes, may create numerical errors in solving the electromagnetic fields when the mesh discretization is applied to a rough topography variation. In this study, a comparison of different mesh discretization, particularly in the vertical direction, was conducted for the MT dataset obtained at a western flank of northern Hakkoda volcano, Aomori Prefecture, Japan, using the 3D MT inversion code WSINV3DMT (Siripunvaraporn and Egbert, 2009). The horizontal size of each cell in the core zone of the interpretation was 150 m by 150 m, while five different sizes, from 5 m to 100 m, were tested for the vertical cell size for the elevation range where the MT stations were located. The frequency used for the inversion spanned from 0.0067 Hz to 115 Hz targeting a normal depth of geothermal reservoirs. The comparison revealed that the smaller vertical cell settings can reduce the computation time in spite of the larger number of cells, generating a smoother and more reliable resistivity model for the shallow underground in the model. The resultant 3D resistivity models by the smaller vertical cell settings showed good consistency with the borehole logging data obtained by past geothermal exploration projects in the study area. These results suggest that the inclusion of topography in the finite-difference modeling is effective for the 3D inversion of MT data at an area where the topography variation is not neglectable.
