em3d-MT: Open-source 3D Anisotropic Forward Modeling of Radio-magnetotelluric Data Considering Displacement Currents

Summary As exploration frequency increases, traditional models based on quasi-static assumptions fail to accurately simulate radio-magnetotelluric (RMT) responses due to the growing significance of displacement currents. While 3D numerical simulations that account for both diffusion and wave phenomena in anisotropic media remain underexplored, we developed em3d-MT, an open-source 3D RMT anisotropic forward modeling algorithm. Based on the vector finite element method, em3d-MT solves Maxwell equations considering both conduction and displacement currents. The algorithm handles arbitrary anisotropy in electrical conductivity and dielectric permittivity and includes a 3D geological modeling workflow with unstructured tetrahedral meshes to model complex geological structures. A direct solver, optimized with a two-layer parallel scheme, is used to solve the linear system, resulting in improved computational performance. em3d-MT aims at contributing to the modeling and inversion of RMT data commonly used in near-surface geophysical exploration projects. We present a detailed discussion on the principles of the em3d-MT program, its algorithmic framework, and numerical case studies.