em3d-MT: Three-dimensional forward modeling of radio-magnetotelluric data considering displacement currents in anisotropic media

ABSTRACT For high frequencies, traditional programs based on quasi-static assumptions fail to accurately simulate radio-magnetotelluric (RMT) responses because displacement currents become significant and cannot be disregarded. However, 3D RMT forward modeling that accounts for diffusion and wave phenomena, especially in anisotropic media, remains underexplored. We have developed an open-source 3D anisotropic forward-modeling algorithm for RMT data, em3d-MT, using an edge-based finite-element (FE) method. Unlike existing 3D RMT modeling algorithms, em3d-MT solves full-wave Maxwell’s equations considering conduction and displacement currents. The program considers the effects of arbitrary anisotropy in electrical conductivity and dielectric permittivity, alongside an advanced 3D geologic modeling workflow with unstructured tetrahedral meshes, to accurately simulate RMT responses of complex geologic models. The FE linear system of equations is solved using a direct solver and a double-layer parallel scheme to improve computational efficiency. We validate the accuracy of em3d-MT using layered-earth and 3D benchmark models. We then analyze the RMT responses of a 3D block model with varying dielectric permittivity and anisotropic parameters. Finally, we apply em3d-MT to the modeling of a complex anisotropic coal mine goaf model to demonstrate its effectiveness in real-world scenarios. Numerical results confirm that em3d-MT can accurately simulate the coupled behavior of diffusive and wave fields, revealing the significant influence of dielectric permittivity and its anisotropic effects on RMT data.