Unstructured Grid Finite Element Modeling of the Three-Dimensional Magnetotelluric Responses in a Model With Arbitrary Conductivity and Magnetic Susceptibility Anisotropies

Three-dimensional (3-D) forward modeling algorithms of conductivity anisotropy with respect to the magnetotelluric (MT) method have been widely developed; however, numerical modeling considering the magnetic susceptibility anisotropy is less studied, and the magnetic susceptibility of the underground medium cannot be ignored in some areas of magnetite-rich rocks. In this study, the nodal finite element (FE) method and the unstructured tetrahedral grid were used to discretize the A-Phi system for a model with both conductivity and magnetic susceptibility anisotropies, which was solved using the biconjugate gradient stabilized method. By comparing the numerical results with the analytical solutions of two-layered models and a previous vector FE solution of a single-block model, the accuracy of the implemented algorithm was verified. Then, the influence of magnetic susceptibility anisotropy on MT responses was analyzed based on different block models, as well as on a quasi-two-dimensional (quasi-2D) model producing out-of-quadrant phases (OOQP). Different types of MT data were considered to conduct these analyses, including the apparent resistivity, impedance phase, phase tensor, and tipper. Our findings point to the same conclusion that the effect of magnetic susceptibility anisotropy cannot be ignored in magnetite-rich areas.