Three-dimensional adaptive finite-element method for time-domain airborne EM over an anisotropic earth

How to generate a reasonable mesh for the anisotropic medium model with complex electrical structure is the key to obtaining high-precision modeling results. In this paper, we study the adaptive finite-element (FE) method for 3D time-domain airborne EM modeling over an anisotropic earth. By combining the time-domain unstructured FE algorithm and the adaptive mesh refinement technology, we successfully realize the 3D time-domain airborne EM adaptive forward modeling for anisotropic media. To deal with the big difference of EM responses at different time channels, we bring in a temporal weighting to achieve a synchronous refinement of shallow and deep meshes. The reliability of our algorithm is verified by comparing our results with 1D semi-analytic solution. The numerical experiments show that the anisotropic conductivity has a big influence on the mesh refinement. The value of the maximum principal-axis conductivity and its distribution directly determines the mesh refinement. In addition, anisotropic conductivity can also seriously affect the distribution pattern and anomalous amplitude of the three-component EM responses. By using the polarity maps for full-domain apparent resistivity, the direction of the anisotropic principal axis can be well identified.