An approach for 3D multisource, multifrequency CSEM modeling

We discuss a practical approach for multisource, multifrequency controlled-source electromagnetic (CSEM) modeling. The approach consists of an efficient iterative multigrid-based solver and an automatic gridding procedure. For a given frequency and a given source location, the automatic gridding procedure ensures that the computational grid is consistent with the discretization of the electromagnetic equations. The conductivity is mapped from the input grid onto the automatically defined computational grid by volume averaging. This mapping changes the conductivity representation. Volume averaging based on the logarithm of the conductivity provides the best result. When the stretching in the computational grid is moderate, we use a multigrid method based on full coarsening. However, when the stretching is more severe, we propose a more robust multigrid method based on semicoarsening. Numerical examples show the usefulness of this approach for survey design and scenario studies over complex heterogeneous structures, when the layered-earth assumption is not satisfactory.