3D finite-difference transient electromagnetic modeling with a whole-space initial field

Abstract

A 3D finite-difference time-domain transient electromagnetic forward-modeling method with a whole-space initial field is proposed to improve forward efficiency and flexibility. The open-source software WFTEM3D is developed based on this method with two language versions: a FORTRAN code and a MATLAB code. First, the scheme calculates the whole-space initial field excited by magnetic dipole sources at an initial time after the current is switched off. Then, the scheme steps Maxwell’s equations in time using a staggered grid and a modified DuFort-Frankel method. Multiple magnetic dipole sources are superimposed to achieve the modeling for large loop configurations. The air is included into the grids and the Dirichlet boundary condition is used on the outer boundaries. The forward-modeling method has three advantages. First, the method can be used to simulate the whole-space model and the half-space model. Second, big mesh growth factors of 1.5–3 are allowed, which significantly improves the computational efficiency and facilitates the simulation of the multiscale model. Third, the approximate initial field can be calculated using a conductivity independent of the model, which facilitates the simulation of the nonflat topography model. These advantages are tested by examples. The model of a conductive brick in a half-space and the model of a complex conductor at a vertical contact are simulated. The run times are 0.48 and 9.72 s, respectively, on an ordinary computer with an Intel Core i7-6700 CPU, and the solutions are consistent with that presented in the literature. Examples of a multiscale model with three conductive bricks, a valley model, and a hill model are simulated, which further verify these advantages.