3D transient electromagnetic inversion based on explicit finite-difference forward modeling

With the development of computers and forward modeling techniques, three-dimensional (3D) transient electromagnetic inversions are being developed. However, there are few 3D transient electromagnetic inversions based on explicit forward modeling methods. Therefore, in this study, we propose a 3D transient electromagnetic inversion method based on explicit finite-difference forward modeling. For forward modeling, we first calculate the whole-space initial field excited by magnetic dipole sources at an initial time after the current is switched off. Then, we step Maxwell's equations in time using the modified DuFort-Frankel method and staggered grids. For inversion, we employ the least squares optimization method and test it using synthetic and field data. Using synthetic data, we invert the model of a conductivity brick in a half-space. The results reveal that the conductivity brick is well recovered. Inversion results using the field data are consistent with known geological conditions. The synthetic and field examples demonstrate that the inversion method based on explicit finite-diff erence forward modeling is reliable.