Finite difference transient electromagnetic migration imaging for grounded sources

The currently major interpretation approach of transient electromagnetics is to provide information on the (apparent) resistivity of the underground medium, and it is challenging to provide detailed information on the geological interface. In this paper, based on the finite-difference principle, the migration imaging technology of the transient electromagnetic pseudo wavefield of the grounded source is achieved. According to the mathematical relationship between the transient electromagnetic diffusion field and the pseudo wavefield, the precise integration method is used to solve the first type of Fredholm integral equation, and then the direct wave is eliminated, and the pseudo wavefield containing only the ground reflection wave is obtained. Based on this, the 2D and 3D wavefield recursion equations in the time-space domain are derived by backward extrapolation of the first-order approximation equation for upcoming wave fields. By solving the wave field of underground space and using the deconvolution method to weaken the wave broadening effect of the pseudo wavefield, the longitudinal resolution of the geological interface can be improved and more accurate information of the geological interface can be obtained. The uniform half-space model, the D type model, the HK type model, the two-layer water-bearing goaf model, and field data are all used to verify the approach proposed in this paper. The results of the migration imaging are consistent with the geological data, indicating the transient electromagnetic finite-difference migration imaging method is feasible in identifying electrical interfaces and stratum features.