Numerical analysis of multicomponent responses of surface-hole transient electromagnetic method

摘要

We calculate the multicomponent responses of surface-hole transient electromagnetic method.The methods and models are unsuitable as geoelectric models of conductive surrounding rocks because they are based on regular local targets.We also propose a calculation and analysis scheme based on numerical simulations of the subsurface transient electromagnetic fields.In the modeling of the electromagnetic fields,the forward modeling simulations are performed by using the finite-difference time-domain method and the discrete image method,which combines the Gaver-Stehfest inverse Laplace transform with the Prony method to solve the initial electromagnetic fields.The precision in the iterative computations is ensured by using the transmission boundary conditions.For the response analysis,we customize geoelectric models consisting of near-borehole targets and conductive wall rocks and implement forward modeling simulations.The observed electric fields are converted into induced electromotive force responses using multicomponent observation devices.By comparing the transient electric fields and multicomponent responses under different conditions,we suggest that the multicomponent-induced electromotive force responses are related to the horizontal and vertical gradient variations of the transient electric field at different times.The characteristics of the response are determined by the varying the subsurface transient electromagnetic fields,i.e.,diffusion,attenuation and distortion,under different conditions as well as the electromagnetic fields at the observation positions.The calculation and analysis scheme of the response consider the surrounding rocks and the anomalous field of the local targets.It therefore can account for the geological data better than conventional transient field response analysis of local targets.