Direct Time‐Domain Numerical Analysis for Transient Electromagnetic Fields

Abstract

AbstractIn order to understand the exploration principle of transient electromagnetic fields, we have made numerical analysis directly in time domain for a two‐dimensional transient field excited by a negative step pulse. The method is to derive the homogeneous diffusion equation of the time‐domain electric field from Maxwell's equations that represent the basic law of electromagnetic fields and to do the difference discretion in the spatial region to be investigated. The source is joined as an initial condition and the boundary in the air is dealt with by quasi‐static approximation, then the time stepping is carried out. Hence the entire process of transient electromagnetic fields diffusing with time in the earth can be demonstrated. The distribution patterns of transient electromagnetic fields at different instants in the earth and the corresponding changes of electromotive force on the earth are calculated, which could enunciate the conductor's concentrating effect on induced eddy current, a decrease in diffusing velocity for conductive overburden and delay effect of transient fields. Therefore, the transient electromagnetic method is sensitive to conductors. Longer time is needed to detect the same depth where conductive overburden exists. Because of the delay effect, the bodies buried at shallow depths could be detected in late time‐periods of transient fields.