On the Computation of underground Electromagnetic Fields Generated by Lightning: A Comparison between Different Approaches

This paper deals with the evaluation of lightning electromagnetic fields inside a finitely conducting ground. We present a comparison between three different approaches that have been adopted by researchers dealing with lightning electromagnetic effects. The first is the Cooray Formula in which, the equations of lightning generated electromagnetic fields below the ground surface are connected to surface fields that can easily be determined. The second approach recently proposed by Delfino et al. consists of numerically evaluating the exact field expressions. Finally, the third approach is based on a numerical solution of Maxwell´s equations using the finite difference time domain (FDTD) method. Two different values for the ground conductivity, namely 0.01 S/m and 0.001 S/m are considered in the computations. The adopted model for the return stroke is the modified transmission line model with exponential decay (MTLE). The results obtained using Delfino et al. algorithm and those obtained using FDTD are virtually identical. It is also shown that the Cooray formula is able to reproduce with a very good accuracy the underground fields, especially for their early- time response.