Full‐wave modelling of early‐time measurements in time‐domain electromagnetics: Consideration of coupling between Tx–Rx antennas and the ground

ABSTRACT Early‐time measurements of time‐domain electromagnetics carry information of the near surface, and therefore, the interpretation of such measurements is important, particularly when utilizing small‐loop acquisition devices. However, these early‐time gates are often distorted by responses from the acquisition system itself, primarily caused by self‐transients associated with the transmission link composed of the transmitter and receiver loops and with the investigated medium electrical properties. Estimating all such potential couplings is crucial for interpreting early‐time measurements and gathering information on the near surface. In this study, a receptor loop made of 20 turns was built for this purpose. Field results and simulations were then compared, which shows how full‐wave finite‐difference method in the time‐domain simulations can be used to accurately calculate the impedance characteristics of time‐domain electromagnetics systems. All simulations were performed using finite‐difference time‐domain software based on a fully explicit three‐dimensional solver. First and second sets of simulations, respectively, were carried out in free space to compare the results to an alternate simulation method with equivalent lumped electrical circuits and on a homogeneous half‐space with varying electrical resistivities. The results were then used to improve the inversion of field data acquired on a test site in Garchy, France.