Full-wave modeling of early times in TDEM: estimation of all potential coupling with numerical FDTD simulations
Résumé
Early times measurements of TDEM carry information on the very near surface, their interpretation is therefore very important in particular when using small-loop acquisition devices. These earliest time gates are however often distorted by a response from the acquisition system itself. This response is caused by self-transients associated to the transmission link composed of the transmission (Tx) and reception (Rx) loops and the half-space variability (investigated medium). Estimating all these potential coupling is crucial to interpret these early times and gather information on the near surface.The simulation method here is a resolution of the full Maxwell equations with 3D finite differences in the time domain (FDTD). This method have the advantage of considering the geometry of the acquisition device as well as the variability of the ground. The main drawback, however, is the important need in computational resources.A first set of simulations were carried out fist in free space to compare the results to another simulation method with equivalents RLC circuits, then on an homogeneous half-space with varying electrical resistivties. The results were then used to improve the inversion of field data acquired on a test site in Garchy (France).