Whole-space modeling of a layered earth in time-domain electromagnetic measurements

In-mine time-domain electromagnetic survey responses can be caused by conductive zones located either in the sequence above the mine drift, in the sequence below or in both. A computer code is developed that computes the whole-space transient electromagnetic field for a horizontal square loop in a layered earth. Four models are considered, which include conductive layer above, below or above and below the transmitter-receiver level. Both in-mine and transmission (transmitter on the surface and receiver at depth) configurations are modeled. The behavior of the vertical magnetic field or its time derivative is identical when the conductive layer is located either above or below the measurement station. On the contrary, the horizontal magnetic field or its time derivative display shape and polarity strongly dependent on the vertical position of the conductive layer, as well as on its conductivity, thickness and distance. Analysis of in-mine field data from the Taquari-Vassouras potash mine (Brazil) shows that no significant conductive layers are present below the mine gallery and that an usual interpretation technique may be used. A transmission test carried out with a transmitter loop at the surface and the receiver coil in the mine 400 in below demonstrates that the geoelectrical distribution above the mine, obtained from in-mine TDEM and borehole logs, is sufficient to satisfy the observed data. (C) 2002 Elsevier Science B.V All rights reserved.