Calibration of time domain AEM systems using a ground loop

Abstract

By using a closed, accurately laid out and surveyed multi-turn, insulated ground loop of known inductance L and resistance R, we can analytically calculate and predict the response of any time domain AEM system. By measuring the current induced in the ground loop, we have tested a two-stage calibration process whereby a system check is made on the transmitter-ground loop coupling and another is made on the ground loop-receiver coupling. Furthermore, in resistive terrain, the ground loop response provides an excellent way to directly measure the dB/dt field of the transmitter. Using this method we analyse the predicted and measured responses of several AEM systems. In every case, the predicted and measured responses differ. Agreement between measured currents and the prediction can be achieved by solving for errors in: a) the altitude of the system, b) the lateral position along the line compared to the GPS reference, c) system tilts. The final but necessary step to achieve a fit to received data required a prediction of the averaging effects of proprietary noise-reduction filters on the predicted response. The method provides a cost-effective way to calibrate time domain AEM systems, and to highlight problems such as transmitter current, receiver window timing and gain.