Resistive-limit, time-domain AEM apparent conductivity

On-time measurements, which can be obtained with existing airborne transient electromagnetic (AEM) systems, are used to map the apparent conductivity of the ground. Such measurements can extend the conductivity aperture of these systems substantially at the low end of the conductivity range. A theoretical analysis is given that provides a simple, approximate, resistive-limit solution for any transient EM waveform. The approximate result is compared to the full-numerical result for the specific case of the GEOTEM system over a conductive half-space. Examples of GEOTEM data reduced to apparent conductivity using the approximate analytical solution are presented from surveys for kimberlites and massive sulfide deposits.