Current channelling in time-domain airborne electromagnetic data

Abstract

Airborne electromagnetic (AEM) surveys for mineral exploration generate vast quantities of data. Rigorous electromagnetic (EM) modelling of geologically realistic structures is very computationally expensive, and interpretation of local AEM anomalies is usually based on approximate or nomogram methods. These methods typically account only for vortex induction in a conductive target, and are only valid if the host medium containing the target is poorly conductive, or if measurements are made at long delay times. When a target is either wholly or partly embedded in a conductive host, current channelling may enhance its response in comparison with that due to vortex induction, and the exciting and secondary fields are modified by 'blanking' effects as they pass through the host. The relative importance of current channelling and vortex induction depends strongly on the target and host parameters, the location of the AEM system relative to the target, and on the measurement delay time. In some instances, galvanic currents may dominate the entire target response, and models that account only for galvanic excitation can be applied. At the relatively early times employed in AEM prospecting, it is necessary to account for blanking of the current channelling response by the host medium, imposed as the secondary fields pass from the target to the receiver. Blanking effects can, however, be ignored if modelling is carried out at the resistive limit of the response. Shallow regolith features, such as palaeochannels, may give rise to observable AEM current-channelling anomalies. A field example from the Lawlers area of Western Australia demonstrates that the AEM anomaly associated with a known palaeochannel can be largely explained by the galvanic response of horizontal thin-sheet targets embedded in the overburden of a two-layered Earth. The host and palaeochannel parameters required to reproduce the observed anomaly are consistent with those derived from drilling, surface EM and DC resistivity surveys, and petrophysical measurements.