Interpretation of Airborne EM (Electromagnetic) Measurements Based on Thin Sheet Models

Abstract

An airborne electromagnetic (AEM) prospecting system is used as a very rapid means of economically searching large geologically potential areas for sulphide ore bodies within the top few hundred feet of the earth's surface. This is possible because massive sulphide ores are usually much more electrically conductive than most host rocks. The system usually consists of a transmitting coil and a receiving coil flown by an aircraft. The transmitting coil generates a time-varying magnetic field in the low audio-frequency range (100-4000 hz). Any small perturbations of a component of the field are then recorded continuously as the aircraft flies along flight lines. The record over a conducting body is called an anomaly profile. The magnitude of an anomaly depends on conductivity3 size and shape of the conducting body causing it3 and the flight direction and height of the AEM system. Interpretation is based on a qualitative or quantitative comparison of the observed response (anomaly) with the responses that would be observed by the system over simple idealized conductors. Many geological conductors are somewhat sheet-like in form and dip into the earth at an appreciable angle. The extent of the sheet is often large compared to the region sensed by the system. Both for airborne and ground EM measurements the single most important model has been the (thin) half-plane. For the multitude of AEM systems actually in use even this simple model has not been fully explored. The only available data are the peak amplitudes of the vertical half-plane for 'small-scale' systems (inaccurate) and the Lockwood quadrature system, and some profiles over perfectly conducting half-planes for a variety of systems plus a few random model profiles for a few systems. The thesis contains systematic studies with several AEM systems over vertical and dipping half-planes, vertical and flat ribbons, strike limited sheets, vertical half-plane under a thin flat overburden and two parallel half-planes. Studies have been restricted to thin conductors because of modelling convenience, and to keep the number of parameters under control. While thick bodies are interesting, the simpler thin case must obviously be done first. Field and model data have been compared in a number of cases and it is found that most of the field data is explicable without further elaboration of the models. However, certain discrepancies are found even in these selected cases which indicate that additional features of the conductors may have to be taken into account (and cast suspicion on the quality of altimetry). In the cases examined, it did not seem that the conductivity of host rocks was having much influence. The study indicates that it is often possible to discriminate flat-lying conductors from dipping conductors with the present systems. Other effects such as finite extent, influence of overburden, and fringe conductors are difficult to identify from a single flight with a current AEM system.