Finding targets in complex hosts using airborne EM

Abstract

Two basic questions lay at the heart of AEM surveys, one concerning the basic physics of the process, the other the interpreter?s capability. Is the equipment capable of exciting the target to the extent that it will produce a detectable signal above background response? Can the presence of a target be deduced from this signal? This study examines the response of a 400 siemen sulphide lens in an altered ultramafic lying at the boundary of a faulted host under a moderately conductive saprolite cover. In the easy case, the surface is flat and in the other, the whole lies under moderately steep topography. The study compares the ability of fixed wing time-domain, helicopter time-domain and helicopter frequency-domain systems to differentiate between when the lens target is present and when it is absent. Constant receiver ground clearance is assumed. In general the effect of the lens was to smooth the response horizontally so that a naïve interpreter might mistake the barren response for the target response and vice versa. For the time-domain systems, the vertical component showed the greatest differentiation whereas the in-phase, in-line coaxial component was the most effective for the idealised HEM system. HEM systems showed better anomaly localisation than did fixed wing systems due to shorter transmitter-receiver separation and lower flying heights. Conductivity depth images and other interpretations based on one-dimensional earth models yield misleading information for the type of model studied.