Disrupted layered lithology can jeopardise airborne electromagnetic interpretation

Abstract

The majority of airborne electromagnetic (AEM) data is processed using stitched consecutive 1D approximations, from which conductivity depth or CDI sections can be produced. Obtaining calculated variable layer depths from AEM is one of the appealing assets of the method. The current induced by an AEM system in the nearsurface circulates preferentially at a radial distance from the horizontal transmitter (commonly called footprint), the section plotted below the receiver is actually generated from currents induced in the general vicinity and not directly below the reviver-transmitter system. Detection of palaeochannels, faults and other laterally varying structures are common geophysical mapping targets. Caution should be taken when interpreting these horizontally-anisotropic targets; especially if smaller that the system 'Annulus of resolution' or close to edges of discontinuous layering. All AEM systems have different transmitter-receiver geometries, moments, wave forms and others specifics. Forward modelling and conductivity-transforms provide a good method to understand CDI sections and for system comparison. When 2D/3D effects of discontinuities are present, 1D modelling incorrectly predicts weak conductors at depth. Within a conductive layer where conductivity changes with facies or geometrical thickness, 1D approximations work well. 2D processing of 2D data is clearly needed, but existing inversion methods are too slow. 2D/3D structures with abrupt conductivity boundaries resolved with an isotropic layered assumption must always be queried.