The resolution of shallow horizontal structure with AEM

Abstract

The minimum size of shallow horizontal structures to be detectable and resolvable with airborne electromagnetic (AEM) systems is discussed. Synthetic data were generated for the helicopter frequency-domain system RESOLVE and the fixed-wing time-domain systems TEMPEST and GEOTEM. The modelled scenarios include conductive prisms in a resistive host and resistive prisms in a conductive host. The EM responses of these models were computed for a range of prism thicknesses, side-lengths and host conductivities. Gaussian noise in amplitude comparable to actual system noise levels was added to the synthetic data followed by the derivation of conductivity-depth sections via three-layer inversion and EMFLOW conductivity-depth imaging. Where these 1-D algorithms failed to indicate the presence of the prism the data were inspected for prism anomalies in order to evaluate if 2-D or 3-D algorithms might be able to map these structures. The results indicate that beyond a minimum prism conductance (conductivity-thickness product), prism thickness and host conductivity are much less important than side-length. For horizontal prisms to be detectable with the RESOLVE system they have to be at least 12 m wide. In order to resolve their thicknesses and conductivities with 1-D algorithms, the prisms must be at least 130 m wide. For the TEMPEST and GEOTEM system the application of 1-D algorithms enables the detection of horizontal prisms wider than 180 m and the resolution of their thicknesses and conductivities for prisms wider than 450 m. Profile inspection shows that horizontal structures as narrow as 24 m might be detectable with more advanced algorithms.