The resolution of shallow horizontal structure with airborne EM

Abstract

The minimum size for shallow horizontal structures to be detectable and resolvable with airborne electromagnetic (AEM) systems is discussed, using synthetic modelling results and analysis of survey data. 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 1D algorithms failed to indicate the presence of the prism, the data were inspected for prism anomalies in order to evaluate whether 2D or 3D algorithms might be able to map these structures. Results from the modelling of synthetic data indicate that beyond a minimum prism conductance (conductivity-thickness product), prism thickness is much less important for prism detectability than side-length. The minimum prism conductances determined for the RESOLVE, TEMPEST, and GEOTEM system are 1, 2, and 8 S, respectively. For horizontal prisms to be detectable with the RESOLVE system on response profiles and 1D conductivity-depth sections they have to be at least 12 m wide. In order to resolve their thicknesses and conductivities, the prisms must be at least 120 m wide. For horizontal prisms to be detectable with the TEMPEST and GEOTEM system on 1D conductivity-depth sections they have to be at least 150 m wide. In order to resolve their thicknesses and conductivities, the prisms must be at least 450 m wide. Profile inspection shows that horizontal structures as narrow as 24 m might be detectable with more advanced algorithms. However, the minimum size for resolving a prism?s parameters is not a result of the application of 1D algorithms but is defined by the system?s minimum footprint, which is a function of the system?s geometry. Some results of the synthetic data were confirmed by analysis of recent RESOLVE and TEMPEST survey data, which suggests conductive structures have to be at least 10?20 and 100?160 m wide, respectively, to be mapped on EMFLOW sections.