Block modelling as a check on the interpretation of stitched CDI sections from AEM data

Abstract

AEM data collected over quasi-layered regolith structures is readily and conveniently interpreted using stitched 1D models. As a check on the validity of this process, a 3D modelling program was used to generate a 'blocky' regolith conductivity profile to characterise the gross conductivity structure seen on a stitched 1D model of a 12-km AEM survey line. The differences between model data based on 'stitched 1D conductivities' and the original field data highlight 'interpretation errors' resulting from a simplistic 1D assumption. In the case tested, large conductive zones (1 km or so in lateral scale) directly interpreted from a stitched 1D conductivity section underestimated the required local conductivity and overestimated the true lateral dimensions. Interspersed resistive areas are thus spatially more extensive than they appear on the stitched 1D model. Simple 3D modelling does not however automatically correct for survey deficiencies. For example, probable errors in bird location resulted in surficial conductors being modelled at a 'false' depth of 10 to 20 m using the nominal but imprecisely known AEM system geometry. Topography should also be modelled to account for all observed features, but this was not possible with the 3D-modelling program used.