Geological interpretation of near-surface induced polarization and superparametric effects in airborne electromagnetic data

Abstract

All available inversion software for airborne electromagnetic (AEM) data can fit a non-dispersive conductivity model to the observed responses. Recent research has attempted to permit this conductivity model to be frequency dependent, commonly using a Cole-Cole parameterisation of the induced polarization (IP) effect; but this parameterisation both slows down and destabilises the inversion. Fundamental inductive responses are controlled by the product of conductivity, magnetic permeability and frequency. Little has been published on inverting data affected by frequency dependent magnetic permeability, or super-paramagnetism (SPM), best characterised by a Chikazumi model, which effects are also seen in AEM. Because both IP and SPM effects are small, and usually only obvious at late delay times, the aim of this research was to determine if these IP and SPM effects can be stripped from the data after being approximated with simple dispersive models. We were able to use a thin sheet model to automatically do this. The stripped data can be inverted using a non-dispersive conductivity model. The IP and SPM parameters fitted independently at each fiducial proved to be spatially coherent, and geologically sensible, and as such are very useful in interpretation in their own right. Chargeable materials are attributed to clays, located in sinuous bands, with some correlation with the higher conductivity palaeochannels where saline waters affect conductivity. High SPM susceptibilities appear on the “uphill” sides of drainage and clay-filled channels, and are attributed to bush-fire generated maghemite nanoparticles.