Selection of a appropriate model for the interpretation of time-domain airborne electromagnetic data for geological mapping

Abstract

A detailed analysis of the time-domain INPUT airborne electromagnetic response to a horizontal layer of variable conductivity and thickness reveals that there are combinations of conductivity and thickness of the layer for which the response behaves like that of a thin sheet (thickness << diffusion depth), those for which it behaves like that of a finite layer (thickness between those of thin sheet and half-space) and those for which it behaves like that of a half-space (thickness >> diffusion depth). Plots of thickness versus conductivity at which the response changes from one category of behaviour to another produces three distinct zones we have referred to as 'thin sheet response zone', 'finite layer response zone' and 'half-space response zone', respectively. The boundaries between these three zones move to higher conductances with increasing sample times. A damped least-squares inversion of the synthetic time-domain airborne electromagnetic response involving all the six channels of the INPUT system and based on singular value decomposition produces a distinct 'boundary' separating pairs of layer conductivity and thickness which can be uniquely resolved from those which cannot. The results further show that conductivity and thickness pairs within the thin-sheet response zone, as expected, cannot be uniquely resolved but those within the finite-layer response zone can be resolved. Using carefully interpreted conductivities and thicknesses of the conductive weathered layer from reconnaissance ground resistivity sounding data from an area flown earlier with an INPUT system, I demonstrate how to apply the general 'response diagram' arising from the above results to select between a thin sheet, finite layer and half-space model for the interpretation of time-domain airborne electromagnetic data for geological mapping.