The choice of systems and interpretation approaches for the exploration for critical mineral systems under a complex and varying regolith cover using airborne electromagnetics, can be informed by forward modelling methods. However, the direct assessment of systems and modelling algorithms using data acquired under real survey conditions can be equally informative. For example, it provides an opportunity to assess the effects of real geological variability and noise, arising in a true survey configuration for different systems, and the artefacts that may result from the use of different inversion codes. Here we discuss the application of 1, 2 and 3D inversion approaches to resolving the geometry and complexity of the geology in an area on the South Australian side of the Musgrave province and consider modelled responses from coincident lines of fixed wing (SPECTREM-Plus and TEMPEST - High Moment), and heliborne (VTEM and SkyTEM) time domain EM systems over a known (from ground EM and drilling) deep, steeply dipping, conductor - the Valen Prospect. All inversion methods and AEM systems contributed to our understanding of geological variability and structural complexity, although all generate smoothed versions of geological reality. Results from the 1D inversions appear to map geological variability and complexity in the near surface (regolith character?) in greater detail compared to those from the 2 and 3D inversions, even though the geology is recognisably 3D in character. The Valen Prospect characterised as a distinct, small, and narrow late time anomaly, is modelled in 1D, albeit deeper than drilling and ground EM suggests. While the 2 and 3D models have good global data fits, in some instances they failed to fit measured data at late time, consequently overlooking Valen. It was suggested that problems with fitting the anomaly at late times may be the result of regolith-related superparamagnetism (SPM) in the near surface which often beset AEM data sets in Australian settings. However, decay-rate analysis of the Valen anomaly suggests a deep conductor response for the SkyTEM, SPECTREM and TEMPEST systems. The decay rate of the corresponding VTEM anomaly suggests an SPM response. However, the shape of the VTEM decay also suggests the presence of deeper conductive material.

Open-Access Online Publication: November 1, 2023