Airborne electromagnetic modelling options and their consequences in target definition

Abstract

Given the range of geological conditions under which airborne EM surveys are conducted, there is an expectation that the 2D and 3D methods used to extract models that are geologically meaningful would be favoured over 1D inversion and transforms. We do after all deal with an Earth that constantly undergoes, faulting, intrusions, and erosive processes that yield a subsurface morphology, which is, for most parts, dissimilar to a horizontal layered earth.

We analyse data from a survey collected in the Musgrave province, South Australia. It is of particular interest since it has been used for mineral prospecting and for a regional hydro-geological assessment. The survey comprises abrupt lateral variations, more-subtle lateral continuous sedimentary sequences and filled palaeovalleys. As consequence, we deal with several geophysical targets of contrasting conductivities, varying geometries and at different depths. We invert the observations by using several algorithms characterised by the different dimensionality of the forward operator.

Inversion of airborne EM data is known to be an ill-posed problem. We can generate a variety of models that numerically adequately fit the measured data, which makes the solution non-unique. The application of different deterministic inversion codes or transforms to the same dataset can give dissimilar results, as shown in this paper. This ambiguity suggests the choice of processes and algorithms used to interpret AEM data cannot be resolved as a matter of personal choice and preference.

The degree to which models generated by a 1D algorithm replicate/or not measured data, can be an indicator of the data’s dimensionality, which perse does not imply that data that can be fitted with a 1D model cannot be multidimensional. On the other hand, it is crucial that codes that can generate 2D and 3D models do reproduce the measured data in order for them to be considered as a plausible solution. In the absence of ancillary information, it could be argued that the simplest model with the simplest physics might be preferred.

Given the range of geological conditions under which airborne EM surveys are conducted, there is an expectation that 2D and 3D methods used to extract models of geological significance would be favoured over 1D inversion and transforms. We analyse data from the Musgrave province, South Australia, used for mineral and for hydro-geological prospecting.

Key words::

• airborne
• electromagnetics
• exploration
• inversion
• target

Acknowledgements

The authors thank Tania Ibrahimi from CSIRO for making maps and taking care of the geographic information systems; Juerg Hauser, Daniel Card and an anonymous reviewer; Musgrave Minerals Ltd for their continued cooperation and making the VTEM data available; and Geoscience Australia for permitting CSIRO to use their layered earth inversion code. The authors would also like to thank Techno Imaging, Aarhus Geophysics, RMIT, University of Potsdam and the Geological Survey of Denmark & Greenland for support of this research. This study was undertaken under the auspices of the Goyder FLOWS Project through the SA Goyder Institute (for more info: www.goyderinstitute.org), and was co-funded by the Water for a Healthy Country and Mineral Resources Flagship.