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

Multiple-point statistical simulation for hydrogeological models: 3-D training image development and conditioning strategies

“…An option would be to use these resistivity data in the MPS modelling as soft conditioning data, such that high electric resistivities indicate sand, while clay corresponds to low resistivity values (see, for instance, He et al, 2014He et al, , 2016. However, EM data have limited resolution capability towards thin layers (Ley-Cooper et al, 2014;Vignoli et al, 2017), especially in the deeper parts of the investigated sequences. The modelled unit is generally present at great depths (from a range between 30 and 80 m, in the east, to a range between 150 and 170 m, in the west), and the resolution of the EM data is consequently quite low in most of the area.…”

Calibrating electromagnetic induction conductivities with time-domain reflectometry measurements

“…Besides the classical regularization matrices based on the discretization of the first and second derivatives, in all the cases characterized by sharp interfaces, we tested a nonlinear regularization stabilizer promoting the reconstruction of blocky features and thus to improve the spatial resolution of EMI inversion results. (Zhdanov et al, 2006;Ley-Cooper et al, 2015;Vignoli et al, 2015Vignoli et al, , 2017. The advantage of this relatively new regularization is that, when appropriate prior knowledge about the medium to reconstruct is available, it can mitigate the smearing and over-smoothing effects of the more standard inversion strategies.…”

Definitive superparamagnetic source identification through spatial, temporal, and amplitude analysis of airborne electromagnetic data

“…This data was presented in Ley‐Cooper et al . (). Note the short‐wavelength anomalies in the smaller amplitudes of Area S compared with Area B, and the “bumpy” appearance of the deep imaged conductor under Area S. Target C is a small sulphide conductor.…”