An information theoretic Bayesian uncertainty analysis of AEM systems over Menindee Lake, Australia

Long-range, active-source airborne electromagnetic (AEM) systems for near-surface imaging fall into two categories: helicopter borne or fixed-wing aircraft borne. A multitude of factors such as flying height, transmitter loop area and current, source waveforms, aerodynamic stability and data stacking times contribute to the geological resolvability of the subsurface. A comprehensive comparison of the relative merits of each system considering all such factors is difficult, but test flights over known subsurface geology with downhole induction logs are extremely useful for resolution studies. Further, given the non-linear nature of the electromagnetic inverse problem, handling transmitter-receiver geometries in fixed-wing aircraft is especially challenging. As a consequence of this nonlinearity, inspecting the closeness of downhole conductivities to deterministic inversion results is not sufficient for studying resolvability. A more comprehensive picture is provided by examining the width of the depth-wise Bayesian posterior conductivity distributions for each kind of system. For this purpose, probabilistic inversions of data must be carried out -- with acquisition over the same geology, survey noise levels must be measured, and the same prior probabilities on conductivity must be used. With both synthetic models as well as real data from over the Menindee calibration range in New South Wales, Australia, we shed new light on the matter of AEM inverse model resolution. Specifically, we use a novel Bayesian inversion scheme which handles fixed-wing geometry attributes as generic nuisance parameters during Markov chain sampling. Our findings have useful implications in AEM system selection, as well as in the design of better deterministic AEM inversion algorithms. Citation: Anandaroop Ray, Yusen Ley-Cooper, Ross C Brodie, Richard Taylor, Neil Symington, Negin F Moghaddam, An information theoretic Bayesian uncertainty analysis of AEM systems over Menindee Lake, Australia, Geophysical Journal International, Volume 235, Issue 2, November 2023, Pages 1888–1911, https://doi.org/10.1093/gji/ggad337