Fixed-wing vs helicopter AEM systems: A Bayesian resolution analysis

Long range, high-power airborne electromagnetic (AEM) systems fall into two categories: helicopter borne or fixedwing aircraft borne. These platforms have their own advantages and disadvantages in terms of flying height, transmitter loop area and current, aerodynamic stability of transmitter and receiver frames, speed of acquisition, stacking times, cost, etc. With the exception of cost, all of these factors ultimately contribute to the resolvability of subsurface geological features. A comprehensive comparison of the relative merits of all these factors is hard, but flights over known subsurface geology with downhole induction logs are extremely useful for resolution studies. However, given the non-linear nature of the electromagnetic inverse problem, especially when it comes to handling transmitter-receiver geometries in fixed wing aircraft, it is not enough to examine closeness of inverted conductivities to available downhole logs. Instead, the width of the Bayesian posterior credibility intervals (CIs) of conductivity with depth for each kind of system, flown over the same geology with measured survey noise levels, sampled with the same priors, provides a more complete picture of resolution. With the aid of synthetic models and real data examples flown over the Menindee calibration range in New South Wales, we shed new light on the uncertainty with depth for different AEM systems. Specifically, we use a novel Bayesian inversion which handles fixed-wing geometries as nuisance parameters during sampling. Our findings have useful implications in AEM system selection, as well as in the design of better deterministic inversion codes.Open-Access Online Publication: May 29, 202