A decade of airborne electromagnetic surveying Lake Menindee (Australia) under varying water levels
Time domain airborne electromagnetic (AEM) surveying is a mature geophysical tool for imaging the Earth's shallow subsurface. It produces images of the electromagnetic conductivity structure of the earth, down to depths of a few hundred metres. The AEM method is fast, with aircraft acquiring data at speeds of 100-300 km/hr, making it an ideal near-surface reconnaissance tool. The physics of the AEM method are sensitive primarily to the subsurface conductivity, which is influenced by a range of geological factors such as mineral content, porosity, and water content and chemistry. In addition, the inferred subsurface conductivity depends on the accurate measurement and modelling of airborne transmitter and receiver geometries. In this work, we present inferences of the subsurface conductivity over Lake Menindee, New South Wales, Australia, using data from various AEM systems over the period 2014-2024. The lake storage has varied dramatically over this time and while this difference in storage volume undoubtedly influences the near surface conductivity, a remarkably consistent interpretation of the regional geology emerges. While the upper ten metres of the modelled depth sections exhibit the greatest time-variability in inferred electromagnetic conductivity, a correlation of lakebed near-surface conductivity with the lake water volume cannot robustly be established. We also provide information theoretic calculations for each inversion result to aid in their quantitative comparison. The implications of our study are that subtle, shallow, hydrogeological changes are difficult to image with repeat overflights. Conversely, we establish that different AEM systems robustly image the regional geo-electric structure of the near surface, validated by known stratigraphy and borehole conductivity logs.