A Decade of Airborne Electromagnetic Surveying Lake Menindee (Australia) Under Varying Water Levels

ABSTRACT 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 rotary‐wing or fixed‐wing aircraft acquiring data at speeds of 100–300 km/h, making it an ideal near‐surface reconnaissance tool. The physics of the AEM method is 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 – a challenging task given the speed of acquisition and variability of wind conditions during an acquisition flight. In this work, we present inferences of the subsurface conductivity over Lake Menindee, New South Wales, Australia, using data from test flights and various AEM systems over a 10‐year period (2014–2024). The lake storage has varied dramatically over this time, and the test flights have coincided with both high and low water levels. While this difference in storage volume undoubtedly influences the near‐surface conductivity, a remarkably consistent interpretation of the regional geology is possible regardless of the hydrologic conditions. While the upper 10 m of the modelled depth sections exhibit the greatest time‐variability in inferred electromagnetic conductivity, the hypothesis that lakebed near‐surface conductivity is significantly correlated with the lake water volume cannot robustly be established. We also provide some 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 AEM overflights from different systems. Conversely, we establish that different AEM systems with minimal extra processing robustly image the regional geo‐electric structure of the near surface, validated by known stratigraphy and associated geological information, as well as borehole conductivity logs.