Time-Lapse Airborne EM for monitoring the evolution of a saltwater aquifer

In this study, a multitemporal Airborne EM survey (2015, 2022, 2024) is conducted to monitor the ground-water changes in the shallow aquifer of the Bookpurnong floodplain, South Australia. Over 200 km of AEM lines were acquired during each survey in this naturally high-salinity aquifer region, with the aim of track-ing the freshwater recharge induced by the River Murray's discharge. A comprehensive comparison be-tween the benefits achieved by applying Time-Lapse modelling to Airborne EM data is provided, show-casing also a synthetic experiment before the real case study. In both instances, enforcing temporal con-straints, characterised by the Asymmetric Minimum Support Norm (AGSM, Fiandaca et al., 2015, doi: 10.1093/gji/ggv350), proves crucial in reducing noise-induced model anomalies that could mislead inter-pretations. Specifically, a Spatial Constrained Inversion approach (Viezzoli et al., 2008, doi: 10.1190/1.2895521), which enables quasi-3D solutions by spatially linking neighbouring EM soundings, is integrated into the unified Time-Lapse framework, where all the dataset are inverted simultaneously using a cascade-style approach. In the latter, the multitemporal models are connected to a reference model, obtained from an initial dataset. Moreover, specific processing procedures are implemented to suppress noise induced anomaly propagation. The Time-Lapse results reveal a clear evolution of the floodplain within the geophysical models, unveiling a different hydrogeological behaviour from the shal-low parts of the models, of primary interest for the groundwater management in the area, to the deep portions (~-120m). A full validation of the results is carried out first employing the available log-EM data for the floodplain and then performing an Independent Hydrogeological Validation (IHV), where hydro-geological indices, calculated following Vonk (2024, doi: https://doi.org/10.5281/zenodo.10816741), are compared to the geophysical outcomes.