4D Airborne EM with Independent Hydrogeological Validation (IHV) – Evolution Study of a Saltwater Aquifer

In this study, a multitemporal Airborne EM survey (2015, 2022, 2024) was conducted to monitor the groundwater 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 tracking freshwater recharge induced by the River Murray's discharge. The river and its flood events are the sole freshwater source sustaining the shallow hydrogeological system, which is crucial for native vegetation survival and for the agricultural activities in the floodplain surrounding (Munday et al., 1997). A Time-Lapse inversion scheme (Fiandaca et al., 2015) is employed to model the data, transitioning from the classic independent inversion approach to minimize model artifacts entering the evolution study. The temporal analysis revealed clear trends in electrical resistivity changes, highlighting a differential behaviour between the shallow and deeper aquifers. A comprehensive validation of the results is performed by first comparing the models with available log-EM data for the floodplain. This is followed by an Independent Hydrogeological Validation (IHV), where hydrogeological indices, calculated following Vonk (2024), are matched with the geophysical results. The comparison demonstrated strong coherence between the AEM models and the hydrogeological indices, thereby validating the spatial information provided by the 3D geophysical models and the quantitative subsurface evolution they reveal.