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

This work presents the Time-Lapse modelling of an Airborne EM (AEM) monitoring datasets acquired over the Bookpurnong area (Southern Australia) to study the evolution and interactions between the Murray River freshwater discharge and the highly saline floodplain aquifers. First, the effectiveness of AEM Time-Lapse modelling is underscored via synthetic experiment, which enhanced data-driven solutions and minimized artifacts compared to the independent approach. Then the same modelling framework is applied on Bookpurnong floodplain data where, SkyTEM datasets were acquired in 2015, 2022, and 2024, each consisting of 200 km of overlapping lines spaced 100 meters apart. Regarding the AEM data processing, a new simultaneous approach was deemed necessary to address and eliminate all noise couplings in the temporal datasets. A comparison between Time-Lapse and Independent inversion models from the field data is then presented, confirming the synthetic test results, with the Time-Lapse modelling producing more compact, conservative, and likely more data-driven results. The interpretation of the Time-Lapse models initially focused on the shallow variations, main interest for this study, which are first validated through the comparison with log-EM measurements that revealed strong agreement with AEM models. Then, a novel Independent Hydrogeological Validation further assessed the Time-Lapse results. This analysis allowed an unbiased evaluation of the hydrological response of the floodplain over time and revealed a solid correlation between this index and the shallow evolution depicted by the AEM models. The remarkable agreement between geophysical and hydrogeological analyses underscored the suitability of AEM surveys to monitor groundwater processes when modelled within a Time-Lapse framework.