SEMINAIRE M. COMTE Jean-Christophe: University of Aberdeen

Direct current resistivity imaging, also known as electrical resistivity tomography (ERT), has become a standard field tool for assessment and monitoring of saltwater intrusion in coastal aquifers. The technique can provide high-resolution spatial information on saltwater distribution in aquifers, including the geometry of the saltwater-freshwater interface. When repeated over time at the same location, it also provides insights into its temporal evolution. This information, when combined with traditional hydrogeological data and integrated within variable-density groundwater models, provides further insights into aquifer heterogeneity and boundary conditions, and importantly, significantly improves model conceptual basis and predictive reliability when applied to sustainable groundwater management. Recent methodological developments involve the use of fully-coupled geophysical-groundwater modelling frameworks which allow for direct, quantitative use of the geophysical information (measured resistivities) as part of automated calibration procedures which can potentially provide even more accurate representation of groundwater flow and salt transport processes, including history matching, and more robust future predictions. These coupled modelling tools, however, also highlight some important limitations associated with their practical use at the present day: they require large computational resources that are usually not available for most practical real-world applications; and they can introduce large bias by producing invalid groundwater models, mostly because of inherent challenges in converting resistivity data into hydrogeological information and error propagation, particularly in heterogeneous aquifer systems. Nevertheless, they do offer highly informative tools for assessing and predicting the value of the geoelectrical information and in turn, for improving the geophysical acquisition procedure in order to maximise that value. Here we will show real-world examples, based on 20 years of research across various coastal aquifer settings, of the key hydrogeological information that can typically be derived from ERT, including aquifer properties, heterogeneity and forcing conditions (recharge, abstraction, evapotranspiration), and how advanced, fully-coupled, multiphysical modelling tools can be further used to assess and improve the acquisition and interpretation of ERT datasets in coastal aquifers, and guide their appropriate utilisation for groundwater modelling and management.