Geophysical imaging of saltwater intrusion
Keywords: Geophysics.

Saltwater intrusion, the process by which saltwater migrates into freshwater aquifers, is an issue that impacts coastal aquifers around the world. Characterization of the locations and extent of intrusion is critical for the sustainable management of coastal groundwater basins. This is commonly accomplished using measurements made in wells, which provide detailed point location data, but may fail to capture important spatial complexity in the presence of heterogeneous geological or hydrological conditions. Increasingly, geophysical imaging methods, including an airborne electromagnetic (AEM) method and electrical resistivity tomography (ERT), have been used to supplement well based monitoring. These methods have been shown to be highly effective for large scale mapping of saltwater intrusion, filling in the spatial gaps where there are no or few monitoring wells. This thesis investigates the application of both the ERT and AEM methods to the problem of saltwater intrusion at the field scale, and further investigated the utility of ERT with laboratory scale experiments. At the field-scale, the AEM and ERT methods are used to understand intrusion along the coast of the Monterey Bay, in central California. In this thesis we present long-offset ERT along the coast of the Monterey Bay, with the goal of resolving large scale saltwater intrusion features and the hydrogeologic controls on them. We develop a region-specific resistivity-to-lithology transform, which is used, in conjunction with other available datasets, to map out and assess the controls on the location of intrusion interfaces. We then present a joint interpretation of offshore AEM data and the onshore ERT data for resolving the location of intrusion interfaces in the northern portion of the Monterey Bay, where in many points, intrusion has yet to be detected onshore. We then conduct a series laboratory and synthetic experiments, with the goal of quantifying the resolution of a saltwater intrusion wedge that could be achieved using ERT data, and determining what approaches should be taken when inverting these data. In the controlled conditions of the laboratory we are able to acquire ERT data on an intrusion wedge whose geometry was known using photometric mapping. We tested a series of inversion approaches, concluding that both an informed inversion using a Tikhonov-style regularization with a flow-model-derived reference, and a parametric inversion approach, maximized our ability to resolve the geometry of an intrusion wedge. The wedge location could be estimated, on average, to within less than one electrode spacing length, and the transition zone width to within an average of ¼ an electrode spacing length. We then used synthetic models to test a variety of scenarios more complicated than the idealized system used in the laboratory. We found that these cases, an informed inversion still out-performed uninformed inversions, but the parametric inversion approach was not as universally successful as it had been in the laboratory. We presented the application of a hybrid parametric informed inversion approach, concluding that this hybrid approach could work as well as a flow-model informed inversion, but without the need to generate a flow model and salinity-to-resistivity transform.