Abstract
Abstract. For reliably predicting the impact of climate changes on salt/freshwater systems below barrier islands, a long-term hydraulic modelling is inevitable. As input we need the parameters porosity, salinity and hydraulic conductivity at the catchment scale, preferably non-invasively acquired with geophysical methods. We present a methodology to retrieve the searched parameters and a lithological interpretation by the joint analysis of magnetic resonance soundings (MRS) and vertical electric soundings (VES). Both data sets are jointly inverted for resistivity, water content and decay time using a joint inversion scheme. Coupling is accomplished by common layer thicknesses. We show the results of three soundings measured on the eastern part of the North Sea island of Borkum. Pumping test data is used to calibrate the petrophysical relationship for the local conditions in order to estimate permeability from nuclear magnetic resonance (NMR) data. Salinity is retrieved from water content and resistivity using a modified Archie equation calibrated by local samples. As a result we are able to predict porosity, salinity and hydraulic conductivities of the aquifers, including their uncertainties. The joint inversion significantly improves the reliability of the results. Verification is given by comparison with a borehole. A sounding in the flooding area demonstrates that only the combined inversion provides a correct subsurface model. Thanks to the joint application, we are able to distinguish fluid conductivity from lithology and provide reliable hydraulic parameters as shown by uncertainty analysis. These findings can finally be used to build groundwater flow models for simulating climate changes. This includes the improved geometry and lithological attribution, and also the parameters and their uncertainties.
Highlights
Climate changes are threatening the groundwater dynamics, at the coast and on barrier islands, where the interaction between freshwater and saltwater is important for water supply
Airborne electromagnetic measurements are important since they provide three-dimensional distribution of electrical conductivity, or its inverse, resistivity (Siemon et al, 2009), which is a key parameter in hydrogeophysical investigations (e.g. Viezzoli et al, 2010)
We have presented a new methodology for investigating layered groundwater systems
Summary
Climate changes are threatening the groundwater dynamics, at the coast and on barrier islands, where the interaction between freshwater and saltwater is important for water supply This is the subject of the international and interdisciplinary project CLIWAT (CLImate and WATer) investigating the impact of sea-level rise on freshwater resources at the coasts of the North Sea and Baltic Sea. In one of the projects, modelling of the long-term hydraulic behaviour of the freshwater lens beneath the island of Borkum is accomplished (Sulzbacher et al, 2012). For density driven flow models several input parameters are required such as porosity, hydraulic conductivity and salinity. Very often these quantities are not available at the catchment scale. There are two main shortcomings of resistivity: (i) we cannot clearly differentiate between clay content and fluid salinity, and (ii) there is no sufficiently reliable relation to hydraulic conductivity, probably the most important parameter needed
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