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- ItemLong-time resistivity monitoring of a freshwater/saltwater transition zone using the vertical electrode system SAMOS(Les Ulis : EDP Sciences, 2018) Grinat, Michael; Epping, Dieter; Meyer, Robert; Szymkiewicz, Adam; Sadurski, A.; Jaworska-Szulc, B.In September 2009 two newly developed vertical electrode systems were installed in boreholes in the water catchment areas Waterdelle and Ostland at the North Sea island Borkum to monitor possible changes of the transition zone between the freshwater lens and the underlying saltwater. The vertical electrode systems, which were both installed between 44 m and 65 m below ground level, are used for geoelectrical multi-electrode measurements carried out automatically several times per day; the measurements are still ongoing. The whole system consisting of a vertical electrode system in a borehole and the measuring unit at ground level is called SAMOS (Saltwater Monitoring System). At both locations the data show a clear resistivity decrease that indicates the transition zone between freshwater and saltwater. The depth of the transition zone as well as the kind of resistivity decrease is very stable since 2010. Temporal changes are visible if single depths are considered. In 2015 Miriam Ibenthal used a vertical 2D density-dependent groundwater flow model to explain the long-term resistivity measurements and showed that the temporal changes at CLIWAT 2 (Ostland) could be explained by variations of the groundwater level, changing groundwater recharge rates and changing pumping rates of the nearby located drinking water supply wells.
- ItemMonitoring freshwater–saltwater interfaces with SAMOS – installation effects on data and inversion(Oxford : Wiley, 2020) Ronczka, Mathias; Günther, Thomas; Grinat, Michael; Wiederhold, HelgaA major problem for the freshwater supply of coastal regions is the intrusion of saltwater into aquifers. Due to extensive extraction of freshwater to suffice increasing drinking water demands and/or in periods of reduced groundwater recharge, the equilibrium state may be disturbed. The result is an upconing or movement of the fresh–saline groundwater interface, which reduces the local drinking water resources at coastal regions or islands. The saltwater monitoring system (SAMOS) is a vertical electrode chain installed in a backfilled borehole. It provides a solution to observe the transition zone in detail, both temporally and spatially. We present monitoring data of the first year from three locations - with different geological conditions that show disturbances in the resistivity distribution that result from the drilling processes. A clayey backfilling, for example, can lead to beam-like artefacts, and a mixed fluid within the backfilling changes its bulk resistivity, both leading to misinterpretations. We performed data inversion under cylindrically symmetrical conditions in full-space in order to separate these resistivity artefacts from the undisturbed background. Data inversion reveals that it is possible to separate drilling effects on the resistivity distribution from the undisturbed background. Thus, an interpretation of the natural transition zones can be made immediately after the installation. © 2020 The Authors. Near Surface Geophysics published by John Wiley & Sons Ltd on behalf of European Association of Geoscientists and Engineers.
- ItemDer Untergrund von Borkum: Geologie und Grundwasser : Ergebnisse des INTERREG-Projektes CLIWAT : Leibniz Jahr 2016(Hannover : Technische Informationsbibliothek (TIB), 2016) Burschil, Thomas; Elbracht, Jörg; Griffel, Grit; Grinat, Michael; Günther, Thomas; Ibentahl, Miriam; Igel, Jan; Simon, Bernhard; Sulzbacher, Hans; Weustink, Andree; Wiederhold, Helga; Winter, Sebastian[no abstract available]