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Author: Siemon, Bernhard × DDC: 660 × Has files: Yes × Publisher: Amsterdam [u.a.] : Elsevier Science ×

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    Geophysical investigation of a freshwater lens on the island of Langeoog, Germany – Insights from combined HEM, TEM and MRS data
    (Amsterdam [u.a.] : Elsevier Science, 2017) Costabel, Stephan; Siemon, Bernhard; Houben, Georg; Günther, Thomas
    A multi-method geophysical survey, including helicopter-borne electromagnetics (HEM), transient electromagnetics (TEM), and magnetic resonance sounding (MRS), was conducted to investigate a freshwater lens on the North Sea island of Langeoog, Germany. The HEM survey covers the entire island and gives an overview of the extent of three freshwater lenses that reach depths of up to 45 m. Ground-based TEM and MRS were conducted particularly on the managed western lens to verify the HEM results and to complement the lithological information from existing boreholes. The results of HEM and TEM are in good agreement. Salt- and freshwater-bearing sediments can, as expected, clearly be distinguished due to their individual resistivity ranges. In the resistivity data, a large transition zone between fresh- and saltwater with a thickness of up to 20 m is identified, the existence of which is verified by borehole logging and sampling. Regarding lithological characterisation of the subsurface, the MRS method provides more accurate and reliable results than HEM and TEM. Using a lithological index derived from MRS water content and relaxation time, thin aquitard structures as well as fine and coarse sand aquifers can be distinguished. Complementing the existing borehole data with the lithology information estimated from MRS, we generate a map showing the occurrence of aquitard structures, which significantly improves the hydrogeological model of the island. Moreover, we demonstrate that the estimates of groundwater conductivity in the sand aquifers from geophysical data are in agreement with the fluid conductivity measured in the boreholes.
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    Comparison of novel semi-airborne electromagnetic data with multi-scale geophysical, petrophysical and geological data from Schleiz, Germany
    (Amsterdam [u.a.] : Elsevier Science, 2020) Steuer, Annika; Smirnova, Maria; Becken, Michael; Schiffler, Markus; Günther, Thomas; Rochlitz, Raphael; Yogeshwar, Pritam; Mörbe, Wiebke; Siemon, Bernhard; Costabel, Stephan; Preugschat, Benedikt; Ibs-von Seht, Malte; Zampa, Luigi Sante; Müller, Franz
    In the framework of the Deep Electromagnetic Sounding for Mineral EXploration (DESMEX) project, we carried out multiple geophysical surveys from regional to local scales in a former mining area in the state of Thuringia, Germany. We prove the applicability of newly developed semi-airborne electromagnetic (EM) systems for mineral exploration by cross-validating inversion results with those of established airborne and ground-based investigation techniques. In addition, supporting petrophysical and geological information to our geophysical measurements allowed the synthesis of all datasets over multiple scales. An initial regional-scale reconnaissance survey was performed with BGR's standard helicopter-borne geophysical system deployed with frequency-domain electromagnetic (HEM), magnetic and radiometric sensors. In addition to geological considerations, the HEM results served as base-line information for the selection of an optimal location for the intermediate-scale semi-airborne EM experiments. The semi-airborne surveys utilized long grounded transmitters and two independent airborne receiver instruments: induction coil magnetometers and SQUID sensors. Due to the limited investigation depth of the HEM method, local-scale electrical resistivity tomography (ERT) and long-offset transient electromagnetic (LOTEM) measurements were carried out on a reference profile, enabling the validation of inversion results at greater depths. The comparison of all inversion results provided a consistent overall resistivity distribution. It further confirmed that both semi-airborne receiver instruments achieve the bandwidth and sensitivity required for the investigation of the resistivity structure down to 1 km depth and therewith the detection of deeply seated earth resources. A 3D geological model, lithological and geophysical borehole logs as well as petrophysical investigations were integrated to interpret of the geophysical results. Distinct highly-conductive anomalies with resistivities of less than 10 Om were identified as alum shales over all scales. Apart from that, the petrophysical investigations exhibited that correlating geophysical and geological information using only one single parameter, such as the electrical resistivity, is hardly possible. Therefore, we developed a first approach based on clustering methods and self-organizing maps (SOMs) that allowed us to assign geological units at the surface to a given combination of geophysical and petrophysical parameters, obtained on different scales. © 2020 The Authors