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Crosshole Ground-Penetrating Radar in Clay-Rich Quaternary deposits: Towards Characterization of High-Loss Media
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  • Espen Bing Svendsen,
  • Lars Nielsen,
  • Bertel Nilsson,
  • Kurt H Kjær,
  • Majken Caroline Looms
Espen Bing Svendsen
University of Copenhagen

Corresponding Author:[email protected]

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Lars Nielsen
Department of Geosciences and Natural Resource Management
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Bertel Nilsson
Geological Survey of Denmark and Greenland
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Kurt H Kjær
Globe Institute
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Majken Caroline Looms
Department of Geosciences and Natural Resource Management, University of Copenhagen
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Knowing the centimeter- to meter-scale distribution of sand in clayey deposits is important for determining the dominating water flow pathways. Borehole information has a high vertical resolution, on the millimeter- to centimeter-scale, but provides poor lateral coverage. For highly heterogeneous deposits, such as glacial diamicts, this detailed borehole information may not be sufficient for creating reliable geological models. Crosshole ground-penetrating radar (GPR) can provide information on the decimeter- to meter-scale variation between boreholes, as the GPR response depends on the dielectric permittivity, electric conductivity, and the magnetic permeability of the subsurface. In this study, we investigate whether crosshole GPR can provide information on the material properties of diamicts, such as water content, bulk density, and clay content, as well as their structural relationships. To achieve ground truth, we compare the crosshole GPR data with geological information from both boreholes and excavation at the field site. The GPR data were analyzed comprehensively using several radar wave attributes in both time- and frequency domain, describing the signal velocity, strength, and shape. We found small variations in signal velocity (between 0.06-0.07 m/ns) but large variations in both amplitude and shape (either order of magnitude variation or doubling/tripling of attribute values). We see that the GPR response from wetter and more clayey diamicts have both lower amplitudes and lower centroid frequencies than the response from their drier and sandier counterparts. Furthermore, we find that the variation in amplitude and shape attributes are better correlated to the diamicts’ material properties than the signal velocity is.