Abstract
Buried tunnel valleys are common features in formerly glaciated areas, and where present, they are very important for the groundwater recharge and flow. Delineation of the structures and modelling of the infill is therefore very important in relation to groundwater mapping. Typically, borehole information is too sparse to enable a detailed delineation of the structures, whereas densely covering airborne electromagnetic data have proven to be very useful for this. In the last decades, the mapping approach has been studied carefully, but the 3D modelling of the valley structures has not been described to the same degree yet. In this study, we create a 3D geological model of an area that is characterised by a complex network of buried valleys mapped with a spatially dense airborne electromagnetic survey. Due to the comprehensive dataset, the modelling requires formulation of an advanced strategy. This contains a number of steps, where the AEM-derived resistivity data are initially interpreted based on the geological background knowledge to identify the buried valleys and build a conceptual geological model. Secondly, the age relationships between the valleys are established from the valley orientations and their internal cross-cut relationships. Thirdly, the deep erosional surfaces are modelled. Subsequently, the interpreted age relationships are utilised to trim the valley floor surfaces, such that younger valleys cut older. Finally, a voxel model is built and populated with lithofacies and stratigraphical units. The model is constructed as a combined layer-based and voxel model in order to map both the overall structures as well as the lithological variations within the 3D model domain. The final model contains 20 buried valleys that show a complex cross-cut setting that indicate the presence of at least eight valley generations. Most of the valley infills show lithological variations, and the final voxel model thus contains 42 different geological units.
Highlights
Buried and open tunnel valleys are common features in former glaciated regions (Kehew et al, 2012; van der Vegt et al, 2012)
Mapping and modelling complex geological environments like buried-valley networks require dense data coverage, which can be obtained through airborne electromagnetic data (Jørgensen and Sandersen, 2009a; Sapia et al, 2014)
We have formulated and followed a strategy for 3D geological modelling of a geological environment characterised by a network of buried valleys
Summary
Buried and open tunnel valleys are common features in former glaciated regions (Kehew et al, 2012; van der Vegt et al, 2012). In places where the buried valleys are eroded into an impermeable substratum, the coarser infill of the valleys typically constitute important aquifers that tend to be deep seated and well protected. Buried valleys can be filled with fine-grained deposits with low hydraulic conductivity, in which case they can act as barriers for the groundwater flow. The buried valleys have typically been influenced by repeated erosional incisions and depositions, and the resulting valley infills are often highly heterogeneous in nature. This heterogeneity affects the groundwater flow paths and may result in potentially scattered recharge areas with greater risk of downward transport of contaminants
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