Highlights

Abstract

Keywords

۱٫ Introduction

۲٫ Selected study areas

۳٫ Methods

۴٫ Data

۵٫ Geological mapping in the selected study areas

۶٫ Discussion

۷٫ Conclusions

Declaration of Competing Interest

Acknowledgements

References

Abstract

The subsurface in areas affected by the Pleistocene glaciations often reveal very complex architectures and because of this, the near-surface geology is generally difficult to map and model in high detail. A number of geophysical methods focus on the uppermost part of the subsurface and are capable of mapping details, but no single method has hitherto been able to provide the detail, the data density and the resolution required to map the near-surface Quaternary geology in 3D. Driven by the demands for high detail in the uppermost parts of the subsurface related to for instance surface water and groundwater vulnerability assessments and climate-change related projects, a new high-resolution electromagnetic survey method, tTEM, has been developed. We present examples and discuss the method and its applicability in four study areas where data from tTEM surveys has been combined with geological data and knowledge to map near-surface geological features that could not be resolved in 3D using other geophysical methods focusing on the deeper subsurface or methods with a wider data spacing.

۱٫ Introduction

The subsurface deposits of many areas that were once covered or bordered by Pleistocene ice sheets reveal very complex architectures. During at least the three latest glaciations and the intervening interglacials, multiple episodes of erosion, deposition and deformation have left a marked impact on the upper parts of the subsurface: Glaciotectonic complexes with structural deformation to depths of up to 300–۴۰۰ m are numerous (e.g. Aber and Ber, 2007; Gehrmann et al., 2019; Pedersen, 2005), and widespread subglacial erosion is evidenced by for instance the occurrence of a large number of buried tunnel valleys eroded to depths of up to 400 m (e.g. Van der Vegt et al., 2012; Jørgensen and Sandersen, 2006). In addition to this, other types of erosion, sedimentation and tectonics have resulted in extensive and complex near-surface sedimentary successions and highly varied terrain morphologies (e.g. Brandes et al., 2018; Houmark-Nielsen, 2007, Houmark-Nielsen, 2011; Lang et al., 2014; Sandersen and Jørgensen, 2015; Winsemann et al., 2018). Due to this complexity, the upper part of the subsurface geology is generally difficult to map and model in high detail.

Extensive groundwater mapping in Denmark since the mid 90’s (Thomsen et al., 2004) created a boost in the development of new geophysical methods that aimed at mapping the uppermost 100–۲۰۰ m of the subsurface. Developed geophysical methods counted for instance Airborne ElectroMagnetic methods (AEM; e.g. SkyTEM; Danielsen et al., 2003; Sørensen and Auken, 2004), Electrical Resistivity Tomography (ERT; Loke et al., 2013), Induced Polarization (IP; Maurya et al., 2018), Pulled Array Continuous Electrical Sounding (PACES; Christensen and Sørensen, 2001), and ElectroMagnetic Induction (EMI; Christiansen et al., 2016; Doolittle and Brevik, 2014). The development of new methods also spurred new modelling workflows and new ways of combining data (e.g. Sørensen, 1996; Sørensen and Auken, 2004; Jørgensen et al., 2003, Jørgensen et al., 2013; Møller et al., 2009; Høyer et al., 2015a, Høyer et al., 2015b; Marker et al., 2015).