Detailed Geophysical Mapping to Support Urban Planning in Nørrestrand, Horsens Denmark

Introduction
Throughout the last century, urbanisation has moved from rural to urban areas (UN, 2019). Continuous urbanisation presents urban planners with great challenges as new land areas have to be developed at high speed, and the functionality of sites is changing. Traditionally, investigations in Denmark prior to an urban devolvement comprise only of geotechnical drillings to avoid foundation on softs sediments, e.g., peat and clay, and to monitor the groundwater level. In Denmark, due to a changing climate pattern where an increase in yearly rain and more frequent cloudbursts are expected to occur (Min et al., 2011; Langen et al., 2019), many development areas need to handle the rain locally using sustainable urban drainage systems (SUDS). Moreover, many households are starting to install near-surface horizontal geothermal systems in order to have a green sustainable energy supply. Both systems operate optimally under specific hydrological and geological conditions. SUDS have the optimal performance in sandy materials with a groundwater level of more than 2 m below ground surface (m bgs). In contrast, near-surface horizontal geothermal systems perform best in saturated settings. Hence, there is a need for urban planners to know the hydrological and geological ground conditions in detail before the devolvement to find the optimal locations for the constructions, SUDS and geothermal systems. This is especially the case in formerly glaciated areas, as many glacial deposits often are highly heterogeneous, and thus, their hydrological conditions can likewise be highly variable (Bockhorn et al., 2017). Therefore, detailed information about the soil is needed, which cannot be achieved with boreholes alone. More and more urban planners use geophysical mapping to achieve high-density information about the sub-surface. For near-surface investigations, electromagnetic induction (EMI) methods have proven to be very effective in successfully mapping the subsurface (e.g. Triantafilis & Santos, 2013; Frederiksen et al., 2017; Brogi et al., 2019; Andersen, 2020).

The objective of this paper is to use high-density geophysical mapping in combination with borehole data and infiltration tests to produce a planning map showing the optimal locations for construction, SUDS and near-surface geothermal systems. The planning map will be conducted from the Nørrestrand area located north of Horsens, Denmark, figure 1. The area is 150 Ha and has been mapped with DualEM-421S as well as detailed lithological soil descriptions from 25 boreholes and 25 infiltration tests.