The role of buried tunnel valleys of the southeastern North Sea for offshore freshened groundwater: New insights from surface-towed time-domain CSEM measurements

The role of large subsurface landforms produced during glaciations of the Pleistocene is still poorly understood with respect to groundwater flow. In particular, so-called tunnel valleys formed beneath ice sheets, acted as drainage systems of glacial meltwater. Their dimensions (up to 5 km width, 400 m depth, 100s of km length) reflect the massive amount of meltwater that incised into and flushed the subsurface beneath ice sheets.

To understand the potential of tunnel valleys as preferential flow pathways of offshore freshened groundwater (OFG) in the southeastern North Sea, we sailed 320 km of marine time-domain controlled-source electromagnetic surveys on 10 profiles using the surface-towed SWAN system on R/V ALKOR. In particular, we aim to answer the following questions: (1) Does the distribution of electrical resistivities indicate the presence of freshened groundwater in the subsurface of the North Sea? and (2) Can we delineate different resistivity distributions inside tunnel valleys?

Here we show our subsurface electrical resistivity distribution from 2D inversions of the TD-CSEM data with and without structural constraints. We compare these results to a dense net of high-resolution 2D seismic reflection data and additional information from core data in similar geological setting, integrating geophysical and geological data.

The subsurface electrical resistivities show good correlation with the structures prevalent in the 2D seismic reflection data, where correlation is strongest for the upper and lower parts of the tunnel valleys. The electrical resistivity distribution also correlates with deeper Paleogene and Neogene sediments showing low electrical resistivities, likely corresponding to brines. These sediments have been updomed into a large anticline due to salt tectonics in the area, which is reflected in the geometry of electrical resistivities. In between the shallow low resistivity Holocene to Pleistocene sediments and the deeper low resistivity Neogene sediments are regions of significantly increased resistivities in Plio-Pleistocene sediments. These regions are interpreted to represent remnant offshore freshened groundwater from the flushing of meltwater below ice sheets during the Pleistocene, likely to be widespread and not limited to the southeastern North Sea.