Screening of seabed geological conditions for the offshore wind farm (OWF) area Nordsøen 1, Område E. Desk study for the Danish Energy Agency Desk study for the Danish Energy Agency

Screening of surface-near geological conditions in Nordsøen 1 area E has established a detailed view of the composition and structure of major Quaternary geological units and the general geological development. The area is characterised by a quite dense network of shallow seismic data and sediment cores, sufficiently for an evaluation of the variability of soil conditions and general suitability for OWF location. Area E is relatively shallow (18-32 m) and flat and the upper c. 50 m or more of the subbottom is characterised by Quaternary glacial and interglacial deposits composed of interbedded sandy and clayey units. Below the Quaternary units, Miocene deposits also of sandy to clayey composition are found. The central to eastern part of the area is characterised by major quaternary buried valleys or channels eroding 300-400 m into the Miocene sequence.

Mapping of the Quaternary unit’s base/top and thickness has revealed that Saalian (penultimate glacial) or older glacial deposits are found very close to the seabed in the central to southern part of the screening area. The glacial sediments can be expected to be of heterogeneous composition and over-consolidated, due to prior ice-loading and subaerial exposure. The sediments consist of clayey or sandy tills with variable stone content, deformed meltwater sediment layers (sand, gravel and clay) and locally glaciotectonically disturbed slabs of Miocene sediments (sands and clays). A similar setting with high-lying Saalian deformed to undeformed glacial units was observed by earlier investigations of the Horns Rev 2 OWF area and parts of the Horns Rev 3 and Thor OWF areas.

Meltwater sediments from both the late Saalian glacial period and the Weichselian (last) glacial period occur as channel elements and as major sheets, which have partly levelled the older glacial landscape. The unconsolidated glaciofluvial and -lacustrine meltwater sediments are varying between sandy and soft silty-clayey subunits. Soft Eemian (last interglacial) marine clayey sediments are confined to the eastern part of the area where the unit reaches a thickness of about 5-10 m. This clay unit is also known from the eastern part of Thor and Horns Rev 3 OWF.

The uppermost marine Holocene unit is mostly composed of fine-medium grained sand but soft silty and clayey or even gyttja like sediments, possibly early Holocene, appear in filled in depressions in the central to southeastern part of the area. Peat layers have only been observed in a limited number of sediment cores, and typically only few decimetres in thickness.

Based on the mapping results of the screening of area E and settings of nearby OWF’s, it is concluded that area E in general is suitable for foundation of wind turbines. However, the area exhibits different character with respect to the level of the glacial surface, the occurrence of several hundred meter deep buried valleys and composition and thickness of surficial sediment units composed mostly of sandy or clayey deposits. The occurrence of soft marine clays with up to 10-20 m in total thickness in the eastern part of the area, as well as minor areas in the western part, is a point of attention, as these areas may be less suitable for wind turbine foundation. The same clay unit was identified by Thor and Horns Rev 3 OWF integrated geological model studies and geotechnical parameters from these projects may thus serve as a guide for potential challenges and foundation solutions. Apart from known positions of newer historical shipwrecks, the probability of making archaeological finds in the screening area is expected to be low.