Geological desk study Bornholm Windfarm cable transects. Geological seabed screening in relation to possible location of cable transects. Client Energinet

Energinet A/S has requested that GEUS undertakes a geological desk study of the Bornholmcable transect. The study has resulted in a general geological description and establishment of a geological model. The study is based on existing data and is to be used as a background for future interpretations of new seismic data, geotechnical investigations and an archaeological screening.

In this study we used a combination of published work, archive seismic and sediment core as well as CPT data, to assess the general geological development of the south-western Baltic Sea region, including the planned Bornholm OWFs and cable transect from Bornholm to Køge Bugt. Detailed information has been acquired from The Baltic Pipe offshore pipeline transect, which for long stretches runs parallel with the planned cable transects. This means that crucial information comes from seismic profiles and vibrocores, reporting The Baltic Pipe offshore pipeline transect studies.

A geological description has been provided and a geological model presented.

A surface sediment map has been compiled using a combination of Emodnet seabed substrate data from the German and Swedish zones and the latest version (2020) of the Danish 1:100,000 seabed substrate map.

Detailed data are presented in the transect sections A–D among others described on the basis of profile sections A–M, modified from Baltic Pipe investigations (Rambøll 2020). Boomer and sediment echosounder data as well as vibrocore data form the background for the interpretations.

In the south-western part of the Baltic Sea, studies of Late Glacial and early Holocene shorelevel changes form the basis for evaluation of the potential to find submerged settlements in the windfarm areas. We consider the early and mid-Mesolithic the most likely.

The detailed studies of the transect A–D provides seabed sediment information that can be used in relation to evaluation of geotechnical challenges to cable deployment as for example in transect A near Bornholm, where Jurassic and Cretaceous bedrock at the seabed as well as glacial boulders may pose problems. However, long stretches may on the other hand be easy to handle due to the existence of sufficient unconsolidated layers.