How understanding past landscapes might inform present‐day site investigations: a case study from Dogger Bank, southern central North Sea

Abstract

The integration of geophysical and geotechnical datasets acquired during a site survey for the Dogger Bank wind farm has enabled a new litho‐ and seismo‐stratigraphy to be established. Although previously believed to be a relatively simple “layer‐cake”, the data reveal that the sedimentary sequence within the foundation zone includes a complex series of buried landscapes with implications for both foundation siting and design. The most significant is a Weichselian glacially derived landscape dominated by a large thrust‐block moraine complex buried beneath a thin Holocene sequence. This glacial landscape profoundly affects the structure and physical properties of sediments within the foundation zone due to locally intense glaciotectonic deformation and the occurrence of sub‐aerially desiccated horizons recording fluctuating palaeo‐climatic conditions. Understanding these landscapes, coupled with the geophysical and geotechnical data, enables the development of a predictive “geo‐model” that may be used to target areas of uncertainty, reducing the requirement for boreholes (over Cone Penetration Tests) at every potential foundation location.

Left High and Dry: Deglaciation of Dogger Bank, North Sea, Recorded in Proglacial Lake Evolution

“…In addition to the preservation of the subglacial landforms, no evidence of ice sheet readvance, such as overriding or reworking of landforms and subglacial till deposition, is seen in the east of the study area, implying the formation of the thrust-block moraine complex and subsequent retreat was a single advance and retreat event, at least in this location. A single, rapid phase of retreat of the ice stream contrasts with observations from Tranche A of the Forewind area (Cotterill et al, 2017b;Phillips et al, 2018), where intense reworking and multiple moraine ridges are a time-transgressive imprint of the ice margin (Stokes and Clark, 1999), formed under active retreat (Figure 11). A stepped ice-sheet retreat in Tranche A, with reworking of previous deposits is shown by at least four retreat moraine ridges.…”

“…A dense grid of 2D shallow seismic reflection data were acquired as part of the site investigation for the Forewind project. A total of 17,000 line km were available in Tranche B, along with an additional 6,000 line km of regional lines (Cotterill et al, 2017b).…”

“…Maps generated from interpreting seismic reflection data in two-way time were converted to depth using the P-wave velocities derived from local geotechnical data as well as the velocity of similar sediments recorded regionally in the North Sea, as given by Cotterill et al (2017a). The sea water velocity was taken to be 1505 m/s (Cotterill et al, 2017b). The velocity used to depth convert each formation is shown on Table 1.…”

Ice sheet and palaeoclimate controls on drainage network evolution: an example from Dogger Bank, North Sea

“…Maps were generated from interpreting seismic grid lines in two-way time (TWT). P-wave velocities derived from local geotechnical data were used to convert TWT to depth (Cotterill et al, 2017a). Seawater velocity was taken to be 1505 m s −1 , and a sediment velocity of 1600 m s −1 was used (Cotterill et al, 2017a).…”

“…Whilst this enables drainage networks to be identified, there is little information to constrain sedimentary and geomorphic processes and therefore the controls on landscape evolution. The availability of new, highresolution datasets from wind-farm site investigation allows more detailed investigation of shallower submerged landscapes (Cotterill et al, 2012(Cotterill et al, , 2017a. Dogger Bank is covered by a large (1500 km 2 ), 2D seismic reflection data grid and geotechnical logs acquired as a site investigation for the Forewind wind-farm projects.…”