Characterising shallow subsurface variability in the UK North Sea: a regional analysis using geotechnical records

Subsea power and telecommunications cables are critical offshore infrastructure, commonly protected through burial within the upper few metres of seabed. Determining appropriate burial depths, and managing anchor-related damage risk, therefore depends on an accurate understanding of shallow subsurface conditions. Current Cable Burial Risk Assessment (CBRA) workflows typically simplify the seabed into vertically homogeneous soil units (e.g. sand or clay), despite evidence that shallow stratigraphy is frequently layered and geotechnically variable.
This report presents an expanded regional assessment of shallow subsurface conditions across the UK North Sea. Building on earlier work (Johnson et al., 2024), geological and geotechnical records from the British Geological Survey (BGS) and the Crown Estate Marine Data Exchange (MDE) have been restructured and standardised within a consistent layer-based framework, increasing the dataset to over 12,000 records. The resulting database enables direct comparison of lithological and geotechnical data information across previously heterogeneous data sources, provides improved spatial coverage, particularly in nearshore and actively developed regions, and supports the mapping of layered soils, coarse deposits, organic-rich sediments and shallow bedrock within the upper 2 metres below seabed (mbsb) presented in this report.
The results demonstrate that layered soils are a dominant characteristic of the shallow subsurface across the UK North Sea. Approximately three-quarters of sites penetrating at least 2 mbsb contain two or more lithological units within this interval, indicating that vertically variable ground conditions are more common than homogeneous profiles. Most sites contain one to three distinct layers, with sand-over-clay representing the most frequently encountered configuration. Clear regional contrasts are evident: the southern North Sea is generally characterised by extensive sand-dominated sediments, whereas the northern North Sea shows a more balanced distribution of sand- and clay-dominated profiles, often with marked internal variability. Coarse deposits occur widely but are most commonly present as thin gravel veneers, whilst thicker gravel layers and shallow bedrock are more localised, particularly in nearshore areas, where they may represent potential burial-limiting conditions. Peat and organic-rich soils are comparatively rare but occur predominantly in the southern North Sea, commonly beneath thin sand veneers where they are not readily identifiable from seabed sediment mapping alone.
These features can influence trenching performance, achievable Depth of Lowering (DoL), and anchor penetration behaviour, highlighting the limitations of representing the seabed as a single homogeneous soil profile within CBRA workflows which can lead to over- or under-conservative burial estimates. The structured dataset used in this report provides a consistent framework for incorporating more complex ground conditions into regional-scale assessments and supports both the physical and numerical work packages of the EPSRC project. In addition, the resulting analyses provide an evidence base for early-stage route planning and risk screening, in line with the Crown Estate’s Cable Route Identification and Leasing Guidelines, while providing a foundation for future work aimed at improving the representation of shallow subsurface variability within offshore engineering assessments.