Updated national 3D model will determine the depth to the solid bedrock beneath our feet

The BGS Superficial Deposit Thickness Model (SDTM) dataset presents the modelled regional variation in thickness of superficial and unconsolidated deposits across Great Britain. These include sediments of fluvial, glacial, marine or anthropogenic origin that were generally formed during the Quaternary (the past 2.6 million years up to today). The thickness of these deposits signifies the depth down to the top of the bedrock, known as geological rockhead.

The distribution of these deposits is not uniform: sometimes they are thin veneers, sometimes they form large, irregular masses, and sometimes they infill entire valleys. The latest SDTM release, the first for over 15 years, includes a raster model of the elevation of rockhead for the first time, providing important data for a variety of different sectors.

The transition between superficial sediments and geological rockhead (and associated bedrock units) is an important zone for geologists, civil engineers, hydrogeologists and environmental scientists because it is where most physical and chemical properties of the deposits significantly change. Properties such as strength, lithology, conductivity, porosity and permeability can be strongly affected by this transition, so an understanding of the depth of the transition zone beneath the ground surface is vital.

The SDTM models have been created using digital mathematical interpolation techniques, combining observations of superficial thickness from borehole logs held in our archives with mapped superficial and artificial geology extents from the 1:50 000-scale digital geological map of Britain.

Version 8 of the SDTM includes the new layer, the rockhead elevation model (RHEM). This is a raster model of the elevation of rockhead relative to Ordnance Datum, the standard vertical reference system used in Great Britain to measure land elevation. This model provides users with an integer value of modelled depth to bedrock in metres and will be of particular use for creating 3D ground models and understanding the geological structure across a site.

The updated SDTM product also contains the observation driven model (ODM), the terrain adjusted model (TAM) and data distance (DataDist).

Observation driven model

The ODM is a raster model of sediment thickness that interpolates thickness values between boreholes within the mapped superficial extents, giving users an integer value of modelled superficial thickness in metres.

Terrain adjusted model

The TAM is a raster model of sediment thickness derived using a method that combines borehole records, the mapped superficial deposit extents and surface elevation data. This differs from the ODM by considering the ground surface elevation, derived from the Ordnance Survey Terrain® 50 model, and gives users an integer value of modelled superficial thickness in metres.

Data distance

DataDist is a raster surface defining the distance of any location relative to the nearest source of thickness information; either the location of a borehole or the mapped boundary of a superficial deposit. This is provided as a pseudo-uncertainty surface relative to the ODM, TAM and RHEM components of the SDTM. The further a location is from a source of thickness data, the more uncertain any values provided by ODM, TAM and RHEM models are likely to be.

New naming conventions

SDTM Version 8 has introduced new naming conventions for the ODM and TAM datasets. These name changes better describe the characteristics of the two datasets and removes confusion or implications relating to the terms ‘basic’ and ‘advanced’ that might imply one model is superior. Both model outputs provide valid options and should be considered in tandem rather than in isolation to understand the models’ constraints.

This dataset release marks an important development for the BGS SDTM and is its first update since 2010. Improved borehole selection criteria have been implemented, allowing the models to benefit from the latest interpretations of BGS-held borehole logs as well as identifying and excluding low-confidence, inappropriate and conflicting boreholes that may have been included in previous versions. As such, the borehole selection underpinning SDTM V8 is considered to be more robust, resulting in a higher confidence model.

Rob Shaw, geospatial analyst and SDTM product developer, BGS.

The intended scale of use of this dataset is 1:100 000, with an average vertical accuracy of +/-5 m elevation. The dataset is intended for use at national, regional and city scales, but not site scales where specific ground-investigation works should be undertaken to determine precise superficial thickness and rockhead elevation.

We are continuing our scientific research on to how to improve the prediction of superficial thickness and will build that into any future update of the SDTM.

For more information on the SDTM datasets, including how to access the data, please visit the STDM product webpage, download the product user guide or get in touch with BGS Digital Data (digitaldata@bgs.ac.uk).