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The 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 geological period (the past 2.6 million years). The thickness of these deposits represents the depth down to the top of bedrock, known as geological rockhead. The distribution of these deposits is not uniform: sometimes they are developed in thin veneers, sometimes they form large irregular masses and sometimes they infill entire valleys.

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. Strength, lithology, conductivity, porosity, and permeability are most strongly affected by this transition, so an understanding of the depth of this transition zone beneath the ground surface is vital.

The SDTM models are created using digital mathematical interpolation techniques, combining information 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: ���ճԹ� Geology 50K.

The SDTM product consists of the following components:

• The Observation Driven Model (ODM): a raster model of sediment thickness which interpolates thickness values between boreholes within the mapped superficial extents.

• The Terrain Adjusted Model (TAM): a raster model of sediment thickness derived using a method which combines borehole records, the mapped superficial deposit extents as well as surface elevation data. This differs from the ODM by considering the ground surface elevation, derived from the

•  The Rockhead Elevation Model (RHEM): a raster model of the elevation of rockhead (the transition between superficial sediments and bedrock material) relative to Ordnance Datum. This differs from ODM and TAM which both provide information on the depth of superficial deposits as a thickness, rather than an elevation.

• The Data Distance (DataDist): 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.

A more detailed overview can be found in the accompanying .

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

���ճԹ� is continuing to do scientific research in to how to improve the prediction of superficial thickness and will build that into any future update of the SDTM model.

SDTM Open

SDTM Open is a generalised version of the Premium ODM dataset and will be freely available under the ���ճԹ� Non-commercial licence . This dataset is for users who are seeking summary information about the average and maximum depths of superficial deposits across Great Britain, i.e. a summary assessment of depth to geological rockhead.

SDTM Open summarises superficial thickness information via spatial statistics on the basis of a regular hexagon grid distributed across Great Britain. The hexagon grid utilises the which provides a hierarchical geospatial indexing system that partitions the globe into hexagonal cells. Summary statistics provided include:

• the maximum and mean superficial thickness values per cell
• the percentage of the cell for which superficial thickness has been modelled
• the number of boreholes per cell that modelled superficial thickness is derived from
• the H3 identifier per hexagonal cell which can be used to join the data to other H3 hexagonal grid datasets
  

A more detailed overview will be provided in the accompanying user guide.