OR/14/039 Datasets used in the model

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Mathers, S J, Terrington, R L, Waters, C N, and Thorpe S. 2014. The construction of a bedrock geology model for England and Wales. British Geological Survey Internal Report, OR/14/039.

The underpinning evidence base for the model is now described in terms of the data types.

Digital Terrain Model

The Digital Terrain Model (DTM) was initially prepared in 2009–10 from the licensed national NextMap 5 m coverage, and sub-sampled with a variable grid spacing comprising 250 m along buffered section alignments and 2500 m in intervening areas. This was then replaced in 2011 with an overall NextMap coverage sub-sampled to 500 m due to the increase in the number of sections. This dataset also includes the Isle of Man SRTM (Shuttle Radar Topography Mission data) data at 75 m resolution.

Geological Map data

Throughout the various stages of construction the model has been built to be broadly compatible with the geological linework of the BGS 1:625 000 scale bedrock mapsheets, UK North and South (BGS 2007a[1]; 2007b[2]; Figure 5). This data is held by BGS as ESRI shape files.

Figure 5 Printed version of the BGS 1:625 000 scale bedrock geological map data published as two mapsheets.

Some of the detail on these mapsheets of faulting, minor intrusions and lithological variations within units was not included in the cross-sections. The same stratigraphical schema was however followed and some 341 units including intrusions and metamorphic suites were distinguished in the GB3D_v2012 version. The geology portrayed on these mapsheets is described in two accompanying booklets (Jackson, 2008[3]; Stone, 2008[4]). Within the cross-sections the two principle groundwater aquifers of England and Wales, the Chalk Group and the Sherwood Sandstone Group, contain additional stratigraphic detail mainly to formation level, where resolvable, at the request of the EA.

Structural contour plots

The structure contour plots (e.g. Figure 6) utilised to inform the cross-section construction are mainly derived from the BGS Subsurface Memoir and 50K Sheet Memoir series together with the published scientific literature. They in turn comprise syntheses of available deep borehole logs and interpreted seismic data for key stratigraphic surfaces and unconformities. These sources are listed in the section tracker database together with a record of which individual sections utilised each of these sources (see Metadata).

Figure 6 Depth to Caledonian unconformity for the south-west Pennine Basin and adjacent area (from Smith et al. 2005[5]). Contours are in 200 m intervals and are offset by geological faults, shown as black lines where they intersect the unconformity. OS topography © Crown Copyright.

BGS 1:50 000 Mapsheet Cross-Sections

The existing BGS 1:50 000 scale mapsheet series contains cross-sections that are held as 3D shapefiles. These were imported into the model workspace as short intersecting segments to guide construction of the GB3D cross-sections. In the GSI3D software section drawing window they are displayed as colour-coded cross ticks or arrows (Figure 7). Data from about 130 individual 1:50 000 scale cross-sections were utilised; these are listed in the section tracker database against the individual sections that they inform (see Metadata).

Figure 7 Section shown in green in the map view (upper left) and under construction in section view (below) showing cross ticks from 1:50 000 section intersections as guides (below). Notice the higher resolution stratigraphy available from in the 1:50 000 section in the 3D view (upper right). NEXTMap Britain elevation data from Intermap Technologies.

Legacy BGS 3D Model Data

Existing 3D Geological Framework Models were used in GB3D cross-section construction to guide the geologist’s interpretations. Figure 8 shows the distribution of models used. A shapefile with attribute information about each of these models is stored in the BGS Geoscience Data Index (GDI), at present this database is only available internally to BGS staff.

Figure 8 BGS 3D geological framework models in green show regional and basin-scale low-resolution models whereas the red areas contain more detailed higher resolution shallower models. Red areas enclosed within larger green polygons contain models of both types. The 625k resolution model with national coverage is GB3D.

The 3D model data is sliced along the lines of the GB3D cross-sections and is displayed as a series of colour-coded traces in a raster back-drop in the GSI3D section construction window (Figure 9) so that the geologist could use it as a guide. In some areas poor model calculation is evident, particularly near to the ground surface where published map data has greater veracity, and in such cases the geologist chose to override the model information (Figure 9).

Figure 9 Attributed surfaces based on the Northumberland-Solway study (Chadwick et al. 1995[6]) displayed along the line of a GB3D section (above), the constructed section is shown below. Note that faults are shown as offsets in the stratigraphic surfaces. The colour schema for units in the two panels is not unified. NEXTMap Britain elevation data from Intermap Technologies.

Slices through BGS 250K resolution GOCAD models based on the BGS subsurface memoir series of sedimentary basins in Northumberland-Solway, East Midlands, Southwest Pennines, Craven and Weald contributed much useful detail. In addition more detailed 50K resolution GSI3D models, mainly located in the London Basin and southern East Anglia-Essex were also utilised (Figure 9). These 50K GSI3D models are in the main shallow and so they only contribute information on the uppermost bedrock units. Details of the data utilised in the construction of these various models are contained in the separate reports and metadata documentation for the individual models. Where multiple resolution models were available the highest resolution model was usually preferred to guide the interpretation unless this was known to be less reliable. In the section tracker database (see below at Section 6) the models utilised in the construction of the individual GB3D cross-section are systematically recorded.

References

  1. British Geological Survey. 2007a. Bedrock Geology UK South 1:625 000 scale mapsheet 5th Edition.
  2. British Geological Survey. 2007b. Bedrock Geology UK North 1:625 000 scale mapsheet 5th Edition.
  3. Jackson, A A. 2008. Bedrock Geology UK South. British Geological Survey 84pp. ISBN 978 085272 586 3 http://nora.nerc.ac.uk/3708/
  4. Stone, P. 2008. Bedrock Geology UK North. British Geological Survey 88pp. ISBN 978 085272 585 6 http://nora.nerc.ac.uk/3709/
  5. Smith, N J P, Kirby, G A, and Pharaoh, T C. 2005. Structure and evolution of the south-west Pennine Basin and adjacent area. Subsurface Memoir of the British Geological Survey, 129pp.
  6. Chadwick, R A, Holliday, D W, Holloway, S, and Hulbert, A G. 1995. The structure and evolution of the Northumberland-Solway Basin and adjacent areas. Subsurface Memoir of the British Geological Survey. (London: HMSO).