OR/18/030 Model datasets

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Burke, H, Martin, C, and Terrington, R. 2018. Metadata report for the City of London 3D geological model. British Geological Survey Internal Report, OR/18/030.

GVS and GLEG files

The generalised vertical section (.gvs) and geological legend (.gleg) files were assembled using Notepad or Excel and iterated as the model expanded and new units were encountered. The GVS was based on DiGMapGB-50 data by identifying all geological units present within the model area to a depth of -100 m below Ordnance Datum. The City of London model GVS and Gleg are based on the existing London Basin geological model.


Colours from DiGMap-50 geological linework

The City of London model uses BGS DiGMap 1:10 000 scale superficial and bedrock geological mapping to inform the modelled units. In the absence of mapped artificial ground within the model area, boreholes were used to determine the distribution of artificial ground in the model.

Digital Terrain Model

The model is capped by an extract from the Ordnance Survey Terrain 50 Digital Terrain Model (DTM). This represents the ground surface in the model, with the full resolution 50 m cell size used. This gives a ground elevation range for the model area of +25.0 m at Holborn in the north-west of the model area to -2.3 m along the River Thames. The DTM is shown in Figure 3 with a blue to red colour ramp (areas of low elevation are coloured blue and high areas are blue).

Figure 3    Ordnance Survey Terrain 50 DTM used in the model shown with a blue to red colour ramp. Areas of low elevation are coloured blue and high areas are shown in red. Contains Ordnance Survey data © Crown copyright and database rights. All rights reserved [2018] Ordnance Survey [100021290 EUL].

Borehole data

A total of 303 non confidential boreholes in the BGS Single Onshore Borehole Index were used to constrain the City of London model (Figure 4). These were coded into text files for use in the 3D modelling software, with the .bid file holds the location information of each borehole (grid reference and start height) and the .blg file stores the downhole information. These downhole interpretations will be uploaded to the BGS Borehole Geology database in due course. Start heights were taken from the OS DTM-50 Digital Terrain Model (DTM) for boreholes where start heights are absent.

Cross-sections

The City of London model area is located entirely within the pre-existing London Basin model area (Burke et al., 2014[1]). Five cross-sections from the London Basin model were used in the City of London model, with an additional 22 cross-sections constructed in the City of London model area to form a framework (Figure 4). The City of London cross-sections are matched to the London Basin cross-sections, with minor amendments made to the London Basin sections using borehole evidence.

Figure 4    Cross-sections and boreholes (green dots) used to constrain the City of London model. City of London cross-sections are shown in blue, London Basin model sections in red.

References

  1. Burke, H, Mathers, S J, Williamson, J P, Thorpe, S, Ford, J, and Terrington, R L. 2014. The London Basin superficial and bedrock Lithoframe 50 model. British Geological Survey Open Report No. OR/14/029. http://nora.nerc.ac.uk/id/eprint/507607/