OR/15/074 Model datasets

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Ambrose, K. 2015. GSI3D model metadata report for HS2 Area 8 (Drayton Bassett to Rugeley). British Geological Survey. (OR/15/074).

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 those geological units that are within a 5 km area of the HS2 route. However some units occur only in subcrop, so additional units in the GVS had to be appended as modelling progressed. The GLEG files were created using the standard BGS colours from DigMap-50. Overall GVS and GLEG files were created for the whole HS2 route, rather than for each individual model area. Thus the units used in this model are only a subset of those available in the overall HS2 GVS file.

Geological linework

The model is covered by 1:50 000 scale geological map sheets 140 (Burton upon Trent) and 154 (Lichfield). A number of problems were noted with the existing mapping of these sheets, which necessitated a partial re-survey of the corresponding 1:10 000 scale standards for the model area. The amended 1:10 000 scale linework is used in the HS2 Area 8 geological model, but is not currently approved for inclusion into corporate datasets. In areas where the mapping has not been revised the model uses published 1:50 000 scale DiGMap geology linework. The revised 1:10 000 scale geological map sheets are shown in Figure 7.

File:OR15074fig7.jpg
Figure 7    Plot of 1:10 000 scale map sheets that were revised (outlined in red) and used in the model. The currently unpublished linework on these maps is used in the model.

Digital terrain model

The terrain model used in this model was the BGS Bald Earth 20 m DTM obtained from the BaldEarth model and trimmed to the project area (5 km buffer of the route shapefile). A NextMap DTM was also included, but not used for modelling.

Borehole data

A review of borehole records in the BGS Single Onshore Borehole Index (SOBI) in the model area was carried out and those that held sufficient geological information were selected for coding in the BGS Borehole Geology database (BoGe). After borehole coding was completed, the boreholes were extracted from the BGS Single Onshore Borehole Index (SOBI) database for use in the 3D modelling software using a set of queries. The borehole log file (.blg) needed to be deduplicated and a borehole filter tool was used to address this. Some boreholes were coded using geophysical gamma ray logs. A plot of borehole data used in the model and the cross-sections constructed from them are shown in Figure 8.

File:OR15074fig8.jpg
Figure 8    Left: plot of available borehole data in the model area, colour coded according to drilled depth. Black boreholes are under 10 m and green boreholes are 10 m and over. Red boreholes are not coded. Right: plot of the 287 boreholes used in the 72 cross-sections constructed to constrain the model.

Some records in the list contained entries that were used to ‘fill-in’ units that were missing, for example TPSF Tarporley Siltstone Formation, Littleworth Beds. A set of priorities were applied to borehole records that were coded by more than one project. The records at the top of this list have a higher priority and the filter tool keeps these records and discards other matching records. This left a total of 884 boreholes coded out of a total borehole count of 2578.

File:OR15074fig9.jpg
Figure 9    Plot of all boreholes in Area 8. Boreholes over 10 m deep are coloured green, under 10 m in black, boreholes in red are not coded. Route cross-section shown in red.

Other datasets

  • Some borehole logs were interpreted from gamma ray logs
  • Sixteen cross-sections from the pre-existing Needwood Basin model were used to constrain the geological units modelled