OR/18/030 Model assumptions, geological rules used etc.

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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.

Wherever possible the start heights (ground levels recorded on the borehole logs) were obeyed during cross-section correlation. Borehole start heights are discussed in more detail in the Model Limitations section below.

Geology polygons from DiGMapGB-10 were used to constrain the model. However, there are occasions where the boreholes and geological mapping contradict one another. In these cases precedence is given to the boreholes and minor edits were made to the geological linework. For example, river terrace deposits may be mapped in an area but are not modelled if a borehole records only artificial ground or bedrock. Similarly, alluvium is not modelled where a borehole proves terrace gravel or artificial ground. Figure 5 shows the geological map data and cross-section COL_WE_3 near Bank underground station, where changes have been made to the modelled distribution of alluvium using borehole evidence.

Figure 5    Top: DiGMapGB-10 geological map data shows alluvium mapped (pale yellow area in the map extract). Boreholes are shown as black and green dots. Cross-section COL_WE_3 is shown in red. Bottom: Alluvium is not modelled at this locality because boreholes used in cross-section COL_WE_3 show the alluvium has been replaced with thick artificial ground.