OR/18/030 Model limitations

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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.

Model specific limitations

Only non-confidential borehole records were used to constrain the model, which reduces the number of available boreholes in the model area.

As stated earlier, borehole start heights have been obeyed wherever possible. With this method of modelling dependent on borehole information to inform the bases of the geological units, obeying or overriding the borehole start heights can have a huge impact on the finished model. This particularly affects the bedrock units, such as the base of the Thanet Sand Formation, because fewer boreholes reach that far down, limiting the number of available boreholes.

Anomalies occur where a borehole site well above or well below the Digital Terrain Model (DTM). Inconsistencies between borehole start heights and the Digital Terrain Model (DTM) can be an issue in highly urbanised areas like Central London, where sites have been developed multiple times and several generations of site investigation boreholes are used. These DTM-start height discrepancies can reflect local anthropogenic changes in the ground elevation, or can be due to issues in the Single Onshore Borehole Index database, such as errors in feet to metres conversions. Each anomalous borehole was examined in the context of adjacent boreholes and nearby boreholes used in crossing sections when deciding whether to accept or overrule its start height. Figure 6 shows cross-section COL_NS_8, where many of the borehole start heights do not fit the Digital Terrain Model.

Figure 6    Cross-section COL_NS_8, showing discrepancies between borehole start heights and the Digital Terrain Model. Boreholes are shown as black vertical lines. Key to geology as per Table 1.

The junction between the base of the Lambeth Group and the underlying Thanet Sand Formation can be difficult to determine in boreholes, particularly where the level of detail recorded in a borehole is minimal. Where green sand above a grey sand unit is recorded in boreholes.

General modelling limitations

  • Geological interpretations are made according to the prevailing understanding of the geology at the time. The quality of such interpretations may be affected by the availability of new data, by subsequent advances in geological knowledge, improved methods of interpretation, improved databases and modelling software, and better access to sampling locations. Therefore, geological modelling is an empirical approach.
  • It is important to note that this 3D geological model represents an individual interpretation of a subset of the available data; other interpretations may be valid. The full complexity of the geology may not be represented by the model due to the spatial distribution of the data at the time of model construction and other limitations including those set out elsewhere in this report.
  • Best endeavours (detailed quality checking procedures) are employed to minimise data entry errors but given the diversity and volume of data used, it is anticipated that occasional erroneous entries will still be present (e.g. boreholes locations, elevations etc.) Any raw data considered when building geological models may have been transcribed from analogue to digital format. Such processes are subjected to quality control to ensure reliability; however undetected errors may exist. Borehole locations are obtained from borehole records or site plans.
  • Borehole start heights are obtained from the original records, Ordnance Survey mapping or a digital terrain model. Where borehole start heights look unreasonable, they are checked and amended if necessary in the index file. In some cases, the borehole start height may be different from the ground surface, if for example, the ground surface has been raised or lowered since the borehole was drilled, or if the borehole was not originally drilled at the ground surface.
  • Borehole coding (including observations and interpretations) was captured in a corporate database before the commencement of modelling and any lithostratigraphic interpretations may have been re-interpreted in the context of other evidence during cross-section drawing and modelling, resulting in mismatches between BGS databases and modelled interpretations.
  • Digital elevation models (DEMs) are sourced externally by BGS and are used to cap geological models. DEMs may have been processed to remove surface features including vegetation and buildings. However, some surface features or artefacts may remain, particularly those associated with hillside forests. The digital terrain model may be sub-sampled to reduce its resolution and file size; therefore, some topographical detail may be lost.
  • Geological units of any formal rank may be modelled. Lithostratigraphical (sedimentary/metasedimentary) units are typically modelled at Group, Formation or Member level, but Supergroup, Subgroup or Bed may be used. Where appropriate, generic (e.g. alluvium – ALV), composite (e.g. West Walton Formation and Ampthill Clay Formation, undifferentiated – WWAC) or exceptionally informal units may also be used in the model, for example where no equivalent is shown on the surface geological map. Formal lithodemic igneous units may be named Intrusions or Dykes or may take the name of their parent (Pluton or Swarm/Centre or Cluster/Subsuite/Suite), or if mixed units Complex may be used. Highly deformed terranes may use a combined scheme with additional rank terms. Artificially Modified Ground units (e.g. Worked Ground – WGR, Landscaped Ground (undivided) – LSGR) are currently regarded as informal.
  • The geological map linework in the model files may be modified during the modelling process to remove detail or modify the interpretation where new data is available. Hence, in some cases, faults or geological units that are shown in the BGS approved digital geological map data (DiGMapGB) may not appear in the geological model or vice versa. Modelled units may be coloured differently to the equivalent units in the published geological maps.