OR/15/074 Model limitations
| Ambrose, K. 2015. GSI3D model metadata report for HS2 Area 8 (Drayton Bassett to Rugeley). British Geological Survey. (OR/15/074). |
Model specific limitations
Although faults were drawn and modelled dipping at around 60° for drawing guidance, all correlated lines are stepped across, either to join into the same unit in the footwall, or if absent there, to join the edge of the polygon at the surface Figure 12 shows the representation of faults in cross- section HS2_Area8_NS_approx_route, which runs north-south through the middle of the model area.
Figure 8 shows all boreholes available in Area 8, with those with drilled depths of 10 m and over coloured in green. Figure 8 also shows the boreholes used in the cross sections with the lines of section shown to indicate where this subsurface data constrains the model. This gives the model user some idea where the model is most and least certain.
The arbitrary limits of the Devensian and Anglian (Mid Pleistocene) glaciations may mean that some of the superficial polygons have been wrongly attributed. However, until there is a definitive method for distinguishing between the glacial deposits of these ages, this will remain uncertain.
There are some uncertainties as to the precise extent of some of the geological units, notably the Wildmore Sandstone Formation, Hopwas Breccia Formation and Littleworth Beds. The Wildmoor Sandstone Formation is largely fault controlled but the limits are in places uncertain. The Littleworth Beds have not been mapped and are not recognised in all boreholes that should prove them. Thus there is some uncertainty as to their precise lateral extent. The Hopwas Breccia Formation has also not been interpreted in all boreholes that may prove it. As it is a breccia sitting on a conglomerate (the Kidderminster Formation), this is the main reason for its failure to be recognised everywhere.
Some of the deep boreholes have not distinguished the Tarporley Siltstone Formation. It is known to be present everywhere at the base of the Mercia Mudstone Group (Warrington et al.,1980) and been extensively mapped on the Lichfield sheet. It has been run through the model at depth with only an approximate thickness in places. This produces a mismatch with the modelled units in Area 7, which at the boundary of the two areas is modelled as Mercia Mudstone Group without being split into formations.
The Warwickshire Group is only depicted in the model at the group level. Although divisible into formations and members, this has not proved possible in all of the boreholes. Outcrops on or close to the model area all indicate the Enville or Alveley Members of the Salop Formation. Because of the similarities between the various members and formations, all comprising dominantly red mudstones and sandstones in varying proportions, their widespread correlation cannot be certain.
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 occasional 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.
- Made Ground (undivided)-MGR, 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.