OR/16/034 Model limitations

Barron, A J M. 2016. Metadata report for GSI3D cross sections along the HS2 route in Area 9 (Birmingham spur). British Geological Survey Open Report, OR/16/034.

Model specific limitations

The Tarporley Siltstone Formation of the Mercia Mudstone Group cannot be meaningfully and consistently differentiated in the Birmingham Spur area borehole logs from the overlying Sidmouth Mudstone Formation. Thus throughout the cross section, the interval is shown as Tarporley Siltstone Formation and Sidmouth Mudstone Formation, undifferentiated (TSSM-MDSS), although it is thought that the Sidmouth Mudstone forms much the greater part and is the unit at rockhead.

Fault planes are presumed to dip at 60° in the absence of other information.

Figure 6 shows all coded boreholes available in Birmingham Spur and Area 7 (in 2013). Figure 7 shows the boreholes used in cross sections with the lines of cross-sections, indicating where this subsurface data may constrain the model. This gives the model user some idea where the model is most and least certain.

The digital geological map data (DiGMapGB-50) used to inform the surface or subcrop (bedrock under superficial deposits or superficial deposits under artificially modified ground) intercepts in cross section drawing is cartographically generalised for publication of the 1:50 000 scale map. In Area 9 this does not significantly affect the (relatively simple) bedrock polygons. In the superficial layer some minor tracts of deposits (e.g. ribbons of alluvium or head) may be exaggerated or removed. In the artificially modified ground layer, however, some areas recorded at 1:10 000 scale may be omitted in DiGMapGB-50, especially if they are embankments/cuttings associated with existing roads and railways (and therefore shown on the OS topography). The Digital Terrain Model (DTM) used is of 2013 vintage and is subsampled to a horizontal resolution 20 m. Therefore some subtleties of the ground surface may not be correctly represented.

In addition, mapping artificially modified ground in urban areas presents difficulties, even where there is no significant mineral extraction, as it is a result of generations of development, and the evidence can be lost or hidden. The vintage of any borehole data compared with development etc. will affect any interpretation. In addition, the extent and nature of artificially modified ground is constantly changing, and so the distribution that is mapped is a snapshot at the time, utilising the available borehole data. In the case of this cross section, the proposed route partly follows existing transport infrastructure, with its consequent artificially modified ground, and together with the limitations of the DTM, it has not been possible to represent all of this in the model.

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 these geological cross-sections represent 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 cross-sections due to the spatial distribution of the data at the time of their 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.