OR/15/044 Model limitations

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Wakefield,O J W, and Barron, A J M. 2015. GSI3D Model metadata report for HS2 Area 6 (Cubbington to Hampton-in-Arden). British Geological Survey Internal Report, OR/15/044.

Model specific limitations[edit]

The model honours the corresponding published BGS 1:50 000 scale maps as closely as possible, with the exception of errors caused by the resolution of the DTM (Figure 11) and the issues highlighted below.

Figure 11    Comparison of published cross-section and DTM resolution.

Significant assumptions on the throw of the various faults have been made. Where possible, fault throws have been calculated, but even where this is done the amount of throw along the lateral extent of the fault is uncertain. The orientation and angle of movement on the faults may also be uncertain. Assumptions have been made that all faults are ‘normal’ and dip at ~60° to horizontal.

As stated above, geological mapping and borehole data have enabled the Warwickshire Group to be fully subdivided in this model into four formations in descending order: the Ashow, Kenilworth Sandstone, Tile Hill Mudstone and Salop formations, the last divided into the Allesley and Keresley members (Table 1). However, within all these units beds of subordinate lithologies are widely mapped, but are not distinguished within the model because of their significant lenticularity, the lack of information in boreholes, and the considerable additional modelling work these would impose. This is contrary to Area 5 where the Warwickshire Group has been modelled due to lack of information.

Minor thin siltstone beds or ‘skerries’ within the Mercia Mudstone Group (MMG) formations are not modelled, nor are the mudstone units in the Arden Sandstone Formation. Additionally, the lithological variation mapped in DiGMap50k has not been retained between the Area 7 model and the Area 6 model and both areas are modelled as AS-SDSM.

As stated in Modelled surfaces/volumes, the area includes part of a highly complex Anglian glacigenic succession, which is in places inadequately mapped (e.g. unclear superposition relationships, unlikely pinch-outs). Lenticular beds and repeated similar lithologies within this, present further difficulties in modelling true/viable stratigraphical units. In addition, the correct identification of the Thrussington Member (THT) till and Oadby Member (ODT) till was doubtful in places and caused misfits so some outcrops as Till, Mid Pleistocene (TILMP) were reattributed (although all Mid Pleistocene units are part of the Wolston Glacigenic Formation). A decision was taken that in part of the area (west centre) it was necessary to conflate the glaciofluvial (GFDMP-XSV) and glaciolacustrine (GLLMP-XCZ) deposits, plus a single outcrop of (dubious) Baginton Sand and Gravel (BGSG-XSV), beneath the main till unit (TILMP) into one Glacial deposits, undifferentiated unit (GDU-XCZS, yellow in Figure 6).

Information from borehole SP37SE773 in section HS2_Area6_NWSE_3 has been interpreted as unreliable. Specifically the recorded detail in the borehole record is not sufficient to adequately differentiate between the Bromsgrove Sandstone (BMS) and the formations of the Mercia Mudstone Group with a significant level of confidence. As such, more reliable information gleaned from nearby boreholes has been used to interpret the geology in this part of the section.

General modelling limitations[edit]

  • 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.