OR/13/050 Model datasets
|Gow, H, Cripps, C, Thorpe, S, Horabin, C, and Lee, J R. 2013. Model metadata report for the Somerset levels 3D geological model. British Geological Survey Internal Report, OR/13/050.|
General caveats regarding BGS datasets and interpretations are:
- Geological observations and interpretations are made according to the prevailing understanding of the subject at the time. The quality of such observations and interpretations may be affected by the availability of new data, by subsequent advances in knowledge, improved methods of interpretation, improved databases and modelling software, and better access to sampling locations.
- Raw data may have been transcribed from analogue to digital format, or may have been acquired by means of automated measuring techniques. Although such processes are subjected to quality control to ensure reliability where possible, some raw data may have been processed without human intervention and may in consequence contain undetected errors.
- All bedrock units have been simplified and faults are not included in the model (i.e. modelled as un-faulted objects).
- A flat cut-off base has been applied to the model at -28 m OD. This should not be interpreted as the base of any of the geological units, simply a level for the extent of the modelling.
- Some discrepancies with the borehole start heights were noted, and these are attributed to errors in data ingestion but additionally could be a result of anthropogenic changes (peat workings, draining of peat areas) or natural processes of soil compaction/ablation.
Digital Terrain Model (DTM)
The geological model uses a capping surface to define the uppermost limit of the model. This Digital Terrain Model (DTM) is derived from the NextMap 5 m DTM (InterMap Technologies) at a subsampled scale of 50 m. Due to limitations of the DTM (such as the inclusion of trees) the whole dataset has been further refined by removing woodland and is stored corporately as the ‘BaldEarth Model’ (Figure 3). ‘BaldEarth’ is a BGS derived dataset produced by combining NextMap data (licensed to BGS) and free Ordnance Survey (OS) Panorama Data. It was generated to cope with deficiencies in the NextMap data associated with forested land.
Borehole records held by BGS are stored and administered within a corporate database called the Single Onshore Borehole Index (SOBI) and this includes basic information such as borehole location, surface elevation and date of drilling. Interpretative and factual geological information obtained from these boreholes (i.e. drill depth, geological sub-division), is held within a separate but parallel database called Borehole Geology (BOGE) (Figure 4).
Within BOGE, a project index file was generated containing the borehole records used within the Somerset Levels model (Figure 5). As per the project coding guidelines, four boreholes per 10 km2 were selected to be interpreted. These boreholes were then attributed according to their lithology using the Unlithified Deposits Coding Scheme (Cooper et al., 2006) creating a series of downhole logs. These logs were then input into the geological modelling software and form the basis of the geological interpretation (See Generalised Vertical Section (GVS) and Geological Legend (GLEG)). Other projects may have interpreted boreholes from the study area but these have not been included in this model.
- BGS 1:50 000 superficial DiGMap GB was used for outcrop and geological unit formation (Version 2.18, released 22/05/2008)
- BGS 1:625 000 bedrock DiGMap GB was used for outcrop and geological unit formation (Version 5.17, released 11/02/2008)
- BGS 1:50 000 Glastonbury 296 Bedrock and Superficial geological map (British Geological Survey, 1973)
- BGS 1:50 000 Wells 280 Bedrock and Superficial geological map (British Geological Survey, 1984)
- BGS 1:50 000 Bristol 264 Bedrock and Superficial geological map (British Geological Survey, 2004)
- OS OpenData 1:50 000 map data was used as the base topographical map
Generalised Vertical Section (GVS) and Geological Legend (GLEG)
GSI3D modelling software uses two files to describe the stratigraphical order and colours of the units contained in the model. These are the Generalised Vertical Section (GVS) file which contains the name and stratigraphical relationships for every geological unit in the model together with its lithology (defined by the BGS Rock Classification Scheme) and a description field (Table 1).
|Model Name||LEX||RCS||Full Name||Age||Description|
|alv||ALV||CZSV||Alluvium||Quaternary (Holocene)||Fluvial Silty clay, but can contain layers of silt, sand, peat and basal gravel|
|tufa||TUFA||CATUFA||Tufa||Quaternary||Calcareous deposits at or near springs and seepages|
|bsa||BSA||S||Blown Sand||Quaternary||Sand dunes|
|btfu||BTFU||CZS||Beach and Tidal Flat Deposits (Undifferentiated)||Quaternary||Composite of 'Beach deposits': Shingle, sand, silt and clay. Beach deposits may be in the form of dunes, sheets or banks, and 'Tidal Flat Deposits': commonly silt and clay with sand and gravel layers; possible peat layers; from the tidal zone|
|peat3||PEAT_3||P||Peat ‘3’, ‘upper peat’||Quaternary||Informal unit of Somerset Levels Formation. Accumulation of wet, dark brown, partially decomposed vegetation|
|head4||HEAD_4||CZSV||Head ‘4’||Quaternary||Head — Colluvium/hillwash (probably post 5000 years BP)|
|tfd2||TFD_2||CZSV||Tidal Flat Deposits ‘2’, ‘upper estuarine clay’||Quaternary (Holocene)||Informal unit of Somerset Levels Formation. Normally a consolidated soft silty clay, with layers of sand, gravel and peat. Characteristically low relief; from the tidal zone|
|peat2||PEAT_2||P||Peat ‘2’, ‘middle peat’||Quaternary||Informal unit of Somerset Levels Formation. See description of peat_3|
|tfd1||TFD1||CZSV||Tidal Flat Deposits 1, ‘lower estuarine clay’||Quaternary (Holocene)||Informal unit of Somerset Levels Formation. See description of tfd_2|
|peat1||PEAT_1||P||Peat ‘1’, ‘lower peat’||Quaternary||Informal unit of Somerset Levels Formation. See description of peat_3|
|sgao||SGAO||VS||Sand and Gravel of uncertain age and origin||Quaternary||probably Devensian — OIS2 or OIS4 in age|
|rtd1||RTD1||SV||River Terrace Deposits, 1||Quaternary||Sand and gravel, locally with lenses of silt, clay or peat|
|rtdu||RTDU||VS||River Terrace Deposits (undifferentiated)||Quaternary||Sand and gravel, locally with lenses of silt, clay or peat|
|unknd||UNKND||CZSV||Unknown Drift (superficial deposits)||Quaternary||Silty clays of unknown age and origin|
|bub||BUB||SV||Burtle Formation||Quaternary (Pleistocene)||Marine sands and gravels with shells — Ipswichian OIS5e in age|
|Lias||LI||MSLS||Lias Group||Jurassic||Lias Group. Mudstone, siltstone, limestone and sandstone|
|trias-mdss||TRIA||MDSS|| Triassic rocks —
mudstone, siltstone and sandstone
|Triassic||Mercia Mudstone and Penarth groups, undifferentiated. Mudstone, siltstone and sandstone|
|trias-scon||TRIA||SCON|| Triassic rocks —
sandstone and conglomerate
|Triassic||Mainly Sherwood Sandstone Group. Sandstone and conglomerate|
|dina||DINA||LSSA||Dinantian rocks undifferentiated||Carboniferous||Avon and Pembroke Limestone groups, undifferentiated. Limestone with subordinate sandstone and argillaceous rocks|
|udev||UDEV||MDSS||Upper Devonian rocks||Devonian||Upper Devonian rocks. Mudstone, siltstone and sandstone|
|sand_lens_top||CHAN||S||Top of channel lens||Quaternary||Top of sand lens found within the tidal flat deposits|
|peat_lens_top2||PEAT||P||Top of peat lens||Quaternary||Top of peat lens found within the tidal flat deposits|
|peat_lens_top1||PEAT||P||Top of peat lens||Quaternary||Top of peat lens found within the tidal flat deposits|
The Geological Legend (GLEG) contains information to provide colour to the model. The software uses a simple Red-Green-Blue reference system, so in Table 2 columns 3, 4 & 5 refer to the Red, Green and Blue values and the final column refers to the transparency value for every unit in the model.
Rockhead Elevation Model (RHEM)
Where insufficient borehole data was available, the BGS Rockhead Elevation Model (version 2009) was used as a guide to determine the depth of the superficial deposits. This data has since been superseded but for continuity with the previous modelling, the 2009 version was used.
- COOPER, A H, KESSLER, H, and FORD, J R. 2006. A revised scheme for coding unlithified deposits (also applicable to engineering soils). British Geological Survey Internal Report, Vol. IR/05/123.
- BRITISH GEOLOGICAL SURVEY. 1973. Glastonbury. England and Wales Sheet 296. Bedrock and Superficial Deposits. 1:50 000 Geology Series. (Keyworth, Nottingham: British Geological Survey.)
- BRITISH GEOLOGICAL SURVEY. 1984. Wells. England and Wales Sheet 280. Bedrock and Superficial Deposits. 1:50 000 Geology Series. (Keyworth, Nottingham: British Geological Survey.)
- BRITISH GEOLOGICAL SURVEY. 2004. Bristol. England and Wales Sheet 264. Bedrock and Superficial Deposits. 1:50 000 Geology Series. (Keyworth, Nottingham: British Geological Survey.)