OR/13/050 Model datasets

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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.

Figure 3    BGS BaldEarth DTM shown as 5 m contour intervals (x10 vertical exaggeration.

Borehole data

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).

Figure 4    Borehole Geology Database.

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[1]) 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.

Figure 5    Boreholes and cross-section locations. Boreholes drilled to a length of 10 m or less are shown as black points, those deeper than 10 m are green points.

Map data

  • 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[2])
  • BGS 1:50 000 Wells 280 Bedrock and Superficial geological map (British Geological Survey, 1984[3])
  • BGS 1:50 000 Bristol 264 Bedrock and Superficial geological map (British Geological Survey, 2004[4])
  • 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).

Table 1    GVS used in Somerset Levels 3D model.
Thick black line represents the superficial to bedrock boundary
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.

Table 2    GLEG used in Somerset Levels 3D model
ALV Description 255 255 0 255
TUFA Description 204 255 204 255
BSA Description 255 221 138 255
HEAD Description 124 64 64 255
PEAT Description 188 130 92 255
TFD Description 153 176 190 255
SGAO Description 255 201 255 255
RTDU Description 246 160 89 255
UNKND Description 130 136 132 255
BUB Description 255 139 61 255
LI Description 176 148 148 255
TRIAS-MDSS Description 255 148 148 255
TRIAS-SCON Description 255 176 201 255
DINA Description 84 255 237 255
UDEV Description 237 176 176 255
CHAN Description 255 249 158 255

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.

References

  1. 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.
  2. BRITISH GEOLOGICAL SURVEY. 1973. Glastonbury. England and Wales Sheet 296. Bedrock and Superficial Deposits. 1:50 000 Geology Series. (Keyworth, Nottingham: British Geological Survey.)
  3. BRITISH GEOLOGICAL SURVEY. 1984. Wells. England and Wales Sheet 280. Bedrock and Superficial Deposits. 1:50 000 Geology Series. (Keyworth, Nottingham: British Geological Survey.)
  4. BRITISH GEOLOGICAL SURVEY. 2004. Bristol. England and Wales Sheet 264. Bedrock and Superficial Deposits. 1:50 000 Geology Series. (Keyworth, Nottingham: British Geological Survey.)