OR/14/035 Model development log

THORPE, S, BURKE, H, WHITBREAD, K. 2013. Model metadata report for the EA Tees 3d Model. British Geological Survey Internal Report, OR/14/035.

This section outlines the initial set-up of the project GIS, Borehole coding and extraction to GSI3D and the 3d modelling of the EA Tees 3D Model. A detailed modelling progress log can be found in Appendix 1.

Phase1: Data compilation and GIS assessment

The project area was defined following meetings with the client (EA). An initial project area shapefile was drawn by ST to include the full extent of Sherwood Sandstone outcrop between West Hartlepool in the north-east and Darlington in the south-west.

Boreholes recorded in SOBI were queried for the project area, with a total of 7066 borehole records, of which 5431 fall directly within the outcrop of the Sherwood Sandstone. In order to prioritise deep boreholes, that record the full sequence of superficial deposits, for coding, a method was derived to identify boreholes that penetrate to rockhead using ARC GIS tools. This process should be viewed carefully, as it can include or exclude some boreholes that may prove useful.

1. Clip the SOBI borehole shapefile and the superficial thickness (BSTM) models to the outline of the Sherwood sandstone (derived from the 250k map)
2. Using the extract values to points function add the total superficial deposits thickness from the BSTM (basic sediment thickness) model to the borehole file
3. Open the dbf file in Excel and used the IF function to select only those boreholes with total length (LengthC) greater than the sediment thickness from the BSTM model, and create a new dbf file.
4. Create a ‘boreholes to rockhead’ shapefile from the new dbf.

The following is a list and description of shapefiles created or viewed within the project GIS (EA_Tees_Superficial.mxd):

EA_Tees_NotCoded_Boreholes_alongSections.shp

boreholes within 200m buffer of section (39)

EA_Tees_NotCoded_Boreholes_below_RH.shp

shows all boreholes which penetrates the RHEM surface from the superficial thickness model (487)

EA_Tees_CodedBoreholes_26-06-12.shp

records extracted from SOBI/BoGe database on this date (2216). This file shows unique borehole (i.e. only id, easting and northing) the majority of these boreholes have multiple coding schemes and so need to be looked at in more detail. Prioritisation tool from ST?

XYTess_Sherbore_ToRock.shp

supplied by Katie Whitbread. Boreholes that penetrate Rockhead using the above procedure

EA_Tees_Sobi_overSSG.shp

all boreholes that overly the outcrop of SSG (5431)

EA_Tees_Sobi.shp

all boreholes within project area (7066)

Borehole_imau.shp

Mineral Assessment borehole records (no strata)

EA_Tees_Sectionlines.shp

shapefile of cross-sections drawn for proposal, these may be subject to slight adjustment

Durham_south, Durham_Permian, Durham_North.shp

three files containing sections from Durham EA model

EA_Tees_Projectarea.shp

currently agreed project area

SSG_outcrop.shp

– outcrop pattern of SSG, supplied by KW and VB

Boreholes layer from GDI Geology layer from GDI Topo base maps from GDI EA_Tees_25m_DTM_ascii.asc

grid generated in GSI3D using Baldearth Model

Superficial Thickness layer from GDI

Phase 2: Borehole prognosis and coding

A review of the level of coding by ST:

Of the 9 categories of borehole records shown in Table 2, all but 2 of them provided very useful data to the project (a total of 3566 records). The top 4 codes provided 1407 boreholes of 'best coding'.

ST performed a narrowing down exercise: from the list of boreholes coded a switch selection was made to select those that hadn’t been coded. From these 2307 the DTM value, BSTM value and borehole total depth (TD) were used to determine which boreholes penetrated the Rockhead surface. This created a file of 2472 boreholes. These boreholes were inspected to assess the quality of the borehole logs (currently some of these are not logged in the SOBI database) and were prioritised using the lines of framework sections specified by the client. The framework section lines were used to create a list of non-coded boreholes within 200m of the lines of section (280 Boreholes). Priority was given for coding of boreholes along these section lines, and to deep boreholes with high quality logs dispersed throughout the model area. Approximately 750 coded boreholes were used in the model cross-sections and many more were used to help delineate the spatial extent of units (envelopes).

Phase 3: Lithostratigraphy and the GVS set up

The GVS presented in table 1 was developed through a literature review with guidance from A Cooper. The lithostratigraphic succession is described in detail in Whitbread et al. (2013). The published geological 50k map for Stockton (Sheet 33), a range of published papers and unpublished reports were consulted to develop this regional lithostratigraphy and these sources are also recorded in Whitbread et al. (2013). Descriptions of the main modelled units are also included in the detailed model development log in Appendix 1.

Phase 4: Model developemnt (summary)

This section provides a summary of the modelling procedure and stages of model development. A detailed log of the modelling process produced by the modelling team during model construction is included in Appendix 1. This detailed log includes information relating to individual modelling decisions (both geological and technical), as well as model checking and amendments.

The project files are located in W:\Teams\UD\TeesSuperficialGeology\Data\GSI3D_Modelling at the time of writing. Borehole and DTM data files used in the modelling are:

Ea_tees_bid.bid

EA_Tees_BLG_v2.blg (prioritised to remove duplicates)

DTM: EA_Tees_DTM_25

Stage 1: Framework section construction

Five framework section lines were requested by the client, one aligned along the length of the model (roughly north-south) and four aligned roughly east to west. These sections were constructed as continuations of cross-sections drawn for the Durham (south) area in previous work for the EA (Price et al. 2007). In the initial stage of the modelling, these five framework sections were correlated by modellers Helen Burke (HB), Steve Thorpe (ST) and Katie Whitbread (KW) working in conjunction with A Cooper to ensure consistent interpretation of the lithostratigraphy was applied across the model area.

Stage 2: Model development

6 Framework sections

21 Model sections

54 Helper sections

The model was constructed in three stages by modellers working consecutively on three model sub-sections; north, middle and south. These model segments were divided by the east-west orientated framework section lines. 17 regularly spaced model cross-sections were constructed to utilise the best available borehole records on an approximate grid pattern between the framework section lines and 4 boundary sections were constructed at the model edges. A further 54 helper sections were constructed during the modelling development and calculation checking.

Phase 5: QA

The Tees Model was reviewed by M Barron following standard BGS model checking procedures (W:\Teams\NGM\Models\Documents\QA & Approval Docs\NGM QA Checklists\ GSI3D_model_corp_check_8-3-13_TEMPLATE.docx). From this review it was noted that the river, although defined by the OS topographic maps, didn’t fit the DTM surface, but showed some of the ‘thalweg’ sections rising and fall across the hills. This is due in part to the resolution of the DTM held at BGS (5 m grid) but more likely to the resampling of the DTM to 50 m. Following discussions with V Banks and S Thorpe it was decided that the river polygon and water-lines in relevant cross-sections should be removed from the model, and the underlying geology reviewed and amended where necessary. Water was retained in the estuary and off- shore areas at the north end of the model. The removal of the river polygon was undertaken by S Thorpe.

Phase 6: Model delivery

Following final approval, the model, together with digital images of the five framework cross sections were prepared for delivery to the client.

The final version of the GSI3D model was encrypted into the Lithoframe viewer using standard procedures.

GIS exports — envelopes (Shapefiles), grid thicknesses were also derived and formed the basis for hydrogeological domain assessment performed by V Banks and M Garcia-Bajo.