OR/13/053 Chalk formational surfaces

Woods, M A, Haslam, R B, Mee, K, Newell, A J, and Terrington, R L. 2013. A guide to a new Geographical Information System for the Chalk of the Thames Basin: The Thames Chalk Information System (TCIS). (FutureThames cross-cutting Programme). British Geological Survey Internal Report, OR/13/053.

The following unfaulted formation surfaces have been derived, and are presented in the GIS as theme layers in the Chalk Group tab and the Subgroup and formational data tab:

1. Top Chalk — referred to as Top CK*
2. Base Newhaven Chalk and Base Seaford Chalk combined — referred to as Base NCK SECK*
3. Base Lewes Nodular Chalk — referred to as Base LECH*
4. Base New Pit Chalk — referred to as Base NPCH*
5. Base Holywell Chalk — referred to Base HCK*
6. Base Zig Zag Chalk — referred to Base ZZCH*
7. Base West Melbury Marly Chalk — referred to Base WMCH*
8. Base Chalk — referred to as Base CK*

* in the GIS and model datasets

The Seaford and Newhaven Chalk formations are combined because of sparse data (only 29 boreholes) for the Newhaven Chalk in the Thames Basin.

Each of the surfaces generated have the following naming convention in the relevant datasets:

b_ = Base

t_ = Top

CK = Chalk

NCK = Newhaven Chalk

SECK = Seaford Chalk

LECH = Lewes Nodular Chalk

NPCH = New Pit Chalk

HCK = Holywell Nodular Chalk

ZZCH = Zig Zag Chalk

WMCH = West Melbury Marly Chalk

‘v1_4’ = relates back to the version of the surface in the GOCAD project.

Source data for surface construction

Source datasets used to construct the formational thickness layers comprise:

• NextMap Digital Terrain Model (DTM) at 50 m resolution
• RockHead Elevation Model (RHEM) at 50 m resolution
• DigMapGB 50k Bedrock
• Contours from the Thames Estuary 250k offshore map (Top Chalk only)
• Correlation points from the GSI3D models (Top Chalk only) listed below:

o  Farringdon Station (Aldiss, 2009)

o  Lower Lea Valley Model (Royse, 2006–2007)

o  London Lithoframe 50k areas 1 to 14 (Mathers et al., 2005–2013)

o  Southern East Anglia (Colchester and Ipswich models; Mathers, 2004–2008)

o  Thames Gateway Area 5 (Royse et al., 2006–2007)

Contours were derived from these correlation points (using Discrete Spatial Interpolation in GOCAD) which were used to constrain the Top Chalk surface. The position of models in the above list shows order of priority for layer construction; highest in list = highest priority and takes precedence over lower priority models where they overlap. The extent of these models is shown on the BGS Geoscience data Index (GDI) models layer.

• Corporate borehole database (Borehole Geology — BoGe).

Work flow for BoGe data in surface construction

The following describes how data from the corporate Borehole Geology (BoGe) database was used in the construction of the GIS.

1. BoGe queried in MS Access for all boreholes with any mention Chalk (‘CHLK’) in the lithology and formation codes. These initial queries excluded combined formations, for example coded LSNCK, because of long processing times. These combined units can be used for future enhancement of formation surfaces (for example addition of faults).
2. Added Single Onshore Borehole Index (SOBI) data.
3. Added pseudo start height to boreholes using NextMap DTM where boreholes did not have a start height recorded in SOBI.
4. Removed exact duplicates and boreholes not proving formational boundaries.
5. Queried borehole top depth, which is used as the reference for determining formation elevation in boreholes. Terminal depth (TD) of boreholes is not consistently recorded in BoGe.
6. A point shapefile for each formation was exported which contained only the uppermost elevation of that formation recorded in a borehole.
7. The thickness of all Chalk formations in a given borehole was calculated by matching common borehole identifications in the shape files of formations, and subtracting the elevations of adjacent formation-tops, or where the formation is at rockhead, the elevation of rockhead from the elevation of the top of the underlying formation.

Work flow for formation surface construction

This work flow describes the sequence of steps used to derive base-formation surfaces shown on the GIS, beginning with the base of the combined Newhaven Chalk (NCK) and Seaford Chalk (SECK).

1. The derived Top Chalk surface spliced with the DTM and RHEM is used as the control surface from which the combined NCK and SECK formation surface would be constrained.
2. Create an isopachyte contour plot for the NCK/SECK generated from the thickness recorded in the borehole datasets (see 4 above for more information).
3. Isolate and smooth out extreme thickness values in the thickness contours caused by Discrete Smoothing Interpolation (DSI) in GOCAD.
4. Clip thickness contours by outcrop and project area for the combined NCK and SECK formations.
5. Vertically project (drape) the combined NCK and SECK thickness contours onto the Top CK surface.
6. Use the thickness values of the contours (saved as a property) subtracted from the projected elevation value relative to OD given by the Top Chalk surface to give the correct vertical position relative to OD for the base of the combined NCK and SECK.
7. Calculate the elevation of the base of the combined NCK and SECK formations for the whole project area, using an initial 1000 m mesh size. This calculation is based on known thickness contours (converted to OD values) and outcrop OD derived by draping of outcrop onto the RHEM/DTM.
8. Increase the resolution of the calculated base of the combined NCK and SECK surface to a 500 m mesh, using quality control in GOCAD structural workflow.
9. Apply thickness range constraints by assessing the range of thickness values found in boreholes (NCK_SECK_V2_0_With_TH_Merged.shp) to control the depth and thickness of the surface.
10. Apply fit to points to tighten the surface against the boreholes picks (in this case the Top_LECH_V2_0 dataset in the Arc and GOCAD projects).
11. Removed cross-overs and smoothed out any anomalies (spikes) caused by the DSI in GOCAD. Some of this is a manual process so not all cross-overs were removed particularly close to outcrop. This explains the majority of negative thicknesses calculated (please see below for more information).
12. Surfaces clipped to combined outcrop and project area in GSI3D or GOCAD.
13. Thickness plots generated in ArcGIS (subtracting the elevation of the base of the combined NCK and SECK from the Top Chalk) were lightly assessed for any extreme values and the process of removing cross-overs was repeated (point 11).

This process was repeated sequentially for underlying Chalk formations. Thus, base SECK forms the top control for the LECH.

NB: the Version number given to the ESRI grid surfaces in the ArcGIS project are the same version numbers given to the surfaces in the GOCAD project. The GOCAD project can be found here:
W:\Teams\SCCP\ThamesBasin\Data\3D\Chalk\CK_Formations\CK_Formations_V16.gprj

Surface control

For each of the surfaces generated, a borehole control multi-buffer extent was created to show areas in which there was a higher density of boreholes and hence greater borehole control, and those areas that had fewer boreholes controlling the surface. The following categories were generated:

Borehole Control	Distance (metres)	Key
Very High	0–200
High	200–500
Medium	500–1000
Low	1000–2000
Very Low	2000–5000
Null (little or no borehole control)	Over 5000

This multi-buffer density distribution plot does not take into account any faulting or folding, so should be used with caution. The distances used are arbitrary and can be refined if necessary. The following datasets were generated:

1. Top_CK_BH_Control
2. Base_SECK_BH_Control
3. Base_LECH_BH_Control
4. Base_NPCH_BH_Control
5. Base_HCK_BH_Control
6. Base_ZZCH_BH_Control
7. Base_CK_BH_Control

The following layer file should be used to visualise the Surface Control layers:
W:\Teams\SCCP\ThamesBasin\Data\3D\Chalk\CK_Formations\CONFIDENCE\SURFACE CONTROL.lyr