OR/15/020 Software used and model workflow

Thorpe, S. 2015. Metadata report for the Knowsley 3D geological model . British Geological Survey Internal Report, OR/15/020.

Software used

• GSI3D V2.6
• ArcGIS 9.3
• MS Excel

Model workflow

The standard GSI3D workflow was used to create the 3D model. (Kessler and Mathers, 2004) The interrogation of the pipeline data with the modelled surfaces was then completed using a series of spatial queries in ArcGIS (for more details see (Price et al., 2008))

A review of the model by Barron in Feb 2015 prompted further investigation into some small modelling errors that came to light. These included a Site Investigation whose boreholes had been entered into the BGS corporate system using a site datum without being converted to the correct level above Ordnance Datum (SI 44928 — corrected on 9th March 2015 by Sally Stolworthy) and a borehole that looked at odds with the rest of the cross-section (Figure below show Knowsley_NS_17 and Knowsley_WE_09).

Figure 3 Sections showing erroneous borehole.

As is clear, the borehole in question (SJ49NW1122.) appears to ‘force’ the superficial geology to a higher elevation than the surrounding correlations. Upon investigation it was revealed that this borehole isn’t located in the correct position by at least 1000m. The cross-sections were amended to exclude this false borehole information and this in turn altered the geological envelopes for the Peat_1 (dark brown — but transparency is set to 50% which allows the other units to show through), Shirdley Hill Sand (in dark orange) and the Weathered Sherwood Sandstone (in orange). The original project is shown on the left, and the amendments are shown on the right.

Figure 4 Corrected sections after removing erroneous borehole.

Cross-section WE_12 also had a borehole at odds with the section. After reviewing the borehole and finding out that the start height had been input into the system incorrectly on loading the borehole appears fit the geology better as shown in the before and after view of the section.

Some cross-sections show correlation lines that appear to be above the DTM. S Thorpe believes that this is because the original model was produced using a version of GSI3D which didn’t store the DTM. Therefore when the NGM checks were completed a new DTM was introduced which was slightly different (due a more recent flying date). This is unfortunate, but the sections have been modified to represent a better fit to this more recent DTM (derived from the BaldEarth DTM model).

Below is a specific example of the changes made: Knowsley_WE_16

This section shows the MGR line floating above the DTM, the corrective action is shown in the figure labelled 'After'