How are geological models created?
The geological modelling methodologies that BGS employ are described at http://www.bgs.ac.uk/research/environmentalModelling/geologicalModellingSystems.html.
Briefly, two methodologies are typically used:
Deep, regional-scale models
Created by interpreting borehole and/or seismic data, which is then used to generate surfaces using mathematical algorithms and manual surface fitting.
Shallow, local-scale models
Created from a dense network of interpreted cross sections and maps drawn by geologists.
In both cases, significant geological interpretation, knowledge of landscape evolution and geological form and structure is incorporated into the models. In many cases the geological units included in our models are equivalent to those shown on BGS geological maps. However, whereas geological maps typically represent the full geological succession, 3D models may omit some geological units or show a higher level of stratigraphical generalisation.
What underlying data are the models based on?
The underlying data may include borehole records, geological maps, geophysical measurements, geotechnical data, geologist's tacit knowledge, digital terrain models and aerial photographs for example.
Any raw data (e.g. borehole data) considered when building geological models may have been transcribed from analogue to digital format. Such processes are subjected to quality control to ensure reliability, however undetected errors may exist. Borehole locations are obtained from borehole records or site plans; best endeavours are taken to minimise errors, but occasional errors may occur. Borehole start heights are obtained from the original records, Ordnance Survey mapping or a digital terrain model. Where borehole start heights look unreasonable, they are checked and amended if necessary. In some cases, the borehole start height may be different from the ground surface, if for example, the ground surface has been raised or lowered since the borehole was drilled, or if the borehole was not originally drilled at the ground surface.
Digital elevation models (DEMs) are sourced externally by BGS and are used to cap geological models. DEMs may have been processed to remove surface features including vegetation and buildings, however some surface features or artefacts may remain, particularly those associated with hillside forests. The digital terrain model may be sub-sampled to reduce its resolution and file size, therefore some topographical detail may be lost.
Geological models may utilise linework from the digital geological maps that range in scale from 1:10 000 to 1:625 000.
How accurate are the geological models?
Geological observations and interpretations are made by geologists according to the prevailing understanding of the available data. The quality of interpretations is affected by the availability of new data, by subsequent advances in knowledge, improved methods of interpretation and improved databases and modelling software. For shallow models, the borehole data considered during model building, and the locations of interpreted cross sections are shown on the 3D model viewer.
Is the surface linework in geological models identical to the geological map?
This linework may be modified during the modelling process to remove some detail or modify the interpretation where new data may be available. Hence, in some cases, faults or geological units that are shown on the digital geological map may not appear in the geological model or vice versa. Modelled geological units may be coloured differently to the equivalent units in the published geological maps.
Geological units may be modelled at group, formation or member level. Where appropriate, informal units may also be used in the model, for example where no equivalent is shown on the surface geological map.
Do the models provide information on the properties of geological units?
Currently, the geological models available only delineate geological boundaries. The properties of geological units can vary, but the geological model gives no indication of this internal variability.
How are interdigitated boundaries represented?
Where geological units are interdigitated, this relationship may be modelled as a sharp boundary.
Synthetic cross sections, horizontal sections and modelled surfaces
Model exports delivered via the 3D data viewer are generated from a grid that is exported from the geological model. The grid spacing is determined by the resolution of the original model, but is typically between 10 and 250 m; occasionally the grid spacing may be such that geological units pinch out sooner (and more vertically) than intended.
Synthetic cross sections
Using the 3D data viewer, synthetic cross sections of between 500 m and 3000 m length can be requested for depths ranging from 5 m to the maximum depth of the model. The specified cross section is fitted into a fixed space in a template and thus a shorter, shallower cross sections will show greater detail near the surface, whereas longer, deeper cross sections will result in the detail of the shallower geology becoming compressed. Cross section depth can be specified as metres below ground surface (depth is measured from the lowest surface elevation along the cross section) or metres above Ordnance Datum. Bespoke cross sections can be generated on an ad hoc basis where the required output cannot be obtained from the 3D data viewer (contact email@example.com for more information).
Synthetic horizontal sections
Using the 3D data viewer, horizontal sections can be generated at any depth through the model, but the total size of the section is limited to between 300 and 500 m. Horizontal sections can be generated at a depth below ground surface or a depth above Ordnance Datum. Where sections are generated at metres below ground surface, the horizontal section reports the geology at that depth across the chosen area and thus does not represent a flat surface if the topography is undulating.
Sections requested at a depth above Ordnance Datum represent the geology across a horizontal plane at the specified elevation. Shallow depth horizontal sections at a height above Ordnance Datum may contain white space where the section is above ground in undulating terrain. Bespoke horizontal sections can be generated on an ad hoc basis where the required output cannot be obtained from the 3D data viewer.
Geological surfaces can be licensed from all models. These can be provided as grids, triangulated surfaces (TINS), point clouds or in propriety software formats such as GOCAD tsurfs.
The geological surfaces modelled are stated in the 3D data viewer. For bedrock models, the entire geological column may not have been modelled, it should therefore not be assumed that the geological surfaces can be stacked to produce a spatially complete geological model.
Where thick superficial deposits occur, bedrock models may not reach the ground surface and therefore these geological models often do not take surface geological mapping into account.
The surfaces in bedrock models may either represent unit tops or unit bases or cross-cutting unconformity surfaces; this is clearly stated in the 3D data viewer.
Other items to note
Where a cutoff depth has been applied during model construction, the apparent base of the lowest unit in the geological model may not represent the true depth of the base of that unit.
Artificial ground is only modelled in some areas and as per natural geological units, its extent in 3D models may differ from that shown on published geological maps.
Where geological models are created from interlocking cross sections and geological maps drawn by geologists, the locations of these cross sections, maps and the boreholes considered are shown within the 3D data viewer. The resolution and reliability of geological models is typically higher where the density of cross sections and boreholes is greater. However, in areas of complex geology, a high density of cross sections or boreholes may not resolve all potentially significant structures or variability in the extent or thickness of geological units.
A synthetic borehole may not match a borehole record at the same location. This may be because: a) the units modelled may be different to those in the borehole record; b) the borehole may not have been considered when the model was built; c) gridding of a model may cause smoothing of geological variation, and d) the surfaces within a model are a result of the interpretation of data from numerous sources, considered together to delineate the most likely geological surface.
All BGS models have been created by professional and experienced geologists using the best available knowledge and data at the time. If you have any queries, get in touch with our geologists via firstname.lastname@example.org.
Geological faults are modelled in some areas. Where faults are not modelled, this does not mean that they don't exist. This may occur where there is insufficient evidence to denote the fault, or where the scale of the fault is too small to be represented in the model.