OR/14/072 Types of 3D geological model data

Kessler, H, and Dearden, R. 2014. Scoping study for a Pan-European geological data infrastructure: D 3.4: technical requirements for serving 3D geological models. British Geological Survey, OR/14/072.

Two types of geological model are considered here:

• Geological framework models, in which stratigraphic boundaries are denoted in space by surfaces.
• Voxel models, in which the 3D space is gridded (voxellated), with each cell or voxel having an attribute to describe the geology within that volume. Voxel models can be generated from geological framework models, or generated stochastically from lithological or geophysical data for example.

Both model types can also be property models; those in which the 3D space is attributed with geological properties. Such property attribution could be made to a bulk geometry in a geological framework model, or to individual cells in a voxel model. The latter is done either manually or via statistical methods.

In order to share the outputs from these models, the type and format of the data must be considered so that it can be viewed and loaded in a standardised way across Europe. This may appear to be a significant challenge because many different software packages are used to create models (Figure 2). However, homogenisation is still a long way off, as most proprietary software systems allow export of geological model data in a variety of standard formats. Such standard formats are listed in

Table 2; note that there is some differences in the standard exported outputs from framework and voxel models. The relative use of methodologies is indicated in Figure 3.

Table 2    Types of data exported from geological models and formats that are commonly in use

		Type of models
Type of data export	Common formats	Framework models	Voxel/property models
Maps	Shapefiles	Yes	Yes
2D cross-section linework	GOCAD P-line format, 2D/3D shapefiles	Yes	Uncommon
Surfaces	Triangular irregular networks Grids (ESRI, accii) Contours (.dxf/.shp)	Yes Yes Yes	Sometimes Yes Yes
Thickness grids	ASCII/ESRI grids	Yes	Possible
3D shells	GOCAD s-grid	Yes	Yes
3D property grids	3D Ascii grid	No	Yes
CAD	dxf, dgn	Yes	Uncommon
Alternative proprietary software formats	Various	Sometimes	Sometimes

Figure 2    Geological modelling software packages used in Europe.

Figure 3    Formats of geological model outputs used in Europe.

Whilst the formats and type of data exported from geological models is fairly standard, it is important that users understand the methodologies by which the original models were created. Each methodology has particular limitations that may affect the form of the final model. The respective modelling methodologies are described in extensive detail in the literature (Mallet, 2002; Kessler et al, 2009, Norden & Frykman 2013) only a few prominent differences in the approaches are listed here:

• GOCAD

o  Mathematical interpolation between known data points. Additional interpretative input varies depending on modellers expertise and resources

• GSI3D

o  Explicit modelling methodology requiring modeller to interpret geology in cross sections and areal unit extents, strongly guided by the modelling geologist.

• Petrel

o  Predominantly based on statistical methodologies, principle format is a geo- cellular 3D grid (voxels)

In addition to geological model data, each model should therefore have a metadata report that explains the data used to create the model, the area covered, stratigraphic complexity, intended use, lineage, revisions, digital terrain model resolution and name, limitations and information about the geological context of the model.

Around one-third of models in Europe are also accompanied by uncertainty information. Because geological models are often derived numerically through the use of algorithms to generate geological surfaces, or to populate voxel cells with parameters, it is important that the relative certainty on the resulting model is known. The type of certainty information provided depends on the modelling methodology employed.

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