Geological Modelling

The geological modeller builds 3D surfaces and bodies from the observations you already have in a project: fault traces and contacts digitised in GIS layers, intervals and structural measurements down drillholes, and points from line databases. You describe what the geology is - which faults exist, which surfaces they cut, which units stack on which, where the ore is - and the modeller works out the shapes that honour your data.

This approach is called implicit modelling. Instead of joining points by hand into triangles, each feature is described by a smooth 3D field: a number defined everywhere in the model volume. The surface you see is where that field equals a chosen value, in the same way a contour line is where a grid equals a chosen height. Because the field exists everywhere, the surface is always closed and consistent, it extends sensibly between your data, and it can be rebuilt in seconds when the data changes.

What It Builds

Every feature takes its data from one or more constraint sets, described in Constraints. Building, mesh resolution and the fit parameters are covered in Building a Model. When something does not look right, see Troubleshooting.

Concepts

Model. A model is saved as a folder ending in .sigeomodel. It holds everything about the model: its features, constraints and relationships, and the surfaces and block models built from them. A model is independent of the project, so you can open the same model in several projects or send the folder to a colleague. In a project it appears as a layer in the Geological Model group.

Feature. Something you want to model: a fault, a surface or a grade shell. Each feature has a name, a colour and an Age. A higher age number is younger. Age decides which features are allowed to act on which: only a younger fault can cut an older surface, and only a younger feature can truncate an older one.

Constraint. One observation the field must honour: a point that lies on the surface, an orientation (a dip and dip azimuth), or a point that must be inside or outside a body. Constraints are copied into the model from their source layers, and each one remembers where it came from. When the source changes you Sync the set and only the rows that changed are updated. Weights, disabled rows and pins you set by hand survive a sync.

Relationship. How features interact: a fault cuts a surface (and offsets it), a younger feature truncates an older one (an erosion surface or a fault that ends another), a fault stops against or splays from an older fault, and a series onlaps an older series. Relationships are what turn a set of independent surfaces into a consistent geological model.

Preview and final. A preview is built quickly on a coarse mesh so you can see the effect of a change straight away. A final build uses a finer mesh with refined vertices. Both use the same fitted field.

Creating or Opening a Model

Hover over the Geological Model icon in the left flyout.

• New Model ... opens the New Geological Model dialog. Give the model a Name and choose a Folder: the model is created as <name>.sigeomodel inside it. Set the Extent, the box the model is built in. The quickest way is From layer: choose any layer, such as the drillholes or a GIS layer of your prospect, and its extent is copied in, enlarged by the Margin %. The model takes that layer's coordinate reference system, or the view's when no layer is chosen, shown as CRS.
• Open Model ... adds an existing .sigeomodel folder to the project.

Double-click the model in the flyout to open the Geological Model dialog.

Hovering over the model in the flyout lists its features, each with an eye button, an opacity slider, its name and a colour swatch. The eye and slider only change how the surfaces are drawn in the scene; the model itself is untouched. The swatch sets the feature's colour, which is part of the model, as on the Features tab. Click a name to select it, Ctrl-click to add or remove one, or Shift-click to select a range: the eye button, slider or swatch on a selected row then acts on every selected feature. A feature that is switched off in the model has no surfaces, so its row is greyed. The eye buttons and opacities are saved with the project.

The Dialog

The dialog has five tabs.

1. Model - the model's name, extent and default mesh resolution, and the switch Refit a preview whenever a feature's inputs change.
2. Features - the feature list with each feature's age, colour and build state, and below it the selected feature's geometry, relationships and Fit parameters. The eye button next to the On tick hides or shows a feature's surfaces without changing the model, handy for seeing one feature at a time; the flyout has the same button and an opacity slider.
3. Constraints - the constraint sets of the selected feature, the rows they hold, and Show constraints in the scene.
4. Stratigraphy - the units of the selected surface, and the Block model export.
5. Build - preview and build buttons, progress, and the build log.

The State column of the feature list tells you where each feature stands:

A First Model

The shortest path to a fault surface:

1. Create a model with New Model ..., taking the extent From layer.
2. On the Features tab, type a name such as Main fault and click Add fault.
3. On the Constraints tab, open Add a constraint set. Choose GIS layer as the Source, pick the layer holding points or lines digitised along the fault, and click Add and sync. If the layer has dip and dip azimuth columns, choose From attributes under Orientation first.
4. On the Build tab, click Preview selected. The surface appears in the scene within seconds.
5. When you are happy with it, click Build selected for a final mesh.

From there, add more faults and surfaces, tell the modeller how they relate on the Features tab, and use Build all to build everything in the right order.

Saving and Undo

Every change is written to the model straight away, so there is nothing to save separately. The model layer itself is saved with the project.

Most changes made in the dialog can be reversed with Undo and Redo: adding features, sets, relationships and units, and editing parameters, geometry and individual constraints. Three cannot:

• Removing a feature, with all its constraints.
• Removing a constraint set, with all its rows.
• Syncing a constraint set. Sync again after reverting the source instead.

The model can also be driven by the Assistant, which can carry out the same actions as the dialog.