Surfaces and Stratigraphy

A surface is a contact or horizon: the base of a formation, an unconformity, a marker bed. Add one on the Features tab with Add surface.

A surface on its own is one field with one mesh, where the field is zero, built from points on it and any orientations you have. Give it units and it becomes a series: one field describing a whole stack of conformable units, with a mesh at each contact between them.

One Field for a Whole Series

Conformable units share their shape: the contacts in a sequence are roughly parallel, thickening and thinning gradually. Modelling them as one field captures that. Each contact is a different value of the same field, so the contacts can never cross each other, and a contact with few picks borrows its shape from its neighbours.

Select the surface and open the Stratigraphy tab. Units of lists the series' units from the bottom up:

Type a name and a thickness and click Add unit on top to add a unit above the others. Up and Down reorder them. The lowest unit has no base contact: everything below the first contact belongs to it.

Thickness only sets the field value at each contact. Use realistic thicknesses so the field grows at about one unit per metre, which keeps the fit well behaved, but the contacts themselves go where the picks put them.

Contact picks

Each contact needs its own picks, and each pick must say which contact it belongs to. Add one constraint set per contact: when the feature has units, the Add a constraint set form shows a Contact choice, and every row of the set takes that contact's value. Base of series (0) is for picks at the base of the lowest unit.

Orientations, such as bedding measurements, can go in any set: they shape the field everywhere.

Truncation

Younger rocks often cut older ones off. An erosion surface removes the tops of the units below it, and an intrusion or fault can end a surface abruptly. Select the older feature and open Truncated by:

1. Choose the younger feature. It must have a higher Age.
2. Choose which part to keep. Against a surface: Keep below its base (erosion) or Keep above its base. Against a fault: Keep its footwall side or Keep its hanging wall side.
3. Click Add truncation.

The older feature's meshes, its own and every contact, are cut exactly where they cross the younger feature's base: a series' lowest contact, or a surface's or fault's own mesh. Build the younger feature first; the older one waits for it and goes stale when it changes.

Onlap

A younger series is often laid down against an older one's irregular top, filling the hollows without eroding it. Select the younger series and open Onlaps. Choose the older series, keep Above its top (onlap), and click Add.

The younger series' contacts then end at the older series' top, its highest contact. In the block model it fills only the space above that top, leaving the older units intact. Without an onlap relationship, a younger series erodes the older one: it claims everything above its own base.

Faulted Series

A series is cut by faults like any surface: list them under Cut by. All of its contacts are offset together, because they are all values of one field.

Block Model

The Block model section of the Stratigraphy tab writes the whole model onto a regular 3D grid, which is added to the project as a volume layer.

• Cell size - the size of each cell. 0 divides the model's extent into about 100 cells along its longest side.
• Write block model - builds the grid, saves it in the model's grids folder and adds it to the scene.

The grid holds these values:

Only built features are included: build everything first with Build all.