Mapping fracture traces
A trace is where a fracture meets the outcrop surface — the line you would draw on the rock face with a marker pen. Traces are the raw observation almost everything else in this tutorial is built from: orientations are fitted to them, P21 intensity is trace length per unit area, and the DFN is validated by comparing its synthetic traces against yours.
This page covers digitising them by hand. The next page covers extracting them automatically from photographs — the two produce the same kind of object, and most studies use both.
Before you start: set the descriptor
On the Interpretations ribbon tab, the Active Descriptor panel carries a Facies and a Horizon dropdown. Whatever is set there is stamped on every object you create next.
Set it now, before digitising. Changing it afterwards means selecting objects and re-applying, and the whole point of the descriptor is that it makes your interpretation filterable and colour-coded later, when you have four hundred traces and need to know which unit each one came from.
Digitising a trace
- On the Interpretations tab, click Digitise Polyline.
- Click along the fracture trace on the model. Double-click to finish.
That is the whole operation. The interesting part is which digitising mode you pick, and the answer depends on the outcrop.
| Mode | When it is the right one |
|---|---|
| Digitise Points | The default. One vertex per click. Right for a fracture with a few clear inflections, and the mode that gives you the fewest vertices to tidy up later. |
| Draw | Hold and drag to lay down vertices continuously. Fast for a sinuous trace, at the cost of a lot of vertices — run Refine Spacing afterwards to thin them. |
| Geodesic Distance | Click sparse points and the line follows the shortest path across the surface between them. |
A straight segment between two clicks cuts through the rock wherever the face is not flat. A geodesic stays on it. That matters more than it sounds: P21 intensity is trace length per unit area, so a trace that chords across a re-entrant is measurably too short, and every trace in the study is short by a different amount.
Keeping the tool armed
Digitising two hundred traces one ribbon click at a time is miserable. Two commands fix it:
- Sticky Interp (Structure panel) keeps the tool armed after each object, so you finish one trace and start the next without going back to the ribbon.
- Pause Digitising (Edit panel) suspends a part-finished line so you can orbit the model, then resumes the same line. It does not end the trace — which is the point, when a fracture runs round a corner you cannot see from one viewpoint.
Turn Sticky Interp on before a mapping session and leave it on.
Tidying up as you go
All of these are on the Edit panel and apply to the trace you just made:
| Command | Use |
|---|---|
| Undo | Drops the last vertex while digitising. |
| Move Vertices | Click a trace to show its vertices, then drag one onto where the fracture actually runs. |
| Erase Vertices | Click vertices to delete them. |
| Snap Vertices | Snap each new vertex onto an existing one — for making two traces meet exactly at an intersection. |
| Join Lines | Ctrl-click two traces and merge them, when one fracture got mapped as two. |
| Split Line | The reverse, at a chosen vertex. |
Refine by Spacing (right-click the polyline → Quality & Refinement) thins a freehand-drawn trace to a chosen vertex spacing in metres. Smooth and Adaptive Refine are on the same submenu. Worth running over a Draw-mode session before you move on.
Classifying what you collect
A trace on its own is just a line. Two things make it a fracture trace as far as the rest of VRGS is concerned:
- Traces digitised as polylines are accepted directly as trace input by the intensity and DFN tools — select the group when the command asks for traces.
- Orientation measurements can be typed, and this is where the distinction between joint, fault and cleavage is recorded. Select them in the Interpretations tree, then right + click → Classification → Bed, Fracture, Cleavage, Foreset or Regional.
Classification is applied afterwards, in the tree — there is no "fracture mode" to arm before digitising. Do it in batches: select a whole group on the stereonet or in the tree and classify the lot in one go.
Right-click in the Interpretations tree → Group to put traces into named folders — by set, by unit, by face. Every downstream command takes a group as its input, so the folders you make here are the units of analysis later. A single flat list of four hundred traces will have to be split up eventually, and it is far easier now.
Scan lines: the other way to record fractures
A scan line is the virtual equivalent of stretching a tape across the outcrop and logging every fracture that crosses it. It produces something traces do not: P10, the number of fractures per metre along a known line, which is the classic field measurement of fracture intensity.
On the Structure panel, Scan Line has two modes:
| Mode | What it does |
|---|---|
| From intersections | Digitise the line; VRGS intersects it with the orientation measurements already displayed and records where each one crosses. |
| From Points | Digitise the line, then mark each fracture position along it by hand. |
Both need at least two points and finish on a double-click.
From intersections gives each logged fracture a measured orientation, so VRGS can report Fisher K, α95, normal set spacing and the Terzaghi orientation-bias-corrected P10.
From Points records positions only. With no per-fracture orientation those statistics are undefined, and VRGS reports them as zero rather than guessing. Raw P10 and direct spacing are still valid.
If you want bias-corrected intensity — and on a fracture set oblique to your scan line you do — collect the orientations first and use From intersections.
Scan-line results appear in the scan line's own properties: P10, Set Spacing, Normal Set Spacing, Fisher K, Mean Dip and Mean Azimuth. Measuring fracture intensity picks this up.
See also
- Basic Interpretation — the complete reference for every digitising and editing tool.
- Measuring — if what you want is a distance rather than an interpretation.
- Stratigraphy — recording which unit a fracture belongs to.
Next: fractures from photographs.