Sedimentary Logging

A sedimentary log in VRGS is a measured section digitised on the outcrop itself. You walk a path up the section by clicking bed boundaries, and each click adds a unit with a grain size and a facies. The result is a graphic log that knows where it came from — so it plots in 3D on the outcrop, corrects its own thicknesses for dip, and correlates against its neighbours.

Two windows are involved:

The 3D view	Where you digitise the log path and edit its geometry. Ribbon tab: Log.
The 2D log panel	Where you draw the profile, edit units, and correlate between logs. Ribbon tab: 2D Log.

Logging a section

1. Make the outcrop the active mesh

The log path can follow the bedding rather than cutting straight lines between your clicks — but only if there is an active mesh to follow. Set the outcrop mesh active before you start.

2. Start the path

Interpretations → Sedimentology → Measured Section creates an empty log, brings up the Log ribbon tab, and arms the digitiser.

3. Click the bed boundaries

Each click adds a log unit running from the previous click to this one, with the active facies applied.

The first click just sets the base. From the second click onward, VRGS tracks the route between your markers across the mesh surface and stores the tracked path as the log's deviation — so the log tube drawn in 3D follows the outcrop rather than hanging in the air between markers. It also samples the mesh's per-vertex attributes along the path into a digital-log curve.

The tool stays armed, so you can work straight up the section.

Set the active facies as you climb

The facies dropdown on the Interpretations tab applies to each new unit as it is created. Changing it as you move between bed types is much quicker than correcting units afterwards — though Replace Facies on the log's tree menu will do a bulk correction if you forget.

No mesh, no tracking

Without an active mesh carrying a tracker, each click simply adds a straight segment. That is fine for a quick section, but you lose the surface-following deviation and the attribute sampling.

Geodesic Path

Log → Geodesic Path changes how the route between two markers is found:

On	The path is straightened onto the shortest on-surface route — a true geodesic.
Off	The path is restricted to the mesh's own triangle edges, so it follows the triangulation.

Edge-restricted is the safe default: it cannot cut across a face and lift the path off the surface. Turn geodesic on when the triangulation is coarse enough that following edges makes the path visibly zig-zag.

Getting the thicknesses right

A section measured on a dipping outcrop is not the thickness of the rock. VRGS holds three depth measures, chosen with Depth on the 2D Log tab:

Mode	What it is
MT	Measured thickness — the raw distance you walked.
TVT	True vertical thickness.
TST	True stratigraphic thickness — measured perpendicular to bedding. This is the one that compares between sections.

To get TST, VRGS needs to know the bedding orientation, and there are two ways to give it one:

• Log → Set Dip Domain applies a dip and dip azimuth to the units currently selected in the 2D log panel. Select a run of beds, set their domain, then do the next run — that is how a section crossing a fold or a fault is handled.
• Set Dip and Azimuth on the log's tree menu applies one orientation to the whole log.

The thickness of every unit is then recomputed by rotating it out of the dipping frame, so TST updates immediately.

TST is only as good as the dip you gave it

An uncorrected log silently reports apparent thickness. If two sections disagree and both were logged on dipping ground, check the dip domain before looking for a geological explanation.

Extracting an image strip from the outcrop

Log → Extract Image Track is the feature that has no field equivalent. It samples the texture of the outcrop mesh along the log path and flattens it into a straight, depth-registered strip — the outcrop equivalent of a borehole image log.

1. Make sure the mesh is textured and the log has a path.
2. Choose Extract Image Track and give a corridor half-width in metres — how wide a band either side of the path to sample.
3. The strip is written as a JPEG into the project's LogImages folder and added to the log, aligned to the depths it came from.

The path used is the tracked deviation where the log has one, and the unit-boundary deviation otherwise — another reason to log against an active mesh.

What the messages mean

"The active mesh has no textures to extract an image track from" — texture the mesh first. "The active log has no path to extract along" — digitise the log path first.

Editing the log

In 3D — the Log tab

Command	What it does
Edit Deviation	Drag the nodes of the log's path to correct its route.
Extend Deviation	Continue the path from either end by digitising further points.
Link Log Unit	Link a unit to an interpretation object in the scene.
Set Dip Domain	As above — the selected units' bedding orientation.

The Defaults panel sets the grain size, carbonate classification and facies that a new unit starts with. It does not change units that already exist.

In 2D — the 2D Log tab

Command	What it does
Digitise Profile	Draw the grain-size profile by defining unit boundaries down the log.
Split Unit	Cut a unit in two at a picked depth.
Add New	Insert a unit.
Sample Point	Add a sample at a picked depth.
Auto Spacing	Arrange the visible log panels tidily.

On the tree

Right-click the log in the Data tree:

Command	What it does
Spreadsheet View	The log as a table.
Statistics	Summary statistics for the log.
Replace Facies	Bulk-swap one facies for another.
Repair	Fix a log with missing or inconsistent units.
Import → From File	Bring in a log from file.
Import → Geophysical Log Tracks	Attach wireline-style curves.
Export	Write the log out.

Log templates

A log template controls how a log is drawn — which tracks appear, in what order, at what widths. Templates live in the Collections tree under Log Templates, and the 2D Log tab's template dropdown picks the active one plus lets you create a new one.

Set a template up once for the study and every log drawn afterwards matches, which is what makes a correlation panel readable.

Correlating logs

With the depth mode set to TVT, the Correlate panel on the 2D Log tab lets you tie horizons between logs:

1. Create a correlation pick (a named horizon) or select an existing one.
2. Use Correlate Logs and click the pick's position on each log.

Picks are shared project-wide, so the same horizon can be traced across every section in the study. Correlation panels themselves are listed in the Data tree under Correlation Panels.

TVT only

The correlation tools are only available in TVT depth mode.

Boreholes

Interpretations → Sedimentology → Borehole creates a borehole rather than an outcrop section — a log with a deviation path defined in space rather than digitised on a mesh. Wireline data comes in through the LAS import.

Tips and troubleshooting

• The log tube cuts through the outcrop instead of following it. There was no active mesh with a tracker when you digitised, so the units are straight segments. Set the mesh active and re-digitise, or correct the route with Edit Deviation.
• Thicknesses look wrong compared with the field. You are probably reading MT or TVT. Switch the Depth mode to TST and give the log a dip domain.
• Set Dip Domain is greyed out. It works on the units selected in the 2D log panel; nothing is selected.
• Every unit came out the same facies. The active facies applies at the moment of the click. Use Replace Facies to fix it in bulk.
• The image track came out blank or the command refused. The mesh must be textured and the log must have a path — the message says which is missing.
• My logs will not correlate. Correlation is TVT-only; check the depth mode.

See also

• Basic Interpretation — the digitising conventions all these tools share.
• LAS Well Log Import — wireline curves.
• Stratigraphy — giving units and markers an age.