Logging: Image Strips and Interpretations

A log can carry a strip of the outcrop photograph straightened along its path, and the interpretations that cross it. Digitising a log and editing its beds is in Geological Logging.

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. Show the log and the textured outcrop it runs across. The mesh need not be active: VRGS takes the mesh the log is attached to, then the active mesh if it is shown and textured, then the nearest shown, textured mesh the path crosses.
2. Choose Extract Image Track and give the image width across the log in metres — the width of the band sampled across the path. It starts at 1 m and remembers the last width you used.
3. The strip is written as a JPEG into the project's LogImages folder and added to the log, the right way up and 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.

The strip's rows are spaced by height up the path, the way the log measures TVT, so a cliff face and a ledge on the way up both land at the right depth. In TST and MT each bed's part of the strip is stretched to that bed's thickness in the mode, so the image stays against its beds in every depth mode.

The outcrop's profile

Resistant beds, such as cemented sandstones, stand out from a weathered face, and recessive ones, such as mudstones, sit back under them. That weathering profile can stand in for grain size where a bed cannot be reached. Extract Image Track measures it while it samples the strip. It records where the face is across the strip at every height, and how steep the face is, and saves them beside the image as a .profile.csv file.

Add an Outcrop Profile track to the panel's template to see it. The track draws the standout of the last image extracted along the log:

• To the right of the middle line, shaded, a bed stands out from its neighbours.
• To the left, it is set back.
• The axis is in metres; the track reaches the largest standout on the log.

The standout is the face's position less its trend up the log. The trend is a smooth line fitted over the track's Trend Window (5 m by default; set it in the track's Properties). It is fitted so that a single bed does not pull it towards itself. The window matters:

• A few beds long shows what each bed does against its neighbours.
• Shorter than the beds fits them away, and the track goes flat.
• Much longer lets the outcrop's overall shape, such as a talus slope at the foot or an overhang near the top, back into the track.

The standout is not measured from the log path, which is digitised on the face, nor from a plane through the base, which would carry the whole outcrop's shape.

An image extracted before this release has no profile: extract it again to get one.

Grain size from the standout

How strongly standout follows grain size depends on the outcrop: its climate, its cements, which way it faces. So VRGS does not assume the relation. It fits it on each log to the beds you have logged by hand, and offers it for the rest.

1. Log some beds by hand: three at least, standing out by different amounts.
2. Select them, right-click one on the grain-size track, and choose Calibrate Outcrop Proxy on N logged beds. The messages panel reports the fit: how many beds it used, its r-squared, and the grain size it gives a bed standing out 0.1 m and one set back 0.1 m. It warns you when the finer beds stand out.
3. Turn on Show Outcrop Proxy in the grain-size track's Properties. The grain size the fit gives is drawn dashed in blue across the track, as a guide to log against.
4. To use it outright, select beds and choose Grain Size from Outcrop Proxy. Each bed's profile is set from the fit: its base, middle and top from the standout over each third of the bed, so a bed that fines upward fines upward. It is one step of undo. A bed where the outcrop has no profile is left as it is.

The fit is saved with the log. It keeps the trend window it was made with, which is the Outcrop Profile track's when the panel has one. Beds you set from the proxy are counted as logged if you calibrate again with them selected, so select only the beds you logged yourself.

Contacts and facies suggested from the outcrop

The same curves can suggest where the contacts are, and which facies a bed is, on the part of a log you could not reach. They read the outcrop profile and the image strip's colour and texture row by row, and they learn what this outcrop's beds look like from the beds you logged by hand on the same log. Nothing is assumed about how a facies looks on a face.

Both are on a bed's right-click menu in the 2D log view when the log has an outcrop profile. They act on the selected beds, or the bed you right-clicked. Each is one step of undo.

• Suggest Contacts from Outcrop cuts the beds at the places where the curves change. It compares how they read over a short window below each height with the window above it. The window and how strong a change must be are fitted to the contacts you logged on the log's other beds: the messages panel reports how many of them the fit finds, and how many of the contacts it finds there are yours. With fewer than three logged contacts it uses a 0.25 m window and says so. A new contact is drawn inferred, queried and grey.
• Suggest Facies from Outcrop gives each selected bed that has no facies the facies of the logged beds it most resembles. A bed is compared by each curve's mean and spread over it, and by its thickness. It needs four beds with a facies you set by hand, in at least two facies. The messages panel reports how often the logged beds, each left out in turn, get their own facies back: read that as how far to trust the rest. A bed whose nearest logged beds disagree, or that is unlike all of them, is left alone.
• Accept Outcrop Suggestions confirms the suggestions on the selected beds. A suggested contact becomes certain.

A suggested facies is drawn with its right quarter washed out on the Facies track, on screen only. Setting a facies or a contact certainty any other way (a number key, the palette, the properties, the Bed Table) confirms or replaces the suggestion.

Beds you have only had suggested do not count as logged: a later suggestion learns from your own beds alone.

More than one image on a log

A log can carry several images: the strip extracted from the outcrop, and a scanned field log or photograph added with Add image on the image track's right-click menu. Where their heights overlap they sit side by side on the image track, each in a column of its own; an image that overlaps no other keeps the whole width. The SVG export places them the same way.

An imported image can be lined up with the beds by dragging it on the image track in Move mode:

• Drag the middle of it to move it up or down the log.
• Drag its top or base edge to stretch it. The cursor turns to an up-down arrow over an edge and a four-way arrow over the rest of the image.
• Esc during a drag puts it back; once dropped, the move is one step of undo.

Its Top and Base can also be typed in the Properties panel, as heights in metres above the log's base (TVT).

An image extracted from the outcrop stays where it was extracted, since its position is the model's: it cannot be dragged, and its Properties show it as Extracted from the model (locked).

What the messages mean

"No textured mesh is visible along this log's path" — show the outcrop the log was digitised on, and texture it if it has none. "The active log has no path to extract along" — digitise the log path first.

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3D interpretations on the log

The 2D log panel can show where the interpretations in the 3D view meet each log: the polylines, orientation measurements, geopolygons, interpreted surfaces and fracture scan lines that cross it or pass near it. Right-click the panel background and tick Show 3D Interpretations. Each one is drawn as a dotted line across the log at the depth it meets it, in its contact's colour, labelled with its name and how far it passed from the log.

Panel right-click	What it does
Show 3D Interpretations	Turn the overlay on or off. It starts off.
Interpretation Range (… m)...	How far from the log an interpretation may pass and still be shown; 2 m to start with.
Rescan Interpretations	Look again after editing interpretations in the 3D view.

Only interpretations shown in the 3D view are included. A log that passes through a geopolygon or a closed surface meets it more than once, and every crossing is shown. A geopolygon's two crossings are labelled base and top.

Right-click a crossing:

• Place '…' pick here places a contact pick on the log at that depth, as Correlate Logs would, and makes the contact the active one, so the next log can carry it on. A trace or a fault gives its own contact. A geopolygon's base and top give the Older Contact and Younger Contact of the interval it lies in, or of the stratigraphic unit in the Interpretation tree for the unit it lies in. With no such contact the item is greyed out and says what is missing.
• Assign interval '…' to the N beds it spans, on a geopolygon that lies in an interval, gives that interval to the beds between its base and top. A bed counts when its middle lies between them.
• Assign facies '…' to the N beds it spans gives the same beds the geopolygon's facies.