2.1.3. Stratigraphy: formations and surfaces

The stratigraphic model is the regional backbone of the ground model: the formations and the surfaces that separate them. It is the first of the three levels, and both the lithological and the geotechnical models hang off it. The formations are named after the Belgian stratigraphic nomenclature maintained by the National Commission for Stratigraphy Belgium (NCS), so the units you build carry the accepted Flemish and Belgian names (Boom Formation, Diest Formation, the Ieper Group, the Quaternary cover).

2.1.3.1. Formations: adding them, and seeding from boreholes

The Stratigraphy stage is where you define the model, the set of formations. There are two ways to bring a formation into being, and they are worth keeping distinct.

• Add a formation by hand. You create it, name it, and place it in the stratigraphic order yourself. This is how you enter a formation you know should be there, or one no borehole has named.

• Seed from boreholes (“Fill from boreholes”). StrixGeo reads the observed stratigraphy of the imported boreholes and creates the formation set from it, across the whole model at once. This is the fast way to get from a set of borehole logs to a first formation list.

The fill step often meets two slightly different names for the same formation: one log wrote it one way, another slightly differently, or a formal and an informal name for the same unit. The fill dialog lets you lump them together into a single formation, so the model has one unit where the field logs had two spellings. StrixGeo also folds obvious variants together on its own (case, dashes, a chronostratigraphic prefix), but the lumping you do by hand is the authority.

When you lump, read the DOV code shown next to each candidate. Beside every informal name the fill dialog prints the DOV formal-stratigraphy code or codes the boreholes paired it with, and how many boreholes each came from. That code is the reliable identity to lump on: two field logs can spell a unit differently, or use an old and a new code for the same member, while the DOV code still says what the borehole actually recorded. A worked example: a layer logged as Ieperiaan may carry the code TtEg, and one logged Formatie van Hyon - Lid van Egem may carry HyEg. Both codes are the Egem member (the 2017 national nomenclature renamed the Tielt Formation to Hyon), so they are the same unit even though the informal names read nothing alike. Where StrixGeo recognises the code it also offers the DOV unit name (here Lid van Egem) as a one-click target, so you can accept the official member name for both instead of guessing which spelling to keep.

Formations carry a stratigraphic order: a lower order is an older formation, deeper in the column. Every layered view paints formations oldest-first so that younger formations render on top, the geological superposition rule. The one deliberate exception is the formation list in the inspector, which reads youngest-first like a borehole log; that is a presentation-only reversal and does not change the order. You can drag to reorder, and a manual reorder pins the formations so a later build never silently reverts your ordering. That matters for unusual stratigraphy (pinch-outs, unconformities, inversions) where an automatic age guess would be wrong.

2.1.3.2. Contacts, and how a unit is defined

A formation contact is a committed boundary: the depth at which one formation meets the next at a specific investigation. Contacts are the durable, audited record of your interpretation; every depth change is timestamped and logged, whichever part of the interface fired the edit.

A unit is defined by its top boundary. Its base is not stored separately: it is simply the top of the next unit below, or, where there is no unit below, the bottom of what the investigation reached. Move a contact and you move the top of the unit under it and the base of the unit over it at the same time; the column stays gap-free by construction.

Because a test’s formation contacts are a property of the test, not of any section, you classify a test once and its contacts appear on every section whose corridor contains it. There is no per-section step to make a formation contact visible.

2.1.3.3. The borehole log on the model

A borehole brings its observed soil log and its samples with it, and the Stratigraphy stage shows that log against your formations, so you can place a contact against what the driller actually recorded rather than against a bare depth axis.

2.1.3.4. Seeding on the model, against seeding per section

The three levels do not all seed the same way, and it helps to know which is which.

• The stratigraphic model seeds on the model: “Fill from boreholes” creates formations for the whole project at once. (A separate per-section verb can reveal already-created contacts on one section, but it never creates a new formation.)

• The lithological and geotechnical models seed per section: you run their suggestions on the section you are looking at, one section at a time, because those interpretations are built where you can see the columns side by side.

2.1.3.5. Surfaces, and the 3-D stratigraphic model

A surface is the interpolated top of a formation, built by triangulating across the contacts you have placed. The triangulation is a Delaunay triangulated irregular network (a TIN) today; other interpolation methods can be added later.

StrixGeo uses on-click build: editing a contact does not silently recompute the surface. It marks the affected surface stale, and the section view shows a stale surface dimmed and dashed, so you can see “this is your last build, refresh to update.” Only the “Build 3-D model” action re-runs the interpolation, so nothing recomputes behind your back. The built surfaces, stacked in the 3-D view, are the stratigraphic model in three dimensions.

Once built, a crossing validator samples each pair of surfaces and flags any point where a younger formation dips below an older one, raising a “surface crossings detected” banner so you can find and fix the inversion.

Faulting and overturned folding are future work. They are deemed unnecessary for the large majority of projects in Flanders, so they are left out of this version rather than half-built.

2.1.3.6. The DEM ground surface, and the elevation-trust rule

StrixGeo can carry a DEM-based ground surface as the ground between investigations. Where a real investigation has a surveyed collar elevation, the surface is warped to meet that collar exactly: it anchors a vertex at each collar and uses the DEM only for the ground in between.

The rule behind this is the elevation-trust rule: a surveyed surface elevation of a test is trusted more than a DEM pulled from the web. A surveyed collar therefore outranks the DEM at that point, and the DEM fills only the ground between tests. One corollary matters in Belgium: a ground level of 0 m TAW is a real value, not a missing-data marker, so StrixGeo treats a genuine 0.0 m collar as data. The 3-D view can also show a footprint ring marking the extent of the interpreted volume.

2.1.3.7. Drawing the canal bed: bed overrides

Over water the DEM is the water surface, not the bed. Where a site was dredged, that difference is large: a canal cut 8 m below the banks reads as if the ground there sat at the water level. A bed override fixes this by hand. You draw a polygon on the map and type one constant bed elevation in m TAW for it. Inside the polygon the ground surface takes that bed level instead of the DEM; outside, the DEM stands. You can draw several polygons, each with its own level, and edit or delete them from the list in the right pane.

The bed level feeds every consumer of the ground surface: the 3-D ground surface, the section DEM line (which shows the bed where the section crosses the polygon), and the surface clip described next. The lithological and geotechnical models follow it too: inside a bed polygon the section fill and the 3-D litho model stop at the bed level, so nothing lithological is drawn or built above the dredged bed (outside the polygon nothing changes). The 3-D fence walls and the fence DXF export are cut at the bed at once as well, but the built 3-D models (litho and geotech solids) and the stratigraphy surfaces cache the ground they were built on, so after you draw, edit, or delete a bed StrixGeo marks them for rebuild and a toast reminds you to Build again to cut them at it (the section and fence views are already cut). A surveyed test collar still wins over the bed at the test’s own location, the same elevation-trust rule as above.

2.1.3.8. When a formation surface meets the ground: the crossing-line clip

On Build, each formation surface is cut where it crosses the ground surface. The crossing is a line, and StrixGeo feeds that line into the surface as new points and drops the part above the ground. So a formation surface ends where it meets the ground: under a dredged canal, a unit that was present on both banks is cut away between them and is simply gone under the water, which matches the site. A surface that lies entirely below the ground, like a deep unit well under the bed, keeps its own top unchanged: it is never flattened to the ground. This replaces an older clamp that pushed every crossing surface down to a flat plateau at the ground level.

Bathymetry and water

Bathymetry from a survey file cannot be imported yet: the manual bed override above is the way to set the bed for now. Over water, the DEM otherwise stands in for the water surface, which makes it an approximation of the water level and very untrustworthy there: it is not the bed of the river or the sea. Full bathymetry import is worth adding later, and not only for a more honest picture of the ground. Recent alluvial and marine deposits are strongly shaped by water: a meandering river deposits on the inner bend and erodes the outer bend, so the two banks carry distinctly different material, and natural or man-made levee breaks and floodplain deposits are usually one-sided. Getting the water surface right is part of getting those deposits right.