Drawing a cross section: correlating layers between boreholes

Updated: 2026-08-13 · ISKRIT

Boreholes are points. A cross section is a surface drawn through them, and everything between the boreholes is interpretation. The drawing rarely says which parts are measured and which are inferred, and that is where it misleads.

Matching layers

Correlation is done on origin and description first, not on depth. Two sand layers at similar elevation in adjacent holes may be the same deposit or may be a channel fill cutting through an older bed. The evidence for the match comes from the material itself: grading, colour, inclusions, the sequence above and below. Depth alone matches layers that have nothing to do with each other.

What the spacing allows

A pinch-out, a channel, a buried surface and a fault all live comfortably between two holes 60 m apart, and all four are invisible on the section that joins them with a ruler.

Vertical exaggeration

Sections are almost always stretched vertically, often ten times, because otherwise the layers are lines. The exaggeration makes gentle slopes look like scarps, and readers who miss the scale note plan foundations around a slope that does not exist. Two habits help: state the exaggeration on the drawing in words rather than only as a ratio, and keep it the same across every section in a report so sections can be compared.

The water surface is not a layer

A water table drawn as a continuous line across the section suggests it was measured continuously. It was measured in each hole, on a date, sometimes on different dates. Where holes were logged weeks apart, the line joins readings from different seasons. The dates belong on the section next to the levels.

What to show as uncertain

The useful convention is simple: solid where a boundary is proved by a log, dashed where it is inferred, absent where there is no evidence. A section drawn entirely in solid lines claims more than the investigation supports, and the reviewer has no way to see which claims are weak.

The cross section tool draws inferred boundaries differently from logged ones, carries the water level with its reading date, and prints the vertical exaggeration on the drawing.