Choosing drill-intercept boundaries
An intercept table is nested because mineralisation is nested — a broad
low-grade envelope, higher-grade zones inside it, and a peak sample or two
inside those. That is the parent / and / incl. shape you see in every
published release.
Choosing where those intervals start and stop is judgement, not arithmetic.
Real intercepts routinely open and close below their own internal peak and
swallow internal waste. An intercept averaging ~0.08 g/t Au can legitimately
open and close on samples below its own average, and carry a quarter of its
length as below-detection material. No grade cutoff reproduces that grouping,
which is exactly why geoDB's
composite() helper deliberately takes no cutoff parameter.
How a geologist does it by hand
Substitute below-detection values (half the detection limit is the usual convention, and it is what your project's Assay Merge Settings already apply), put grade in a column next to depth, and turn on Excel data bars. The column becomes a downhole profile you read vertically. You are looking for where mineralisation thickens — not where it crosses a number.
The multi-scale sweep
Petra reproduces that eye with a sweep across several scales at once. A single rolling window cannot work: one wide enough to see a 130 m envelope smears out a 6 m zone, and one narrow enough for the zone is blind to the envelope.
At each scale, Petra computes a length-weighted rolling mean of grade and keeps the contiguous runs that exceed the hole's own length-weighted mean:
| scale | window | multiplier | finds |
|---|---|---|---|
| envelope | 100 m | 1.3× | the broad swell — the parent row |
| zone | 30 m | 1.5× | and rows |
| shoot | 10 m | 2.0× | incl. rows |
| peak | 3 m | 3.0× | tight high-grade |
The tightest incl. rows are then taken as individual samples, not from a
window — a half-metre of high-grade sitting inside a multi-metre shoot is not
a statistical feature, and no amount of smoothing will find it.
Enrichment is always measured relative to that hole, never against an absolute cutoff, because grade populations differ from hole to hole and deposit to deposit.
What it is and is not
Benchmarked against a hand-built reference table, the sweep matched about two-thirds of the intervals closely — including the broad envelopes that simpler single-window methods miss entirely, and several boundaries to the metre with identical grades. It still misses isolated shallow hits that sit far from any envelope.
So these are candidate boundaries offered for review, not answers. Petra will tell you which scale produced each row. Move any of them and it will recompute; give it your own boundaries and it will use those instead.