# Choosing drill-intercept boundaries

> intercepts are nested — sweep scales, never a cutoff

Source: https://geodb.io/guide/topic/drill_intercept_boundaries/ · Category: Understanding your data
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.