Grade Shells from Drill Assays: What an Exploratory Envelope Is (and Isn't)

A grade shell built from drill assays is a 3D surface drawn around the volume where interpolated grade is above a cut-off. It shows where the drilling says the better grade sits and how the holes connect. It is an exploratory envelope: no geological domains, variography, top cuts, density or economic limits are applied, so it implies no tonnage or contained metal, and a public figure should say so on its face.

Build a shell on the sample project. Turn on the grade shell in the Layout step and change the cut-off.

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What a grade shell is

Drill holes are lines through a volume. A grade shell joins them up: it estimates grade between the holes and draws the surface where that estimate crosses a chosen cut-off, so the inside of the shell is the part of the drilled ground that interpolates above it. On a figure, a shell makes a set of intercepts read as one body, which is why companies like them. It is also why they need care: the shell looks like a solid fact, and between the holes it is an interpolation.

Two kinds of shell

A modelled shell comes from geological modelling software, built by a geologist with domains, structural controls and judgement, and often reviewed as part of a technical report. An exploratory envelope is a quick, rule-based interpolation of the assays alone. Both have a place on an investor figure. They should never be confused, and a figure should name which one it shows.

Exploration Maps 3D does both jobs separately: a modelled shell exported as an OBJ mesh is loaded as a model shape and drawn as supplied, while the grade shell option builds an exploratory envelope from the assays. If you are weighing the two approaches, the comparison with desktop 3D software covers where each fits.

The method in plain words

This is how the grade shell in Exploration Maps 3D is built, step by step. The same description is in the tool, under How the shell is built.

Composite: each hole is split into 10 m length-weighted composites of the grade column the holes are coloured by. Only assayed lengths count, and a composite needs at least 5 m assayed.
Grid: a regular grid is laid over the block, with cells about one fortieth of its width, between 5 and 25 m.
Group the collars: holes whose collars are within 20 m of each other form one collar group and count once, so a fan of holes from one pad does not outvote the rest.
Interpolate: at each grid point, the nearest composite from each group is found; the nearest groups within the search radius, at most four, are combined with inverse-distance-squared weights, with a 5 m minimum distance.
Require support: at least three groups must be in range, or the point is left unestimated.
Set the reach: the search radius is 75 m, or 1.5 times the median spacing between collar groups for wider drilling. Nothing is estimated above the terrain or collar surface.
Draw: the surface is extracted at the cut-off with marching cubes and lightly smoothed.

Example Ridge sample 3D figure with a translucent teal grade shell at 0.2% Cu interpolated from the drill assays, with the disclosure line in the footnote — A 0.2% Cu grade shell built from the sample assays. The footnote line is added automatically (illustrative data).

What it is not

No geological domains, variography, top cuts, density or economic limits are applied, and the distances above are screening choices, not parameters fitted to your deposit. So the shell is not a geological model, not a mineral resource, and implies no tonnage or contained metal. Where drilling ends, the shell can close off or break into pieces; those edges are where the data stops, not geological limits. A shell that stops at the last hole says "we have not drilled further", not "the mineralisation ends here".

Choosing the cut-off

Pick a cut-off that matches how the release describes the mineralisation, usually the broad-interval cut-off used for the callouts. Then try a cut-off either side. If a small change makes the shell grow or shrink dramatically, the drilling is not constraining it and the figure should not lean on its shape. The Advanced settings let you set the search radius and cell size; leave them automatic unless you can explain the change, and record any change you make.

How to label it on a public figure

When the shell is shown, Exploration Maps 3D adds this line to the figure's footnote: "Grade shell: exploratory envelope interpolated from drill assays (≥ cut-off); not a geological model or mineral resource." The legend names it with its cut-off. Keep both. If you crop the figure or hand the layers to a designer, make sure the disclosure line survives, because the image will be shared without the release around it.

Disclosure note

This page explains a visualisation method; it is not legal or regulatory advice. Whether a grade shell may appear in a public figure, and how it must be described, is for the issuer and its Qualified Person to decide under the rules that apply to them. Mineral resources are estimated and disclosed under their own standards, which this method does not attempt.

A checklist before you publish a shell

To see the effect of the cut-off yourself, load the sample in Exploration Maps 3D, turn on Grade shell from assays in the Layout step, and change the value. The full figure workflow is in how to make a 3D drill hole figure.

The figure says it is an exploratory envelope, with its cut-off
The grade column is the one the release reports (Cu, or a stated CuEq)
The shell was checked at cut-offs either side of the one shown
Edges are not described as limits of mineralisation
No volume, tonnage or contained metal is quoted from the shell
The QP has reviewed the figure

Show the drilling, labelled honestly. The shell's disclosure line is added to the footnote automatically.

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Frequently Asked Questions

Is a grade shell a mineral resource?

No. An exploratory grade shell interpolates assays to show where higher grade sits between holes. It applies no geological domains, density, top cuts or economic limits, and implies no tonnage or contained metal.

Why does the shell break into pieces at the edges?

At least three collar groups must be within the search radius for a point to be estimated. Where drilling thins out, that condition fails and the shell closes or breaks. Those edges mark the limit of the data, not of the mineralisation.

Why is nothing estimated above the ground?

Grade cannot exist in the air. Points above the terrain or collar surface are excluded so the shell never pokes out of the topography.

Can I show a shell from our geological model instead?

Yes. Export it as an OBJ mesh and load it as a model shape. It is drawn as supplied; label it with what it is and who built it.