Geological modelling glossary: key terms defined
GEOREKA publishes this glossary of geological modelling and resource estimation terms, each defined in a short, self-contained entry.
The terms below are the ones that come up most often when building a 3D geological model, estimating a resource and reporting it under JORC or NI 43-101. Each definition stands on its own, and links to the GEOREKA page or guide where the term is applied.
Terms
A short glossary for geological modelling
Block model
A block model is a 3D model that divides a deposit into a regular grid of blocks, often with smaller sub-blocks at boundaries, where each block stores estimated values such as grade, density, domain and resource class. Tonnes and grade are calculated from the blocks.
Compositing
Compositing combines the sample intervals along a drillhole into intervals of equal length, or into intervals bounded by bench or geology, so that every sample used in estimation has the same support. Intervals are length-weighted when they are combined.
Declustering
Declustering assigns weights to samples to correct for preferential drilling, where high-grade areas are drilled more densely. It makes statistics such as the mean and the histogram represent the whole domain instead of the most heavily drilled part.
Domain
A domain, or estimation domain, is a volume of rock that is geologically and statistically homogeneous enough to be treated as one population. Domains are modelled and estimated separately, and the contacts between them can be hard or soft.
Geological modelling in GEOREKA →
Implicit modelling
Implicit modelling builds geological surfaces and volumes as isosurfaces of a scalar field that is interpolated from data, for example with radial basis functions, instead of digitising each section by hand. The model updates when the data changes.
Inverse distance weighting (IDW)
Inverse distance weighting estimates a value as a weighted average of nearby samples, with weights that fall as distance rises. It is simple and fast, but it uses no model of spatial continuity and gives no estimation variance.
JORC Code
The JORC Code is the Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves. It defines the Inferred, Indicated and Measured resource categories, and requires a Competent Person to take responsibility for the reported classification.
Classification and reporting in GEOREKA →
Kriging
Kriging is a geostatistical estimation method that gives the best linear unbiased estimate at a location, using a variogram model of how values correlate with distance. Ordinary kriging assumes an unknown but locally constant mean, and it also returns a kriging variance.
Resource estimation in GEOREKA →
NI 43-101
NI 43-101 is the Canadian National Instrument that sets standards of disclosure for mineral projects. Technical reports must be prepared by or under the supervision of a Qualified Person, and resources follow the CIM definitions.
Resource classification guide →
Orebody
An orebody is a body of mineralisation that can be mined and processed at a profit under the assumed conditions. A mineralised body that has not been shown to be economic is usually described as a deposit or mineralised zone instead.
Resource classification
Resource classification assigns each part of a Mineral Resource to the Inferred, Indicated or Measured category, according to the confidence in geological and grade continuity. Inputs include drill spacing, the number of informing holes and estimation quality. The Competent or Qualified Person signs off the result.
Classification support in GEOREKA →
Top-cut (grade capping)
A top-cut limits extreme high grades to a chosen value so that a few outliers do not dominate the estimate. The cut is usually chosen from the histogram or a log-probability plot of the domain’s samples, and applied before or during estimation.
Variogram
A variogram describes how the difference between sample values grows with the distance between them. It is summarised by a nugget, a sill and a range, and by anisotropy where continuity depends on direction. Kriging uses a fitted variogram model.
Wireframe
A wireframe is a triangulated surface or closed solid that represents the boundary of a geological body, such as a vein, a lithology or a weathering horizon. In GEOREKA, domain volumes are built from the implicit model and carried through to estimation.
Keep learning
From definitions to practice
Read the step-by-step guides, or see how each term is applied in the GEOREKA features.