Once the deposit has been deemed economically mineable, the next step is to define the technological and economical framework for its exploitation.. The total in situ resources of a deposit are seldom entirely mineable, both for technological reasons (depth, accessibility) and for economic reasons (mining cost, cut-off grade). To define recoverable reserves within a given technological and economic context, or, conversely, to define the type of mining which will result in maximum profit from the deposit it is necessary to have a detailed knowledge of the characteristics of this deposit. These characteristics include the spatial distribution of rich- and poor-grade zones, the variability of thicknesses and overburden, the various grade correlations (economic metals and impurities).
To determine these characteristics, it is usual to sample on a small grid, at least over the first production zone. The results of this sampling campaign are then used to construct block estimations within this zone. These local estimations set the usual problem of determining estimators and evaluating the resulting estimation error. There will also be problems associated with the use of these local estimations: the evaluation of recoverable reserves depends on whether the proposed mining method is block-caving with selection units of the order of 100 m x 100 m x 100 m, or open-pit mining with selection units of the order of 10 m x 10 m x 10 m.
From experience, the mining engineer knows that, after mining, the actual product will differ from the estimations of the geologist. In general, he knows that tonnage is underestimated and, more importantly, quality is overestimated. In practice, this usually results in a more or less empirical ``smudge factor'' being applied to the estimation of recoverable reserves, which may reduce the estimated quantity of recovered metal from 5 to 20%. This is not a very satisfactory procedure, because it does not explain the reasons for these systematic biases between the geologist's estimations and actual production; in addition, it is far too imprecise when evaluating a marginally profitable new deposit.
Given the size of present day mining projects and the cost of their investments, the risks of such empirical methods are no longer acceptable, and it is imperative to set and rigorously solve such problems as those given below.