naturally occurring deposits in such forms and amounts that economic extraction is currently or potentially feasible (Wood et al., 1983). Coal reserve is a more restrictive term describing the part of the coal resource that can be mined economically, at the present time, given existing environmental, legal, and technological constraints (Wood et al., 1983). Coal reserve estimates are often considered the more important parameter because they quantify the amount of recoverable coal. However, coal resource estimates are also important because they are the basis for reserve estimates, and in areas where the data required for defining reserves are missing or inadequate, they provide an indication of the amount of coal in the ground.

The coal resource and reserve classification system currently in use in the United States (Figure 3.1) has undergone more than a century of development. The current system was adopted in 1976 by the U.S. Geological Survey and the U.S. Bureau of Mines (USDOI, 1976) and modified in USGS Circular 891 (Wood et al., 1983). Circular 891 established a uniform foundation for coal resource and reserve assessments by providing standard definitions, criteria, guidelines, and methods. Circular 891 defined coal resource and reserve classes according to their degree of geological reliability (horizontal axis) and economic feasibility (vertical axis) (Figure 3.1), with reliability categories based on the distance from data

FIGURE 3.1 Definition of coal resource and reserve classes based on the geological reliability (horizontal axis) and economic viability (vertical axis) of resource estimates. This diagram, often referred to as the McKelvey diagram after a former director of the USGS, represented the state of the art for resource depiction at the time of its publication. SOURCE: Wood et al. (1983).



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