Evaluation of coal quality is traditionally done on borehole cores in the laboratory; it is expensive and time consuming. Now, using a microcomputer-based geologic work station, geologists can obtain inexpensively reliable coal-quality estimates in minimal time. Typical coal borehole logs (gamma, density, and resistivity) are calibrated with a few core analyses from a specific area by correlating dry BTU content (measured in the laboratory) with measured log values for each zone analyzed. Multiple regression techniques describe the relationship between logs and BTU content. The regression surface of BTU on gamma and density, or the regression hypersurface of BTU on gamma, density, and resistivity is used to estimate the BTU for user-specified depth zones in boreholes that have not been cored. Confidence surfaces nested around the regression surfaces and hypersurfaces bracket the estimated BTU values within preselected confidence intervals.

The estimated BTU content of a coal seam is expressed as an expected value per depth increment. Each is easily qualified with user-specified confidence limits. Expected values can be averaged over any zone of interest, either an entire coal seam or a zone within the seam. Expected coal-quality values for numerous boreholes can be contoured to produce a coal quality isograd map. The volume of coal within any isograd can then be estimated. With a stand-alone geologic work station, geologists can make estimates and relevant maps on site; they no longer need to wait for core analyses as results are immediate.

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