Mapping Coal Quality Parameters for Economic Assessments
M. E. Hohn, C. J. Smith, K. C. Ashton, G. H. McColloch, Jr.
This study recommends mapping procedures for a data base of coal quality parameters. The West Virginia Geological and Economic Survey has developed a data base that includes about 10,000 analyses of coal samples representing most seams in West Virginia. Coverage is irregular and widely spaced; minimal sample spacing is generally greater than 1 mi. Geologists use this data base to answer public and industry requests for maps that show areas meeting coal quality specifications.
Local variability in parameters, such as high total sulfur, results in ordinary contour maps showing many anomalies. Contour maps are smoothed through use of linear kriging to estimate expected values. With kriging, the geologist controls smoothing to a degree appropriate to each variable. Variabilities in Btu require less smoothing compared with sulfur or ash content.
Smoothed maps give a geologist the average quality of coal in a number of samples or over an area. These samples or different parts of a mine show variability around this average. By contouring the probability of exceeding one or more cutoffs, the geologist can measure this variability. In one study of the Pittsburgh coal, large parts of the area mapped met specified limits for sulfur percentage, but some coal that did not meet the specifications could be expected in all areas. This approach is used to compute frequency distributions for coal parameters in areas such as 1-mi2 blocks.
Geostatistical analyses help the geologist determine optimal mapping procedures for individual parameters and lead to a better understanding of the effects of statistical variability, number of samples, and sample spacing on map quality.
AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.