N.A.C. Cressie, 1993, Statistics for Spatial Data,
Wiley.
Gomez, M. and Hazen, K. (1970). Evaluating sulfur and ash
distribution in coal seems by statistical response surface regression
analysis. U.S. Bureau of Mines Report RI 7377.
see also fields manual:
http://www.image.ucar.edu/GSP/Software/Fields/fields.manual.coalashEX.Krig.shtml