“Roke published a theoretical coal analysis method in 1978.
The method consistently produced results for ash content and thermal value within 2% and 400 Btu, respectively, of the average values determined in the laboratory”
– Geophysical Borehole Logging Handbook for Coal Exploration
G.L.Hoffman, G.R.Jordan, G.R.Wallis,
The Coal Mining Research Centre,
Edmonton, Alberta, 1982.