Coal Resource Classification System of the U.S. Geological Survey
By Gordon H. Wood, Jr., Thomas M. Kehn, M. Devereux Carter, and William C. Culbertson
GEOLOGICAL SURVEY CIRCULAR 891
Table 5. - Areas of reliability from figure 15 listed in proper overburden and coal thickness categories
| Geologic assurance category |
Coal thickness | ||
| 14 to 28 inches | 28 to 42 inches | 42 to 84 inches | |
| 0 to 500 feet overburden: | |||
| Measured | A, D, E | B, Q | C, R |
| Indicated | J, K, L | I, H | G, D |
| Inferred | O | N | M |
| 500 to 1,000 feet overburden: | |||
| Indicated | P | None | None |
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