Posted maps of coal bed thickness, overburden, aquifers, etc were made from reconnaissance drilling of a 15 mi² (39 km²) prospect. These data were then interpreted and hand-contoured to form individual maps of the several geologic factors that influenced the feasibility of in-situ gasification. Contours from each individual map were then digitized, thus allowing expert human interpretation, as reflected in the hand-drawn contours, to be entered into the computer. The computer then formed individual geological maps, structure surface and isopachs, from the sets of contour data. By devising various algebraic combinations of the maps and assigning various favorability weighting factors to the geologic conditions embodied within maps, several alternative interrelations and evaluations of the combined maps could be formed quickly within the computer. These alternate interpretations embraced a range of uncertainties in geological, engineering, and processing conditions.

Several features of this computer method are beneficial to geologic evaluation. The system of computerized combined map interpretation allows both continuous-valued and quantized data to be brought together in one analysis. Human insight and experience are included insofar as original data are input to the computer as complete hand-contoured maps, rather than as isolated points left to a "non-interpreting" gridding algorithm. To produce a computer evaluation of several maps, the geologist is obliged to organize his ideas and think in a quantitative and reproducible fashion about the interrelations of his data sets. Finally, the computer allows the geologist to explore and compare alternative hypotheses in far less time than is required by conventional methods.

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