More precise recovery factors can be calculated by determining the total coal in place before mining occurred and the total coal mined in any given area. Those remaining coal resources that are determined to be unminable because of current land-use or technologic restrictions. Stratigraphically, in underground mining, the rock immediately overlying a coal bed. Often the upper part of a coal bed is left unmined to serve as a roof to the mining operations. A coal bed that is completely below drainage and does not crop out at the surface nearby, so that for all underground mining of that coal bed, the coal could only be reached by shaft.
Costs associated with shaft mining are higher than any other conventional method.
Those coal resources usually within a few hundred feet of the surface that would most probably be mined by surface mining methods. The restrictions could change with advances in science or modifications in the law. Geological factors are included as technologic restrictions in this report. Deep mine is synonymous with underground mine. This may occur before or after burial.