REMNANT ROOF COAL THICKNESS MEASUREMENT WITH PASSIVE GAMMA-RAY INSTRUMENTS IN COAL-MINES

Current underground mining practice often requires that a predetermine d amount of coal be left on the roof of the mined-out area. The need t o leave such coal occurs on both continuous miner and longwall section s and is derived from considerations of ground control, quality contro l, machine guidance, or simply good operating practice. Efforts at mea suring boundary coal thickness have been employed mechanical, nucleoni c, and energy adsorption and reflection methods. The nucleonic methods have found application in operations in the United Kingdom, the Unite d States, the former Soviet Union, and Poland. Natural gamma devices a re currently the instrument of choice, and several successful installa tions exist. The calibration of natural gamma background (NGB) instrum ents must be carefully maintained, and they cannot be used in areas wh ere a NGB radiation is not present. This radiation is ordinarily prese nt in the fine-grained sedimentary rocks that bound many coal seams.