Remnant roof coal thickness measurement with passive gamma ray instruments in coal mines

The need to leave coal on the roof of a mined-out area occurs on both continuous miner and longwall sections and is derived from considerations of ground control, quality control, machine guidance, or simply good operating practice. Efforts at measuring boundary coal thickness using the nucleonic method is described. Natural gamma devices are the instruments of choice, and the use of gamma-ray backscatter sensing is discussed. The calibration of natural gamma background (NGB) instruments is described, and it is concluded that they cannot be used in areas where NGB radiation is not present. This radiation is ordinarily present in the fine-grained sedimentary rocks that bound many coal seams