Electromagnetic Technique of Measuring Coal Layer Thickness

An FM-CW microwave system was investigated for measuring coal layer thickness. Measurements were made in three different mines near Pittsburgh, PA, Fairview, WV, and Coffeen, IL. Microwave frequencies in the range 1-2 GHz were used to measure samples up to 55-cm thick. All samples were backed with a naturally occurring shale. Measurements were also made on coal and shale samples compounded in the laboratory at the Bureau of Mines Pittsburgh Mining and Safety Research Center near Bruceton, PA. The results indicate that layer thickness can be determined in most cases, although large anomalies may, in some cases, produce misleading results. Many anomalies that were detected with the FM-CW system were verified visually by drilling into the coal layer. The dielectric constant of coal apparently varies significantly within a coal seam. It appears that this technique has the potential of measuring changes in the dielectric constant of a coal seam and providing an output that can be used for real-time corrections in layer thickness measurement. Key words include automation, coal, coal mine safety, dielectric constant, energy, microwave measurement, nondestructive testing, and thickness of coal layer.