Electrical Properties of Eastern Bituminous Coal as a Function of Frequency, Polarization and Direction of the Electromagnetic Wave, and Temperature of the Sample

The development of electromagnetic systems to detect and monitor an in situ underground coal gasification process requires a knowledge of the electrical properties of coal. Using a two-path interferometer at microwave frequencies (≅9 GHz), the samples of solid eastern bituminous coal were tested as a function of temperature, electric-field polarization, and direction of the coal samples. There were slight decreases in the dielectric constant and conductivity of coal as a function of temperature from ambient up to 700° F. However, there were significant variations in the conductivity as a function of polarization, with the vertical polarization possessing lower conductivity. Only slight changes in conductivity were detected as a function of direction, with the face cleats exhibiting somewhat higher losses. The dielectric constant remained essentially constant as a function of polarization and direction of the electromagnetic wave. Measurements of electrical properties of coal were also made in the 0.5-100-MHz region using capacitance techniques. In general, there was a decrease in the values of dielectric constant and an increase in conductivity as the frequency increased.