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.
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