Propagation of low and medium frequency radio waves in a coal seam

Abstract

A theoretical model has been developed to explain the unexpectedly long communication ranges of over 300 m obtained, and the marked polarization effects observed, during recent radio transmission measurements in a conductor‐free area in a coal mine at frequencies in the range of 57.5 to 920 kHz. The model is based on a dipolar cylindrical TEM mode of propagation, with E vertical and H horizontal, in a low‐conductivity horizontal coal seam bounded by high‐conductivity rock. The best overall fit to the experimental data over the frequency range 57.5 to 920 kHz is found for coal and rock conductivities of 1.4 × 10 −4 mho/m and 1.0 mho/m, respectively, for an assumed coal dielectric constant of 7. A better fit to the data at 57.5 kHz can be obtained if, in the model, the conductivity of the surrounding rock is allowed to be high for only a short distance above and below the coal seam and of a substantially lower value elsewhere. If the coal conductivity is changed from 1.4 × 10 −4 mho/m, which is near the lower end of the range of expected values for bituminous coals, to 1.0 × 10 −2 mho/m, which is near the upper bound of values for such coals, the attenuation is markedly increased with corresponding reductions in communication range.