Measurements and Modeling of Through-the-Earth Communications for Coal Mines

This paper presents modeling results from the National Institute for Occupational Safety and Health research into through-the-earth (TTE) communications technology for underground coal mines. Research focuses on the factors controlling the propagation and coupling of radio signals between transmit and receive antennas separated by earth or coal. Most TTE systems use single or multiturn loops of conductor for the transmit antenna. We compare the magnetic field distribution predicted from analytical formulas to the predictions of a method of moments computational electromagnetic (CEM) code. The predictions are compared in free space, in a homogeneous earth, and with the effect of the presence of the surface of the earth. The evaluations are done with the transmit loop buried in the earth and with the loop above the surface. The analytic results are shown to agree reasonably well with the more detailed CEM predictions for the situations considered, reducing the need for expensive and complicated CEM codes in analyzing simple TTE configurations. The predictive methods are applied to TTE measurements made in 94 different coal mines by the Bureau of Mines in the 1970s, and the implications for the apparent conductivity of the earth are discussed.