At low frequencies, the attenuation factor for an electromagnetic field in solid rock is governed by the eleetrical conductivity. In developing methods for experimental investigation of an electromagnetic field in the solid rock, it is therefore necessary to take account of the electrical conductivities of the individual seams and the solid rock as a whole in each case. A knowledge of these parameters facilitates the processing of the results. At the frequencies in question, the electrical parameters of the rock are characterized by the effective conductivity of the medium, which does not reflect even the average behavior of the true conductivity of the solid rock, but only its geoelectrical structure.We shall describe the most practicable methods of estimating the conductivities of rocks.Measurement of the True Conductivity of a Rock. Laboratory measurements are made less reliable by the fact that the electrical parameters of rocks depend on the geological conditions of their occurrence -moisture content, type and amount of salinity, pore water, temperature, and rock pressure. For measuring the electrical conductivities of the seams in underground workings in a coal mine, the most convenient method is the four-probe method. This forms the basis of a miniaturized measuring device [1], the schematic diagram of which is shown in Fig. 1.Measurement of the electrical conductivity reduces to measuring the potential difference between electrodes 6 and 7 and the current in the circuit of electrodes 4-5 [2]: