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]:

UDC 622.002 ~le rocks opened up by a coal mine constitute an inhomogeneous medium, owing to the large number of seams with differing electrical and physical properties. In some cases, rough calculations of a theoretical or engineering nature can be limited to a homogeneous model of the propagating medium. The energy losses incurred in transmission through a conductorless channel in a medium with the structure of a real rock mass are due to losses in the side rock strata, the electrical properties of which are similar owing to dependence on the moisture, temperature, and rock pressure. As an example of the use of a homogeneous model of the solid rock in an engineering calculation, we can take a conductorless link between different horizons across the stratification.The emission, propagation, and reception of an electromagnetic field in a homogeneous semiconducting medium have been quite exhaustively studied: the starting-point was the field of elementary antennas in a homogeneous medium with arbitrary characteristics. The general expression for Hertzian oscillators of the electrical and magnetic typos, used for transmitting longitudinally polarized electromagnetic fields,can be written as Me=~ -~t' [1 + j k r --k 2 r t) e -/k" sin O, (1) where /~I 0 is the longitudinal component of the electric field of an electric antenna or the magnetic field of a magnetic loop, i ~ is the dipole moment of the antenna, k is the wave number of the medium, r is the distance, and 0 is the angle between the axis of the antenna and the direction to the reception point.In the long-and superlong-wave ranges, an elementary oscillator of the Hertzian electrical type consists of a grounded electric dipole. End loads level out the current distribution along the antenna. The moment of the grounded dipole can be found from the equation:where I e and l are the current and length of the dipole, ~ is the complex dielectric permittivity of the medium, and w = 2r] is the frequency of the radiated field.In conduetorless mine communications systems use is made of loop antennas of the magnetic type, with a dipole moment ofwhere Ip is the current in the loop, and n and S are the number of turns and area of the loop.The electrical conductivity of the main mass of rocks in coal pits is o = 10-1-10 -3 mho/m [2, 3]. The dielectric permittivity is in the range (10-40)e 0. With such rock characteristics, the displacement currents in the Institute of Mining, Siberian Branch of the Academy of Sciences of the USSR, Novosibirsk. Khar'kov Institute of Radioetectmnics.