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]:

Mine communications and telemechanics systems, such as mine rescue communications, mine announcement systems, communications with the engineers of conductor-rail and battery locomotives, communications with a moving cage, etc., are systems of mobile communications and telemechanics in which the best method is frequency and frequency-phase modulation.Other conditions being equal, frequency modulation gives superior radiotelephone range and quality. This is because in frequency modulation the amplitude of the modulated waves does not vary, and consequently the mean power is equal to the maximum power of the generator. This is all the more important because in a mine the radio transmitters of the systems must be low-power and "intrinsically safe" from sparks which might cause explosions.Service and mass-production requirements for the standardization of components and the simplification of the design and construction of various mine radio systems oblige the designers to make extensive use of previously developed components and subsystems. In the Control Systems Laboratory of the Institute of Mining of the Siberian Branch of the Academy of sciences of the USSR we have developed an FlVl transmitter circuit for mine communications and telemechanics systems.The transmitter has the following technical characteristics: carrier frequency 100 kHz; output power 4 W; frequency instability 5 9 10 -4 for AUfeed = + llYr] o and t = + 10-50"C; nonuniformity of frequency characteristic less than 3 dB in the 300-3000 Hz range; equivalent antenna resistance Req u = 80-280 ~; frequency deviation + 3 kHz; nonlinear distortion less than 6% at 400 Hz. Figure 1 shows the block circuit of the transmitter. It consists of four stages: a microphone amplifier, a frequency-modulated master oscillator, an externally excited IF oscillator and an externally excited push-pull output generator based on a common-emitter circuit. The output generator circuit reduces even-harmonic emission and yields the required power under load. The output stage makes use of medium-power germanium transistors T4, Ts, of type P602. To improve the heat balance, the output-stage transistors are fixed to a common heat sink via an insulating mica sheet.The IF oscillator is based on a common-emitter circuit with a P15 transistor with transformer coupling, since it is necessary to ensure not only a given excitation power but also a given excitation voltage amplitude to the out-