Electrical measurements on soil with alternating currents

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The electrical properties of the earth play an important part in many modern applications of physics and electrical engineering. In the early portion of this paper a summary is given of the published information on the electrical properties of soil for alternating currents at frequencies from 50 to nearly 200 million cycles per sec. Particular interest is attached to the properties at radio frequencies, and it is shown that, while a certain amount of knowledge has been acquired by the use of field methods depending upon some characteristic of electric wave propagation, the results available apply to certain rather scattered ranges of frequencies and to some fairly localized portions of the earth's surface. The later portions of the paper are concerned with the investigation of the conductivity and dielectric constant of samples of soil by a laboratory method of measuring resistance and reactance at radio frequencies. This method was used for the majority of the measurements which were carried out at frequencies between 100 and 10 000 kilocycles per sec, but, by the aid of a standard type of capacitance bridge, the work was extended through the audio-frequency range to a power frequency of 50 cycles per sec. A series of preliminary measurements showed that reliable results could be obtained which were independent of the size and shape of the fixed condenser, in which the soil was packed as the dielectric. The conductivity and dielectric constant were both found to be very dependent upon the moisture content of the soil, except when this was within the normal range for the site under consideration. In a typical case the conductivity for dry soil, at a frequency of 1 200 kilocycles per sec, was of the order of 105 electrostatic units (corresponding to a resistivity of 9 megohms per cm cube), while at a moisture content of between 12 and 26 per cent a limiting value of between 108 and 2 × 108 electrostatic units (resistivity 9 000 to 4 500 ohms per cm cube) was obtaine