Some Measurements of the Electrical Constants of the Ground at Short Wavelengths by the Wave-Tilt Method

摘要

The paper gives an account of some measurements of ground electrical constants ? and ? made by the tilting-rod apparatus, being a continuation, on shorter waves, of the experiments described in an earlier paper. The employment of higher frequencies involved some modification of the apparatus and also an extension of the theory to allow for the effect of the dielectric constant, which begins to influence the results under these circumstances. The paper divides the measurements into four groups: (1) Measurements made at one site over prolonged periods to investigate seasonal variation. (2) Measurements on selected sites over a large range of wavelengths to determine ? and ?. (3) Summary of all measurements made at a number of different sites to determine ? only. (4) Measurements made on sites with abnormal ground surface, singled out in a separate table for the purpose of emphasis and comment. A theoretical section is devoted to the extension of the analysis of the field of a plane wave advancing over the surface of an imperfect conductor, in order to derive a simple relationship between wavelength, tilt angle, conductivity, and dielectric constant. Values of ? and ? are obtained by plotting a suitable function of tilt angle against frequency and choosing from a series of theoretical curves, plotted for different values of ? and ?, that which most nearly fits the results. It is concluded that no seasonal change of ? was detectable, but that changes from wet to dry weather had an appreciable effect and that the conductivity was not appreciably different from its long-wave value. The series of measurements taken on a large area of ice over deep fresh water probably constitute the most interesting results, giving values of 100 for ? and 1.4 x 108 for ?. Theory indicates that the results may be regarded as applying to the water rather than to the ice. This is confirmed by the value of ? obtained, which corresponds approximately to the normal ?static? value of the dielectr-nic constant of water and is many times greater than that of ice.