Measurement of resistivity of large masses of undisturbed earth

Abstract

The method and equipment for measuring the resistivity of large masses of undisturbed earth, primarily in connection with earth‐current potential measurements at the Carnegie Institution of Washington, magnetic observatories at Watheroo (Western Australia), and at Huancayo (Peru), are described. Measurements, made at four sites in the vicinity of Washington, D.C., during the development of the method, are reported.Method and procedure are described in Part I. Four contacts to earth are provided at equal intervals along a straight line at the surface. A measured current is sent through the earth between the extreme points and the resulting difference of potential between the intermediate points is observed. These data, together with the length of interval between electrodes or contact points, yield the average resistivity of a body of earth whose linear dimensions are of the same order as that of the electrode interval.Part II summarizes the results of series of measurements with electrode intervals ranging from 5 feet (1.5 meters) to 1,800 feet (540 meters). These give some knowledge of the variation of resistivity with depth and determine approximately the depths of ground‐water and of changes in the geological sub‐structure.