Earth‐resistivity survey at Huancayo, Peru

This survey, carried out near the Huancayo (Peru) Magnetic Observatory of the Department of Terrestrial Magnetism of the Carnegie Institution of Washington in the high Andes, is one of a series of earth‐resistivity surveys made in connection with the study of earth‐currents at observatories where potential‐gradient registration is in progress. The general resistivity of the region and its variation with position, depth, direction of current‐flow, and rainfall, were determined. All may affect potential records and are indicative of the geological structure. The resistivity of soil near the surface varied from over 100,000 to less than 2,000 ohm‐cm, depending on the character of the overburden. The mean values tended to converge to a value around 10,000 ohm‐cm as earth to depths of 200 to 300 meters was included in the measurements. These results are typical for an underlying structure of sedimentary rocks. A local area of high resistivity (values three times the mean for the region as a whole) was found near one earth‐current line and explains the high potential‐gradient records obtained from that line. The results were found to be independent of the orientation of the survey‐lines, indicating that the region is laterally isotropic. The seasonal variation was small and relatively unimportant.