Earth‐resistivity measurements in the copper country, Michigan

During the summer of 1927 the Department of Terrestrial Magnetism of the Carnegie Institution of Washington and the Michigan College of Mines and Technology engaged in conducting jointly a series of earth‐resistivity measurements in the Copper Country of northern Michigan, where the geological structure is well known, to determine the value of measurements of resistivity and its variations as indications of the geological structure below the surface. The work done may be roughly divided in two parts: First, measurement and study of the variations in resistivity with depth, or with volume, to establish such general relations as exist between the two; and, second, the determination of the resistivity of specific rock‐flows, in order that more fundamental data be made available as an aid to the interpretation of the results of the former type of measurement. The results of the first part of the investigation bear out the evidence from previous work that resistivity‐determinations may be used to advantage in determining the location of certain underground discontinuities. The data obtained in the second part on the resistivity of specific rocks are also in line with previous results. The resistivity of porous, sedimentary rocks was usually under 20,000 ohms per centimeter cube, while traps and associated rocks of volcanic origin were found to have resistivities from five to twenty times as great.