Ground conductivity considerations

During 1964, RCA developed the H/I conductivity measurement technique as the most appropriate method of determining the effective earth conductivity for the siting of an ELF transmitting system. In the fall of 1964 and the summer of 1965 and 1966, measurements by this technique were carried out at 43 different locations in Northern Wisconsin and Northeast Minnesota. Most of these locations were in the area where the geology was indicative of low conductivity or in areas adjacent to the indicated low conductivity area to determine if the region of low conductivity extended beyond the geological boundary. The measurements indicated a high degree of correlation between low conductivity and those areas for which the surface geology projects a shield area of Pre-Cambrian origin. Twenty-seven sample measurements within the shield area of Northern Wisconsin give an effective value of conductivity of1.6 \times 10^{-4}mhos/m at 78 Hz with upper and lower standard deviation limits of1.9 \times 10^{-4}and1.25 \times 10^{-4}mhos/m, respectively. 1968 measurements along the Test Facility lines give values in agreement with the above.