On the measurement of ground conductivity at VLF

The applicability of the four electrode methods of measuring ground conductivity at very low radio frequencies is discussed. The general theory is extended to include anisotropy in the substrata. In view of the spurious coupling between the current and potential line in conventional configurations, an alternative array is proposed which is arranged so that the inductive coupling is zero. A number of charts are computed which facilitate the interpretation of the measured or apparent conductivity in terms of a two-layer earth.