Earth resistivity and geological structure

N connection with inductive coordination problems it is frequently necessary to estimate low-frequency ground-return mutual impedances between power and communication lines. The distribution of currents in the earth is a major factor in the determination of these impedances. This distribution is controlled by the resistivities of the component parts of the earth's crust and the arrangement of these parts. In impedance formulas that are customarily used the effect of the earth is taken care of by the inclusion of a single parameter — the earth resistivity. For a homogeneous earth this would be the actual resistivity of the material composing it. But the crust is nowhere homogeneous; hence, the resistivity used in such formulas is always of the nature of an average of the resistivities of the several parts of the crust — it is termed the effective earth resistivity.