Mutual impedance of circuits with return in a horizontally stratified earth

During recent years considerable attention has been given to investigation of the mutual impedance between two circuits with earth return. The problem is of importance. On a knowledge of this coupling depends the predetermination of the telephone interference which may arise from earth-fault currents or residual currents of harmonic frequency in power and traction systems. Theoretical formulae, which have been confirmed by field experiments, are available for its calculation where the earth can be considered to be homogeneous and of uniform conductivity. Experience has shown, however, that this condition is not always fulfilled. Where it is not, the assumption of an earth structure consisting of two horizontal layers of different conductivity would in nearly all cases give the accuracy required for engineering purposes. Curves have been prepared to facilitate calculation of the mutual impedance in the case of stratification of this type. They give an equivalent conductivity which, when introduced into the formulae relating to a homogeneous earth, will yield a mutual impedance equal in magnitude to that resulting from the given stratified conditions.