Numerical model of the selectivity effect and the ΔV/L criterion

Numerical solutions of Maxwell equations for a model earth with a reasonably conducting channel indicate that the electric field values are intensified within a certain region only (i.e., above the end of the channel), thus explaining the observed selectivity effect. In this region, the electric field may reach detectable values (5–10mV/km), while the magnetic field still remains low (10 −2 nT). The results are compatible with those obtained recently by analytical solutions ( Varotsos et al. [1998]). Both the numerical and the analytical solutions lead to a natural explanation of the ΔV/L≈const criterion. This criterion, however, should not be applied over an area with strongly inhomogeneous electrical structure.