Numerical model of the selectivity effect and the Delta V/L criterion

Numerical solutions of Maxwell equations for a model earth with a reasonabl y conducting channel indicate that the electric field values are intensifie d within a certain region only(i.e., above the end of the channel), thus ex plaining the observed selectivity effect. In this region, the electric fiel d may reach detectable values (5-10mV/km), while the magnetic field still r emains low (10(-2)nT). The results are compatible with those obtained recen tly by analytical solutions (Varotsos et al. [1998]). Both the numerical an d the analytical solutions lead to a natural explanation of the Delta V/L a pproximate to const criterion. This criterion, however, should not be appli ed over an area with strongly inhomogeneous electrical structure.