The natural electromagnetic field suggests some different types of measurement to determinate the electrical conductivity of the Earth's crust and upper mantle. In particular, if a measurement at two levels is possible, we can obtain the impedance from purely electric or purely magnetic data (Berdichevskii et al., 1969). In this note we study this second case which can have interesting developments in some applicative fields, as e.g. offshore or in presence of tunnels.