ELECTROMAGNETIC INDUCTION IN A SPHERICAL EARTH WITH NONUNIFORM OCEANS AND CONTINENTS IN ELECTRIC CONTACT WITH THE UNDERLYING MEDIUM .2. BIMODAL GLOBAL GEOMAGNETIC SOUNDING OF THE LITHOSPHERE

The vertical electric currents flowing between the inhomogeneous surface layer and the conductive layers of the Earth's mantle are substantially dependent upon the transverse resistivity of the Earth's crust and upper mantle. Although these currents do not contribute straightforwardly to a surface magnetic field, they change the vortex sub‐surface currents which contribute to it. Thus, there exists an indirect influence of leakage currents upon the magnetic field. This makes it possible to evaluate resistance of the high‐ohmic layers, which could not be evaluated if the Earth was laterally homogeneous and only the induction mode of the field was induced. This paper presents the first results of the bimodal sounding based on Sq‐variation data. An estimate of the lithosphere's transverse resistance T∼ 3 × 109Ω m2 has been obtained. Copyright © 1990, Wiley Blackwell. All rights reserved