ON THE COMBINATION OF THE MAGNETOTELLURIC AND THE GEOMAGNETIC DEPTHSOUNDING METHOD FOR RESOLVING AN ELECTRICAL-CONDUCTIVITY INCREASE AT 400 KM DEPTH

The simultaneous use of two different passive electromagnetic (EM) sounding techniques for resolving the upper mantle electrical conductivity structure is emphasized: The geomagnetic (GM) method uses magnetic field variations at periods between 3 hours and 600 hours, yielding penetration depths between 300 km and 1000 km. The magnetotelluric (MT) method is applied to field variations at periods between a few seconds and 16 hours and yields penetration depths between a few km and 600 km. This is an extension of both methods, using additional Dst-data for MT and additional S(q)-data for GM. In application to field data from Western Europe the resulting overlap of the target depths includes an electrical conductivity increase from 10(-2) S/m at 300 km to 1 S/m at 700 km depth while a single conductivity jump at either 400, 500 or 700 km cannot explain the broadband data. Although the olivine to spinel transition is a possible explanation of the conductivity enhancement at 400 km depth, another mineralogy could also occur. This conductivity increase has not been resolved in other sub-continental profiles and lateral heterogeneity is probable in this depth range.