OBSERV A nONS OF THE ELECTRICAL ASTHENOSPHERE BENEA TU SCANDINAVIA

The recent recognition that long period (i.e., of the order of hours) electromagnetic induction studies could play a major role in the detection of the asthenosphere has led to much interest amongst the geophysical and geological communities of the geomagnetic response functions derived for differing tectonic environments. Experiments carried out on the ocean bottom have met with considerable success in delineating the "electrical asthenosphere", i.e., a local maximum in electrical conductivity (minimum in electrical resistivity) in the upper mantle. In this paper, observations of the time-varying magnetic field recorded in three regions of Scandinavia, northern Sweden (Kiruna-KIR), northern Finland/northeastern Norway (Kevo-KEV), and southern Finland (Sauvamaki-SAU), are analysed in order to obtain estimates of the inductive response function, C ( w), for each region. The estimated response functions are compared with one from the centre of the East European Platform (EEP), and it is shown that the induced eddy currents, at periods of the order of 10 3-10 4 s, in the three regions flow much closer to the surface than under the platform centre. Specifically, at a period of- 3000 s, these currents are flowing at depths of the order of: KEV-120 km; KIR-180 km; SAU-21O km; EEP-280 km; implying that the transition to a conducting zone, of 0=0.2 S/m, occurs at around these depths. One-dimensional inversion of CKEV and CK1R shows tha