1. Department of Geology; University of Tartu; Ravila 14A 50411 Tartu Estonia
2. Geological Survey of Finland; P.O. Box 1237, Neulaniementie 5 FI-70211 Kuopio Finland
3. Solid Earth Geophysics Laboratory; Physics Department; University of Helsinki; P.O. Box 64, Gustaf Hällströminkatu 2 FI-00014 Helsinki Finland