1. GEUSOstervoldgade 10, DK-1350 Copenhagen K, Denmark (sg@geus.dk))
2. Department of Geophysics, Niels Bohr Institute, University of CopenhagenDK-2100 Copenhagen 0, Denmark
3. Department of Earth Sciences, Uppsala UniversitySE-752 36 Uppsala, Sweden
4. Present address:Geological Survey of SwedenSE-751 28 Uppsala, Sweden
5. Institute of Geophysics, University of WarsawPasteura 7, PL-02-094 Warsaw, Poland