1. Dept. of Geophysics UFPa, Campus do Guamá, P.O. Box 1611, CEP 66017-900 Belém, PA, Brazil
2. Geophysical Institute, Karlsruhe University, Hertzstr. 16, D-76187 Karlsruhe, Germany
3. IMECC/UNICAMP, Dept. of Applied Math., P.O. Box 6065, CEP 13081-970 Campinas, SP, Brazil