1. National Institute of Geophysics, Geodesy, and Geography, Bulgarian Academy of SciencesAcad. G. Bonchev Street, Block 3, 1113 Sofia, Bulgaria
2. Institute of Geophysics, National Academy of Sciences of Ukraine32 Palladin Avenue, 03142 Kiev, Ukraine
3. National Antarctic Scientific Centre, Ministry of Education and Sciences of Ukraine16 Taras Shevchenko Boulevard, 02000 Kiev, Ukraine