1. Geophysical Institute; Bulgarian Academy of Sciences; Bonchev Street, Block 3,; 1113; Sofia; Bulgaria
2. National Institute of Archaeology and Museum; Saborna Street 2,; 1000; Sofia; Bulgaria
3. Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography; University of California, San Diego; 9500 Gilman Drive,; La Jolla; California; 92093-0225; USA