1. Lund University; Centre for Mathematical Sciences; Box 118, SE-22100 Lund Sweden
2. Institute of Petroleum Geology and Geophysics SB RAS; Pr. Ac. Koptyuga 3 Novosibirsk 630090 Russia
3. Novosibirsk State University; St. Pirogova, 2 Novosibirsk 630090 Russia