1. Norwegian Radiation and Nuclear Safety Authority , P.O. box 55, 1332 Østerås , Norway
2. Vestfold and Telemark County Council, Fylkeshuset , Svend Foynsgt. 9, 3110 Tønsberg , Norway
3. The Njord Centre , Departments of Geosciences and Physics, , 0313 Oslo , Norway
4. University of Oslo , Departments of Geosciences and Physics, , 0313 Oslo , Norway