[1] ENEA, SSPT, Rome, Italy
[2] Ist Nazl Geofis & Vulcanol, Rome, Italy
[3] Univ Bologna, Dipartimento Sci Biol Geol & Ambientali, I-40126 Bologna, Italy
[4] Univ Aldo Moro, CONISMA, Dipartimento Sci Terra & Geoambientali, Bari, Italy
[5] Univ Cagliari, CONISMA, Dipartimento Sci Chim & Geol, I-09124 Cagliari, Italy
[6] IAMC CNR Oristano, Torregrande, Oristano, Italy
[7] Univ Padua, Conisma, Dipartimento Geosci, I-35100 Padua, Italy
[8] Univ Trieste, CONISMA, Dipartimento Matemat & Geosci, I-34127 Trieste, Italy
[9] Lesia Observ Paris, Sect Meudon 5, Meudon, France