1. Institute for Geosciences and Earth Resources-CNR Section of Florence; Via La Pira, 4 50121 Florence Italy
2. Centro Enrico Fermi, Piazza del Viminale 1; 00184 Rome Italy
3. Department DICAM, Alma Mater Studiorum; University of Bologna; Via Terracini 28 40131 Bologna Italy
4. Department of Physics and Astronomy, Alma Mater Studiorum; University of Bologna; Viale Berti Pichat 6/2 40127 Bologna Italy