GNGTS 2013 - Atti del 32° Convegno Nazionale

(velocità e Q ) che da quello elettromagnetico (conducibilità elettrica) potrebbero essere usati per verificare la relazione di cross-properties tra le proprietà sismiche ed elettromagnetiche. Riconoscimenti. Gli autori ringraziano il Dr. Alberto Michelini per le utili discussioni fatte durante questo lavoro. Per tutti i casi d’inversione è stato utilizzato il software tomografico CAT3D. Bibliografia Carcione J. M., Ursin B. and Nordskag J. I.; 2007. Cross-property relations between electrical conductivity and the seismic velocity of rocks . Geophysics 72 , E193-E204. Carcione J. M., Gei D., Picotti S. and Michelini, A.; 2012. Cross-hole electromagnetic and seismic modeling for CO2 detection and monitoring in a saline aquifer. Journal of Petroleum Science and Engineering, 100 , 162-172. Quan Y. and Harris J.M.; 1997. Seismic attenuation tomography using the frequency shift method . Geophysics 62 , 895– 905. Rossi G., Gei D., Böhm G., Madrussani G. and Carcione J.M.; 2007. Attenuation tomography: an application to gas- hydrate and free-gas detection . Geophysical Prospecting, 55 , 655 – 669. Vesnaver A. and Böhm G.; 2000. Staggered or adapted grids for seismic tomography? The Leading Edge, 19 , 944-950. Wielandt, E.; 1987. On the validity of the ray approximation for interpreting delay times; in Seismic Tomography , edited by G. Nolet, pp. 85-98, D. Reidel, Norwell, Mass. First results of Waterborne Geophysical surveys around the Malpasso site (Tuoro sul Trasimeno, Italy) for geological and archeological characterization L. Borgia 1 , C. Colombero 2 , C. Comina 2 , F. Del Bianco 3 , L. Gasperini 3 , F. Priore 4 , L. Sambuelli 5 , G. Stanghellini 3 , S. Trippetti 6 1 Assessore alla cultura, Comune di Tuoro sul Trasimeno, Italy 2 DST - Università di Torino, Italy 3 ISMAR - CNR, Italy 4 DiFeST - Università di Parma, Italy 5 DIATI - Politecnico di Torino, Italy 6 Archeologa, Rotta dei Fenici/Cammino di Annibale, Itinerari Culturali del Consiglio d’Europa, Italy Introduction. Geological characterization of shallow sediments in water-covered areas is a difficult task with traditional survey techniques. Direct investigations are indeed often neither cost-effective nor reasonably quick and adequate in number to cover the whole surface of a basin and to obtain a reliable correlation of data over a wide area. Geophysical techniques can conversely be very useful to investigate sediments which are entirely located beneath a water- covered area. This could allow for an uniformly spaced, even if indirect, investigation of the interested site. The integration of seismic and non-seismic waterborne surveys can be of great aid in defining related geological properties; the use of non-seismic methods to study shallow inland water is relatively recent but is becoming more and more diffused. Indeed electrical resistivity is very sensitive to fluid phases and clay content, while seismic waves are sensitive to the mechanical properties of the soil skeleton. Therefore a combination and integration of the two methodologies can offer a more complete geological characterization. In this respect we present in this paper preliminary results of waterborne seismic and electrical geophysical surveys on the nothern shore of the Trasimeno Lake in an area around the Malpasso site (Fig. 1). The Trasimeno lake, located in the Umbria Region of Central Italy, is the broadest lake of the Italian Peninsula, covering about 120 km 2 . The extremely low depths, the flat bottom morphology, and the absence of natural outflows and dams along its shorelines caused periodical floods since ancient times, only partially attenuated by creation of an artificial outflows. In contrast to most lakes in Central Italy (Bolsena, Vico, Bracciano, Monterosi, Albano) Trasimeno lake does not fill a volcanic crater, but its origin is related to tectonic processes. In fact, it underwent a complex geological evolution, from a marine gulf in the Early Pliocene, to a subsiding tectonic depression starting in Middle Pleistocene to present (Gasperini et al. , 2010). 186 GNGTS 2013 S essione 3.3