GNGTS 2013 - Atti del 32° Convegno Nazionale

The dominant structure in the study area is the Ispica (known also as Pozzallo-Ispica- Rosolini) fault zone that extends NE-SW for about 20 km adjoining the marls and calcarenites of the Ragusa Formation (lower Miocene) to the west with the blue clays of the Tellaro Formation (middle Miocene) to the east (Ghisetti and Vezzani, 1980; Carbone et al. , 1982; Grasso et al. , 2000) (Fig. 1). Fault planes dip on average ~80° toward the SW with a maximum vertical offset of 100 m, forming the Ispica-Capo Passero depression to the west. The NE-SW slope above which stands the convent corresponds to the Ispica fault plane (Fig. 1). Methodologies. In the last twenty years, down-hole seismic surveys has proven to be a valuable tool for the physical and dynamic characterization of soils. The down-hole technique allows to determine the average and interval seismic velocity of compressional (P) and shear (S) waves. In a down-hole seismic survey, a seismic source is placed on the surface near a borehole, and two tri-axial geophones are placed at selected depths in the borehole, to measure seismic velocity at fixed space intervals (Sambuelli et al. , 2004). The raw data obtained from a downhole survey are the travel times for compressional and shear waves from the source to the geophones and the distance between the source and geophones. The down-hole techniques do not require that seismic velocity increases with depth, because the arrival times are estimated as far as waves penetrate into the medium. Therefore, these methods are suitable to detect velocity inversions along the hole, as well as fractured volumes of rocks (Cosma et al. , 2001). We have carried out a down-hole survey in the 30m deep mechanical drillings S1-S3 (Fig. 2). Two 3-D geophones with natural frequency of 4.5 Hz are fastened to the walls of the hole through a pneumatic system. To measure shear waves velocity, the soil is energized by a hammer that hits transversely an iron plate placed at 2 meters of distance from the hole axis. Seismic signals are recorded in digital format and analyzed in the time domain with a dedicated software for P and S waves picking (CWP/SU). The arrival time of the S wave train is identified using the procedure of Boathwright (1987) that consists in rotating the two horizontal components of the ground motion until the maximum amplitude of the waves is reached, i.e. the source and the receiver are aligned. This point coincides with the arrival of the S waves recorded on the two horizontal components that will have opposite peaks at the same time. Fig. 2 – Lithostratigraphic profile reconstructed along the mechanical drillings S1-S3 (Imposa et al. , 2010). 47 GNGTS 2013 S essione 3.1