GNGTS 2016 - Atti del 35° Convegno Nazionale

562 GNGTS 2016 S essione 3.2 there is no evidence of the flysch complex, probably because its depth in this part would be greater than those of the stratigraphic logs. In that sub-area, the prevailing lithology achieved by drilling are the Pleistocene grey-blue clays, present in almost all considered logs, except for those located in a restricted eastern area (Catalano et al. , 2013). Execution and interpretation of HVSR measures. Between December 2015 and January 2016, 49 single-station microtremor measurements were carried out, in coincidence with or in the immediate neighborhood of the stratigraphic surveys, so as to link the HVSR inversion to the thicknesses of layers investigated by logs, allowing to characterize the seismic velocity of near surface deposits (Fig. 1). Most of the HVSR curves have clearly identified peaks, according to the SESAME criteria (SESAME, 2004). Anyway some curves have not entirely clear peaks (H/V ratio not higher than 2) but they are however considered likely to occur resonance effects, starting from a comparison with the log stratigraphy. Near surface stratigraphic and seismicmodelling. The stratigraphic and seismicmodelling was carried out with the help of Rockworks 15 software, by creating a geo-referenced database of lithological and seismic data relating respectively to geognostic logs and inverse models from HVSR curves. The data thus organized were interpolated with geostatistical analyses to build discrete 3D models, represented successively by means of sections and fences (Figs. 2 and 3). The parameter used for the seismic modelling (2D and 3D) is the shear waves velocity Vs. For the lithological modelling instead, the lithological data were considered according to the above cited reclassification. In particular, the 3D model of Vs velocity was used to derive vertical sections with depth of 150 m b.s.l. but also more detailed sections till a more limited depth of 30 m. Discussion of results. To verify the reliability of the 3Dmodels developed, 2D sections were extracted in areas with large numbers of data in order to create a highly reliable distribution of the various layers. Two of these sections are parallel to the coastline in approximately ENE- WSW direction and two ones are transversal. From comparative analysis of the 3D models we note that the most superficial portions (1.5-2 m) in the western zone are characterized by seismic layers with low density and poor mechanical properties. This is confirmed by low shear waves velocity values (about 200 m/ s). This first seismic layer is attributable to the overburden. Stratigraphic data confirm that approximately at depth of 2 m there is the transition between the incoherent deposits represented by the overburden and the calcarenite-sand complex, characterized by Vs about 350-430 m/s. In the western area at about 8-10 m depth from the ground level is evident a decrease of Vs characterizing the transition to a less rigid lithotype. This limit corresponds to the transition Fig. 2 – Representation by vertical fences of the lithological model of the Bandita plain (Palermo).