GNGTS 2013 - Atti del 32° Convegno Nazionale

imposing a constraint of a depth equal to zero in areas where geological strata with elastic characteristics of seismic bedrock outcrop. The kriging interpolation algorithm was used, obtaining a lattice with equidistance between nodes equal to 10 m. The map of the bedrock depth contours was obtained by extracting from the Digital Elevation Model of the area a grid of points equidistant 50 m. From these the corresponding lattice of the depth of the bedrock were subtracted. This choice was made in order to obtain a trend of the isobaths that was not excessively tied to topographic detail but which, however, did not present unrealistic areas with higher elevations of the topographic surface. It should however emphasize that the real detail of the maps can’t be higher than the sampling density of the HVSR measurements, which generally is not less than 300 m. Considering the geological configuration of the subsoil and the values of shear-wave velocity (deduced from direct bore-hole measurements) a preliminary identification was made of the geological structures that can cause seismic resonance. For the above mentioned reasons the inversion of HVSR curves and evaluation of the thickness of the cover did not take into account the available seismic down-hole data and instead, the mean values of shear-wave velocity of the cover, available in literature, were considered. In particular, we considered about 200 to 250 m/s for alluvium and about 350 to 400 m/s in correspondence of silty sands. Geological-technical model. The geological-technical model of Oliveri plain’s subsoil foresees the presence of a surface layer of clastic deposits consisting of incoherent sands with variable levels of gravels and silts. The thickness is defined only in the proximity of the Torrente Elicona, where is 16 - 17 m. In this area, below the alluvial layer, there are mixtures of fine sands and silts with bivalves. The groundwater level is placed in the borehole at a depth of 7.5 m. In the wide area of the plain which extends to the west of the Torrente Elicona data from direct tests are not available. However, based on hydrological maps added to municipality building plane, the presence of the groundwater level is attested at depths less than 15 m across the floodplain. The HVSR show the presence of a seismic bedrock at depths up to 70 m near the outfall of the Torrente Elicona. Fig. 3 – Oliveri. Map of the thickness of the sedimentary cover (left) and map of the altitude of the seismic bedrock above sea level (right). 239 GNGTS 2013 S essione 2.2