GNGTS 2017 - 36° Convegno Nazionale

348 GNGTS 2017 S essione 2.2 In this work, we focus on five seismic stations (CMP0, CDCA, ROM9, SANR, LAV9) installed in a different geological context. We show the strategy adopted for assessing the geological setting and velocity profile below the site and in the estimation of the soil class category. CMPO, SANR and CDCA are situated in alluvial environment (Reno alluvial plain, Veneto-Friuli plain, Alto-Tiber plain, respectively) where the soft deposits show significant thickness (> 100 m), whereas ROM9 and LAV9 are characterized by the presence of volcanic deposits belonging to the Colli Albani hills. At ROM9 the thickness of the volcanic deposits is the order of 50 meters, at LAV9 the thickness is larger (> 100 m). As first step, a conceptual model has been derived by geological field surveys and collecting the available geological information (scientific agreement between ISPRA and INGV). The results of this step are basically 2D geological models and a lithostratigraphic and lithotechnical classification of the outcropping units. Further, a geophysical survey at each site was carried out using surface-wave methods. We deployed 2D arrays of seismic three-components stations recording ambient vibration (or ambient seismic noise) in proximity of the target site to measure the dispersion curve following the recent guidelines (Foti et al. , 2017). Passive 2D arrays recorded ambient noise for a total duration of some hours at each site. The array geometry was defined according to the logistic, and when possible two geometries with a progressive larger aperture were used at a same site (Fig. 1). The maximum aperture of the 2D arrays varies approximately from 100 to 400 m using a number of seismic stations from 8 to 14 depending on the site. At LAV9 site, we combined passive 2D array with a linear array of geophones equally spaced and using an active source (a weight body of 50 kg falling down from a height of about 2 m). Frequency-wavenumber and spatial auto-correlation methods derive a dispersion curve (Fig. 2). The inversion of the dispersion curves jointly with the horizontal-to-vertical spectral ratio (H/V curve) provides the local shear-wave velocity (Vs) profile. The soil class was finally Fig. 2 - Example of unpicked and picked dispersion curve in the velocity-frequency plan for the small (top) and big 2D passive array (bottom panel) deployed nearby ROM9. The resolution and alias limits are also reported. On the right panel, the picked dispersion curves derived from the two arrays of ROM9 are over-imposed.