GNGTS 2015 - Atti del 34° Convegno Nazionale

and is completely composed by limestone and dolomite. PALA seismic station is installed on a rock outcrop. H/V spectral ratio. Seismic noise was recorded in the four sites: GEMO, GESC, POLC and PALA. The acquisition was performed with a Lennartz LE-3D/5s seismometer for about one hour. The analysis was performed with Geopsy software, developed during Site EffectS assessment using Ambient Excitations (SESAME) European Project, for ambient vibrations processing and site characterizations (Bard et al. , 2004). For each site, the resonance frequency and the polarization of seismic energy were determined. Furthermore, the application of a relation that links the resonant frequency to sediment thickness (Haskell, 1960) allowed estimating the bedrock depth. The fundamental frequency, f 0 , of a layer over a half-space is given by f 0 = Vs /4 H , where Vs is the shear wave velocity of the sediments and H is the total thickness of sediments. At GEMO site, located close to the borehole, at the border of the alluvial fan, a resonant frequency of 1.0 Hz was determined, with a significant local amplification. Knowing the bedrock depth from the borehole and applying Haskell relation to the resonant frequency, it was possible to determine the mean shear wave velocity of the sediments above the bedrock. The resulting velocity is 445 m/s, value compatible with the materials of the area. For GESC site, located at the middle of the alluvial fan, the determined resonant frequency is 0.8 Hz. The amplification of that frequency is significant. Since GEMO and GESC sites present similar geological conditions, the shear wave velocity used for the determination of the bedrock depth was the same found in GEMO. The depth of the bedrock evaluated with Haskell low is about 140 m. This depth is in agreement with the value found by a gravimetric survey (Furlanetto, 2004). In POLC site, located on San Floriano hill, the determined resonant frequency is about 1.1 Hz. Also for this site, the relative amplification is very high. For the bedrock depth estimation, it was supposed that the shear waves velocity is similar to those evaluated in GEMO site. The obtained bedrock depth is 102 m. For PALAsite, located directly on the carbonate bedrock, the seismic response do not present any resonant frequency, as expected. F-K. Four seismic arrays were performed in GEMO, GESC, POLC and PALA sites in order to characterize the surface layers. The geometry of the array used in each site was triangular. Each array was made by two set of three sensors arranged in two circles, with respectively 25 and 50 m of ray, having an angular distance equal to 120°, and one sensor in the center (Fig. 1). In each case, seismic noise was recorded for, at least, thirty minutes. The acquisition was performed with a Reftek acquisitor, model 130-01, and 7 Lennarts LE-3Dlite. The analysis was performed using Geopsy software. In particular, were used the tools for F-K analysis (Asten and Henstridge, 1984; Horike, 1985; Yamanaka et al. , 1994) and for the dispersion curve inversion (Sambridge, 1999a, 1999b). For each site, the theoretical array response was evaluated in order to determine the array resolution (Kmin/2) and aliasing (Kmax) limits (Fig. 2). The performance of an array with respect to the determination of phase velocity depends on its geometry and on the wavefield characteristics. In this paper, the results obtained for the two sites located in Gemona del Friuli (GEMO and GESC) will be described. For GEMO site, the velocity spectrum obtained with F-K method is reported in Fig. 2. The dispersion curve of the fundamental mode is apparent in the figure. The portion of curve used for the inversion, highlighted in black, has a limited frequency range. At low frequency, it is limited by the energy loss, the array resolution and the corner frequency of the geophones used, while at high frequency by aliasing. The velocity profiles obtained from the inversion and the respective dispersion curve are shown in Fig. 2. The velocity profiles of P and S waves reveal, for both cases, a sharp increase in the velocity at a depth of about 115-120 m. The P waves velocity increases from about 1200 m/s to 2200 m/s while the S waves from about 850 m/s to 1300 m/s. The depth of discontinuity is in agreement with the depth of the bedrock in the borehole: 115 m. GNGTS 2015 S essione 2.2 185