GNGTS 2015 - Atti del 34° Convegno Nazionale

In GESC site, the velocity spectrum obtained was more difficult to interpret and the identification of the dispersion curve was not easy. However, from the inversion of the dispersion curve, the P and S velocity profiles show an important increment in the velocity at a depth of about 130 m. This discontinuity is probably related to the sediment-bedrock contact. Such result is in agreement with the H/V spectral ratio analysis and previous studies made in Gemona del Friuli: gravimetric survey (Furlanetto, 2004) and seismic noise investigation (Pavan, 2007). Active seismic. An active seismic investigation was performed in GESC site in order to determine the propagation velocity of compression and shear waves. Two NNO-SSE seismic profiles were acquired: one with vertical geophones and the other with horizontal ones. The P waves profile is made of 23 vertical geophones for a total length of 44 m, being the geophone interval equal to 2 m (Fig. 3). The S waves profile is made of 18 horizontal geophones, having a constant separation equal to 2 m, for a total length of 34 m. Several shots were executed on both sides of the profile in order to obtain conjugate profiles. For the P waves profile, eight off-end shots were executed at respectively 2, 4, 10 and 20 m from the external geophones, while, for the S waves profile, only six shots located at respectively 2, 4 and 10 m. The data was analyzed with Plus-Minus (Hagedoorn, 1959) and General Reciprocal (Palmer, 1980) methods. A three layers model was used for the interpretation of the P and S waves travel times. The reflector depth under each geophone and the velocity fields along the seismic profiles were estimated. Fig. 1 – On the top, a schematic representation of Gemona del Friuli alluvial fan form a geological and geomorfological point of view (Maselli, 2007). The location of the two investigated sites (GEMO and GESC) is also reported. On the bottom, the picture on the left displays the geometry of the seismic array applied in GEMO site. The location of the borehole is shown. The picture on the right represents the result of H/V spectral ratio. Each colored curve corresponds to the H/V spectral ratio determined for each window analyzed, the black line to the average H/V spectra and the dotted lines to the standard deviations. The resonant frequency is 1.0 Hz. 186 GNGTS 2015 S essione 2.2