GNGTS 2015 - Atti del 34° Convegno Nazionale

For the P waves, the first layer presents a regular trend, with a depth increasing in SSE direction, while the second one presents a more irregular trend. In Fig. 3 is represented the 2D velocity profiles, where it is possible to estimate the depth of different refractors. The obtained velocity model exhibits a velocity gradient both vertical and horizontal, with a velocity increase in south direction. The first layer has a velocity between 400 and 540 m/s, with a thickness of 4-6 m. The second layer shows a velocity increase from 600 to 700 m/s and a thickness variable from 3 to 7 m. The deepest layer presents a maximum velocity close to 1100 m/s and a depth of about 13 m. The obtained velocity values are compatible with alluvial fan materials and the lateral velocity variations are coherent with the geomorphological setting of the area. It is important to highlight that the model obtained using refracted waves is correct only if there are no low velocity channels (i.e. velocity inversions) in the surface. Fig. 1 – On the top, the velocity spectrum (slowness-frequency) obtained with F-K method for GEMO site. The dispersion curve of the fundamental mode is easily identifiable. Only the portion highlighted in black was used for the inversion, it is limited by the energy loss and the spatial aliasing. On the bottom, the P and S waves velocity profiles and the respective dispersion curves. The different colors refers to the misfit value. The grey band contains the whole space of parameters where the algorithm searches the solutions. GNGTS 2015 S essione 2.2 187