GNGTS 2013 - Atti del 32° Convegno Nazionale

To evaluate the heterogeneity of the seismic layers, we determined the true interval velocity V i , i.e. the velocity the wave would propagate vertically between two adjacent positions Zn and Z(n-1) of the geophone: V i = S z / t v (1) where S z is the partial distance equal to Zn - Z(n-1) . A seismic transient correspond to an impulsive stress of low energy; therefore, soils and rocks traversed by seismic waves undergo elastic-type deformation; P and S waves velocity are related to the elastic and mechanical properties of the geologic medium through which they propagate. From the estimated seismic velocities Vp and Vs , we have computed the following elastic and seismic parameters: γ din , geophysical density, indicating the status of aggregation of soils; G , rigidity modulus; K, compressibility modulus; E , Young or elasticity modulus; M , contrast modulus; σ , Poisson ratio. The seismic parameters describe the dynamic behavior of a litho-stratigraphic unit in terms of transmission of seismic energy from a formation to another. I.S., seismic impedance, defined as the shear wave velocity for the density of the medium, related to the local amplification of ground motion; refraction ( t ) and reflection ( ξ ) coefficients, related to the rigidity of the ground, representing the ratio between the amplitude of the reflected and refracted wave with respect to the incident wave. They are related to the amount of the energy transmitted from one lithotype to the other. To classify the soils according to the current regulation O.P.C.M. 3274/2003 (“First elements on general criteria for the seismic classification of the national territory and technical regulations for construction in seismic areas”) we have identified 4 seismic zones characterized by different values of horizontal acceleration ( α g / g ) of the response spectrum as follows: seismic zone 1, with acceleration 0.35 g; seismic zone 2, with acceleration 0.25 g; seismic zone 3, with acceleration 0.15 g; seismic zone 4, with acceleration 0.05 g. The down-hole technique is the most reliable because it allows to get evaluations of the seismic amplifications using as datum of base the profile of the Vs with the depth. In absence of a specific study of the site, the parameter Vs30 , introduced from the „D.M. 14/01/2008”, is used for defining the interaction between the structure and the studied area, it represents the average shear wave velocity in the first 30 meters of depth. In the mechanical drillings S1-S3 we have computed the weighted average shear wave velocity Vs 30 in the first 30 m of depth, defined as: (2) where h and V indicate thickness (in meters) and shear wave velocity in the i -th of the N layers that have been drilled in the first 30 meters. The Vs 30 is used to classify the rigidity of a soil, since shear wave velocity is directly related to the rigidity modulus μ as: μ = ρ * V s 2 (3) where ρ is the density. In addition, we used the seismic tomography method to distinguish highly fractured volumes of rocks from intact or little fractured rocks. We used a cross-up hole configuration of multishots type, with 21 vertical geophones, located along the calcarenitic cliff, with natural frequency of 14 Hz and interspacing of 5 meters; was covered a total distance of 105 meters. A seismic gun has been used as seismic source. Have been carried out seven shots, inside the mechanical drilling S2, with interspacing of 4 meters, up to a depth of 28 meters (Fig. 3). Data were analyzed using the G.S.A.O. (Generalized Simulated Annealing-Optimization) method (Pullammanappallil and Louie, 1994). The G.S.A.O. allows a nonlinear optimization of the inversion of the arrival times of the direct and refracted seismic waves. The advantage of this 48 GNGTS 2013 S essione 3.1