GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 2.2 243 sediments and Pliocene marine deposits, respectively located at 95 m (Medolla) and about 115 m (Mirandola) from the surface. This initial check showed that the method adopted for calculating the average V S values up to a depth h from interval V S profiles of borehole seismic tests may significantly affect ouctomes of the procedure. Specifically, two methods are compared: 1 – by using the formula (3) where h i and V i are respectively thickness and V S interval velocity of the i -th seismo-layer. In this case, the best fit among experimental S V ( h ) profiles and those deduced from Rayleigh dispersion curves was obtained with c = 0.8. So, to modelize the trends of this six S V ( h ) profiles (two cross-hole and four arrays) a power law pattern with V 0 = 80 and x = 0.37 was considered; 2 – by considering the formula (4) (arithmetic weighted average). In this case, best fitting is obtained with c = 0.7 and the relevant parameter of the power law are V 0 = 80 and x = 0.42. To check best performing approach, the depth of the resonant layer was estimated with Eq. (2) and considering f 0 = 0.8-0.9 (these are resonance frequency values estimated from HVSR measures closer to the cross-holes). One can see that when Eq. (3) is considered, Eq. (2) provides values for the depth of seismic bedrock at about 70 m for Medolla and 70-85 m for Mirandola. When Eq. (4) is considered, the estimates result 90 m for Medolla and 90-110 m for Mirandola: it is evident that, in this way, it is possible to obtain a better calibration of the whole method with real data. In view of this, the method 2 has been used to obtain the S V ( h ) profiles relative to remaining borehole seismic tests and to calibrate the dispersion curves for the whole study area; the trends obtained revealed to be rather similar up to a depth of 150 m, with differences between average velocity values less than 100 m/s. On the basis of the above results, two power-law patterns were determined to bound observed S V ( h ) profiles (Fig. 3) and capture observed variability. The lower bound is marked by a power law with parameters V 0 = 55 and x = 0.43; upper bound is marked by a power law with parameters V 0 = 120 and x = 0.3. In this way, the resonant surface linked to the main peak has a depth variability about 40 m for each frequency taken in account; in particular, in the zones 1 and 4, the seismic bedrock is located at depth in the range 55 m and 110 m (Tab. 2). As concerns the lower frequency peak (0.25 - 0.3 Hz), using the same method, the suggested depth is of the order of several hundreds of meters (350-500 m). Tab. 1 - Minimum ( H min) and maximum ( H max) depth values obtained using respectively minimum power law ( V 0 = 55, x = 0.43; Fig.3) and maximum power law ( V 0 = 120, x = 0.3; Fig.3). These depth values are related with main HVSR peak frequency values. Frequency (Hz) H min (m) H max (m) 0.8 65 110 0.9 55 95 1 45 80 1.1 40 70 1.2 35 65 1.3 30 60