GNGTS 2019 - Atti del 38° Convegno Nazionale

474 GNGTS 2019 S essione 2.2 For the NS2 profile deep stratigraphic information is available from a seismic reflection profile (Pöyry, 2017). Here we expect the H/V peak frequencies to correlate with the sediment cover thickness and we therefore attempted to convert the H/V curves from the frequency to the depth domain by using a Vs model aiming at reproducing the reflection results (see also Ibs-von Seth and Wohlemberg, 1999). We obtained the H/V cross-section shown in Fig. 3 from which we estimate a maximum thickness of the sedimentary cover of about 400 m, corresponding to an H/V peak frequency of 0.5 Hz. Fig. 3 - H/V contour plot of the NS2 profile (located within the Bolzano basin; Figure 1) overlaid with the seismic reflection profile (Pöyry, 2017). The red line indicates the position of the sediment-bedrock interface inferred from seismic reflection and used to constrain the H/V model. The arrows indicate the positions of the single H/V curves. Each H/V curve is individually normalized. On the other profiles across the Adige valley, where we observe both 1D resonance peaks above 0.4 Hz on the edges and 2D resonance peaks at 0.3 and 0.4 Hz in the valley center, constraining the model in depth is more problematic, especially in the central part. However, we expect the maximum sediment thickness to be greater with respect to the other profile within the Bolzano basin, both because the resonance frequencies observed are lower and because the 2D effects are likely to be generated along the deeper Adige valley. This is supported also by direct stratigraphic information from a borehole located in Sinigo, 15 km north west of Bolzano, where the Adige valley is 700 m deep (Bargossi et al. , 2010). Conclusions. We collected 60 H/V measurements along three profiles in the Bolzano sedimentary basin and we identified H/V peaks that are generated by 1D and 2D resonance. We recognized a number of features that allow us to discern between 1D and 2D resonance peaks (Sgattoni et al. , 2019): - 2D resonance peaks are generated in the H/V ratio by local peaks in the longitudinal and transversal horizontal spectra at different frequencies (the longitudinal frequencies being smaller than the transversal ones); 1D H/V resonance peaks are generated by local minima in the vertical spectral component; - 2D H/V frequencies across the valley are constant, while 1D frequencies increase with decreasing sediment thickness. Note that 1) both 1D and 2D resonances may be observed along the same profile, depending on the complexity of the bedrock geometry and 2) H/V curves acquired along profiles oblique to the valley may yield spatially varying 2D frequencies if the 3D geometry of the valley is complex (in particular when the geometry changes along the valley axis); - 2D H/V peak amplitudes are larger in the valley center and decay towards the edges. We underline that in order to detect 2D effects on H/V curves it is mandatory to orient the