GNGTS 2022 - Atti del 40° Convegno Nazionale

286 GNGTS 2022 Sessione 2.2 recordings at each site preferably in a couple of distinct but close locations, rather than at a single station. In this way, the check of the representativeness of the results for the area surrounding the measurement site is facilitated. Previous studies reported that, while the resonance frequencies resulting from different acquisitions are generally consistent, variations of environmental conditions can determine strong changes in the amplitude of the H/V spectral peaks, likely for changes in the composition of the noise wavefield (cf. Del Gaudio et al. , 2021). Furthermore, sometimes a poor ground/sensor coupling can lead to an underestimate of the amplitude of the resonance effects. Therefore, on the final map of site resonances, we suggest to report the mean value of frequency and amplitude of the H/V peaks obtained from the couple of measurements if they appear consistent. When the two measurements provide rather different amplitudes, we advocate the selection of the major peak for the most relevant resonance frequency. H/V results and comparisons. The first H/V results pointed out some peculiarities in the pattern of resonance properties in the different macro-areas so far investigated. Regarding the Daunia Mountains, there are rapid spatial variations in resonance in relation to the heterogeneities of mechanical properties of different flysch materials even within the same geological unit. However, in general, H/V ratios do not show high values and very clear peaks, likely for the lack of strong impedance contrasts between surface materials and the underlying units. Indeed, a competent rocky substratum is often found at considerable depths. Figure 1 shows an example from the Municipality of Volturara Appula (FG), where clear H/V peaks with amplitude ≈ 3-4 at frequencies of 3-4 Hz characterize sites within outcropping quartzarenites of the “Flysch Numidico” and on the pelitic facies of the “Flysch di San Bartolomeo”. No evident peaks were observed at sites located on the arenaceous facies of the latter Flysch or on the chaotically deformed clay-rich “Flysch Rosso”. Locally on slopes, the presence of significant resonance can be also related to the impedance contrast between landslide bodies and a stiffer substratum. Fig. 1 - Geological map of the municipality of Volturara Appula involved in the SM studies. Red dots represent the locations of ambient noise measurement sites on the outcrops of: arenaceous (HV06) and pelitic (HV17) facies of the Flysch di San Bartolomeo, the “Flysch Rosso” (HV12), consisting of clays/clay-shales, and the quartzarenitic “Flysch Numidico” (HV14). The results of the HVSR analysis are shown as insets (data from Gruppo di Lavoro ASSET, 2022).