GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 3.3 491 as in other previously-published (e.g. Dal Moro, 2019; 2020), only the presence of Love waves can properly channel the inversion procedure towards the correct solution. It should be clearly underlined that in both the accomplished procedures reported in Fig. 1 and 2, the observed dispersion curves were not interpreted in terms of modal curves but modelled according to the mathematics of the effective curve (Tokimatsu et al. , 1992; Ikeda et al. , 2012). A different approach to surface-wave analysis is possible through the computation of the group velocities and their holistic analysis jointly with the Rayleigh-wave Particle Motion (RPM) curve (describing the actual particle motion due to the Rayleigh-wave propagation and quite useful in further constraining the subsurface model) and, in case we intend to investigate deeper strata, the HVSR (Dal Moro et al. , 2017; 2019; Dal Moro, 2018; 2020). Group velocities are computed via frequency-time analysis (Levshin et al. , 1972) and can be obtained both from passive (e.g. Fang et al. , 2010) and active data (Ritzwoller and Levshin, 1998; 2002; Dal Moro et al. , 2019). Differently than phase velocities, group velocities can be obtained considering a very limited field equipment which is fundamentally based on just one or two 3-component geophones, depending on whether we are considering passive or active data. Data considered in the present study were recorded by means of a 3-component geophone deployed at a distance of 44 m from the source (a 10-kg sledgehammer). Due to the high noise level, stack was fixed to 25. Fig. 3 shows both the extracted data (group velocity spectra of the Z and R components as well as the RPM curve) and the solution of the holistic analysis of surface waves (HS), i.e. the joint analysis of multi-component group velocities together with the RPM and the HVSR (this latter is useful to extend the investigated profile in depth). It should be underlined that in the HS approach, dispersion data (group-velocity spectra) are not analysed Fig. 3 - Holistic analysis of the group velocities of the vertical (Z) ( a plot) and radial (R) ( b plot) components (FVS approach – background colours represent the field data while the overlying black contour lines the obtained model) jointly with the RPM ( c plot) and HVSR ( d plot) curves. The obtained V S profile ( e plot) is entirely similar to the one obtained while considering the joint analysis of the phase velocities of the Z and T components (see Fig. 1).