GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 3.3 489 DETERMINATION OF THE VS PROFILE IN A NOISY INDUSTRIAL SITE: FURTHER EVIDENCES ABOUT THE IMPORTANCE OF LOVE WAVES AND THE OPPORTUNITIES OF THE GROUP VELOCITY ANALYSIS G. Dal Moro Institute of Rock Structure and Mechanics - Academy of Sciences of the Czech Republic, Prague, Czech Republic Introduction. In the last decade, the analysis of surface-wave propagation has become extremely popular especially in the framework of seismic-hazard studies although, as a matter of fact, the determination of the shear-wave velocity (V S ) profile is useful for any geotechnical or geological application that requires the knowledge of the subsurface conditions. It is well known that the accuracy of the V S profile depends on the number of observables considered in the inversion process and on the kind of analyses actually put in place. In fact, in spite of the popularity of the approach based on the interpretation of the modal dispersion curves of the vertical component of Rayleigh waves (MASW – Multichannel Analysis of Surface Waves), a wide range of further options are possible and capable of providing better results, free from major ambiguities and pitfalls that characterize the standard MASW approach. For the present illustrative study, we considered a set of multi-component active and passive data gathered in a NE-Italy heavily-industrialised area home to many industries related to metalworking and therefore characterized by an extremely-high level of microtremors. Data and analyses. With the goal of defining the best procedures necessary to unambiguously define the subsurface model in a very noisy industrial area in NE Italy, we collected a comprehensive series of active and passive seismic data. Active data were recorded by means of a single 3-component sensor in order to work with the Holistic analysis of Surface waves (HS) (Dal Moro et al. , 2019; Dal Moro, 2018; 2020) while passive data were recorded so to define the HVSR (Horizontal-to-Vertical Spectral Ratio), the dispersion curve of the vertical (Z) component of Rayleigh waves via Miniature Array Analysis of Microtremors (MAAM - Cho et al. , 2006a; 2006b; 2013; Tada et al. , 2007; Dal Moro et al. , 2015a; 2018) and the Love-wave dispersion curve via ESAC (Extended Spatial AutoCorrelation - Ohori et al. , 2002). MAAM was accomplished considering a triangular geometry with a radius of 1.7 m while data for the ESAC were collected considering various multi-offset linear arrays with total lengths ranging from 44 to 60 m and with different orientations (for a series of clarifications about the performances of the MAAM and ESAC techniques see Dal Moro, 2020). Since it was systematically observed that Rayleigh-wave phenomenology is extremely complex and therefore prone to significant ambiguities and pitfalls (Safani et al. , 2005; Dal Moro et al. , 2015b; Dal Moro, 2020), first of all we accomplished the joint inversion of the HVSR together with the effective dispersion curves of the Z and T components (i.e. the vertical component of Rayleigh waves and Love waves) as obtained fromMAAM and ESAC, respectively (see data and results shown in Fig. 1). Sinceamorecommonapproach isbasedonthe joint analysisofRayleigh-wavedispersionand HVSR (e.g. Arai and Tokimatsu, 2005), in order to compare the outcomes we also accomplished this kind of simpler approach (in other words, unlike before, now we are not considering the Love-wave dispersion). Fig. 2 shows the obtained results. Although the overall misfits appear quite good and would inevitably represent a very satisfactory result, the comparison with the solution obtained while considering both Rayleigh and Love waves (see Fig. 1 and related text) demonstrates that the use of Rayleigh waves alone can lead to erroneous solution which are necessarily associated to higher V S values. This is easily and plainly demonstrated if we compute the Love-wave dispersion from the V S profile shown in Fig. 2c and compare it with the field data (i.e. the velocity spectrum shown in Fig. 1b): the Love-wave phase velocities of the model are significantly higher than the observed ones.