GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 3.3 137 By fully and efficiently exploiting the data that can be collected through a simple 4-channel (triggerable) acquisition system, we are then able to collect a considerable amount of active and passive data then used to perform a joint analysis based on the following objective functions: 1) Horizontal-to-Vertical Spectra Ratio (HVSR) [3-channel passive seismics]; 2) phase-velocity Rayleigh-wave dispersion curve from Miniature Array Analysis of Microtremors (MAAM) [4-channel passive seismics]; 3) Active HS data recorded by means of the 3-component geophone and used for the holistic analysis of the group-velocity spectra of Rayleigh and Love waves (defined according to the Multiple Filter Analysis - e.g. Bhattacharya, 1983) also jointly with the RVSR (i.e., altogether, four components). Since the phenomenology of Love waves is typically by far simpler than that of Rayleigh waves (Dal Moro, 2014; Dal Moro et al. , 2015c) but their analysis via miniature array techniques does not provide highly accurate results (Tada et al. , 2009), we decided to analyze their propagation from the active dataset (THF component depicted in Fig. 1). By properly combining the data acquired by means of these three procedures (for which we just need a 4-channel acquisition system), we can potentially jointly analyze up to 6 components . Since this would result in a so-to-speak extreme inversion procedure, here we decided to limit the presented analyses to just three components, chosen with the goal of anyway ensuring the most robust solution. The here-considered components are the HVSR curve, the effective dispersion curve of the vertical component of Rayleigh waves as defined via MAAM and the Love-wave group velocities (from the active acquisitions) here analyzed according to the FVS approach. Fig. 2 – MAAM (passive seismics): a) acquisition setting (in this case the four vertical-component geophones are deployed along a 2-m radius circle); b) Rayleigh-wave effective dispersion curve (vertical component) determined while considering the data acquired for the case study illustrated in the present paper. The 3.5-λ line reports the upper frequency limit determined by spatial aliasing effects. An urban case study. The site considered for this pilot study is located in the city center of La Spezia (NW Italy) where, as common for any urban area, logistical problems (related to the lack of available spaces) and a considerable amount of noise (related to several human, commercial and industrial activities), make the acquisition procedures quite challenging. Local stratigraphy is superficially dominated by a mixture of soft alluvial and marine sediments that, at deeper layers, turn into gravel-like materials. The active (HS) and passive (MAAM) data to consider for the previously-described joint analyses were acquired according to the parameters reported in Tabs. 1 and 2. Passive data used