GNGTS 2015 - Atti del 34° Convegno Nazionale

136 GNGTS 2015 S essione 3.3 - the Radial-to-Vertical Spectral Ratio (RVSR) describing (clearly in relative terms) the amplitude ratio of the two components as a function of the frequency (thus enabling us to go behind the analysis of the velocities alone). Since these four components are capable of fully describe the surface wave propagation, such a methodology [which can be used as a purely-active stand-alone application - see Dal Moro et al. (2015a, 2015b)] is here indicated as Holistic analysis of Surface waves (for the sake of brevity hereafter HS). It must be clearly underlined that the radial and vertical components of Rayleigh waves are, in general terms, different, and such a difference is a crucial point that enables more-constrained analyses based on the multi-component Full Velocity Spectrum (FVS) approach (Dal Moro et al. , 2015b, 2015c). On the other side, while considering the multi-channel data recorded for instance by 24 or 48 vertical-component geophones for MASW ( Multichannel Analysis of Surface aves - see e.g. Dal Moro, 2014) or ESAC ( Extended Spatial Auto-Correlation - Ohori et al. , 2002), we are actually dealing with less information with respect to the described HS approach, since such classical multi-channel techniques actually provide information on just one component (in this case the phase velocities of the vertical component of Rayleigh waves). Being an active methodology, the depth investigated through the HS approach will clearly depend on the adopted offset (Dal Moro et al. , 2015a, 2015b). From the logistical point of view it must be anyway considered that moving a single geophone is definitely faster and easier than moving an array of geophones. Considering for instance an urban environment, it is clearly possible to set the source on one side of a road and the geophone on the opposite, without significantly interfering with the circulation of traffic. Fig. 1 – Active seismics (HS multi-component approach): the components acquired in case a single 3C geophone is used to record the signals produced while considering both Vertical and Horizontal forces (Rayleigh and Love waves, respectively). Further details in Dal Moro (2014) and Dal Moro et al. (2015b). In addition to such a multi-component active dataset, for the present work we also considered two passive datasets/components acquired with the same 4-channel acquisition system: the HVSR curve (Arai and Tokimatsu, 2004) and the Rayleigh-wave (vertical component) effective dispersion curve (phase velocities) retrieved via Miniature Array Analysis of Microtremors (MAAM - see Fig. 2) (Cho et al. , 2013). This latter technique relies on the passive data collected by an uneven number of vertical- component geophones deployed along a small-radius circular array (the triangle is the easiest and quickest solution), with an additional central geophone used for the noise compensation (Cho et al. , 2013). The frequency range where the retrieved phase velocities are properly defined is a function of the adopted radius (Fig. 2). For the most common near-surface applications that require the investigation of some tens of meters, the typically radii range from 0.5 up to about 5 m.