GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 3.3 135 2013). In the last decade, several methods have been actually proposed in order to acquire data to analyze according to different (active and passive) procedures. The critical point that must be considered in order to retrieve a reliable V S profile, is that because of the intrinsic non-uniqueness of the solution that inevitably characterizes any methodology, the use of a single component cannot provide a fully constrained subsurface model (e.g. Scales et al. , 2001; Dal Moro, 2014). For this reason, over the years, a number of solutions capable of handling and jointly analyze various datasets and components have been proposed (e.g. Arai and Tokimatsu, 2005; Picozzi and Albarello, 2007; Dal Moro, 2014; Dal Moro et al. , 2015a). As a matter of facts, the design of acquisition and analysis procedures which are efficient (limited equipment and straightforward field procedures) and capable of providing a sufficiently large amount of data necessary for a robust inversion process, represents a challenging task which can be seen also from a so-to-speak aesthetic point of view. The work and philosophical approach of the well-known German architect and designer Mies van der Rohe, who adopted minimalistic elements with multiple functional purposes, inspired the holistic acquisition and analysis system here briefly introduced. The pursued goal is to obtain the maximum possible outcome out of an extremely-light equipment and field efforts. The proposed system consists of a simple 4-channel seismograph, one 3-component (3C) geophone and four vertical-component geophones. The acquired (active and passive) data, are used to determine up to six independent (but mutually related and complementary) objective functions that, altogether, fully describe the surface-wave propagation. It is important to point out that, in general terms, an acquisition system is composed by the combination of several elements: the A/D (Analog-to-Digital) conversion unit (usually referred to as seismograph), a certain number of sensors (geophones or accelerometers), the seismic cables that transmit the signal to the A/D unit and the acquisition software that handles the data also possibly computing some quality check to ensure that no major problems occur during the acquisition procedures. The overall efficiency of the adopted acquisition system is determined by the combination of all these elements and actually depends on the quality of the weakest component so that, for instance, very high-quality geophones risk to be quite useless when a poorly designed and engineered A/D unit is used to convert and acquire the signals. Data acquired while considering the considered system, are eventually jointly used to set up an extremely-constrained inversion, thus giving no room to ambiguities in the obtained V S subsurface model. After briefly introducing the fundamentals of the designed system and approach, we also present the analyses performed on a dataset acquired in a NW-Italy urban area. The designed system. The design of the acquisition and analysis system described in the present paper represents the concrete realization of the paradigmatic van der Rohe’s motto: less is more . Differently than the standard multi-channel approach (24 vertical geophones actually provide a single component ), here, the mindful use of a simple 4-channel seismograph, one 3C geophone and four vertical geophones, allows the determination of four components from active acquisitions and two from passive seismics. In order to properly appreciate the value of the four components extracted from the active acquisitions, a clear understanding of the concept of component (with respect to the one of number of geophones ) is crucial, and Fig. 1 will help clarifying the point. If we consider the data that can be acquired by a 3C geophone in the framework of an active survey, we can define up to four components (for further details see Dal Moro et al. , 2015b): - the THF (Love waves) group velocity spectrum; - the group-velocity spectra of the Radial (RVF) and Vertical (ZVF) components of Rayleigh waves;