GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 2.2 441 between 0.15 – 1.8 sec twt corresponding to a maximum penetration of about 400 m below the ground surface (Fig. 1). Subsurface seismic velocities can be obtained by seismic borehole measurements: vertical seismic profile (VSP) and/or by cross- hole survey (CH). However if boreholes are not available to perform direct measurements, shear and compression wave velocities can be obtained through non-invasive seismic surveys. The picking of first arrivals (direct and refracted waves) performed on the shot gathers gives information about velocity but, typically, only about the shallower layers. Also reflection seismic data contain information about the seismic velocities. In this case data processing requires to perform the normal-move-out (NMO) correction to obtain the best stack of the reflected events over the available offset range. The choice of the stack velocity (Vstack) can be rather subjective, however in presence of horizontal layers with no lateral variations, small offsets and good S/N ratio, Vstack approximates Vrms and Dix formula can be reasonably applied. The acquired data meet these conditions so the interval velocities computed by Dix equation are reasonably reliable. Figure 2 shows the stack section converted in depth with the superposition of SH-wave velocity model and the stratigraphic units. Conclusions. The SH-wave reflection seismic method was successfully applied in the study area. Data present clear reflections and the absence of surface wave noise that, at some sites, can inhibit the signals detection. Recent acquisition performed by OGS nearby Pieve di Cento also confirms the results obtained in the Cavezzo area. The velocity model obtained by SH-wave reflection data shows SH-wave velocity lower than 500 m/s until the depth of 240 m below ground level. In correspondence of the marine sediments (QM – Quaternario Marino) a velocity increasing can be observed with values greater than 700 m/s. Table 3 shows the shear wave velocity associated to the different stratigraphic units. Tab. 3 - SH-wave interval velocities (m/s) within main stratigraphic units. AES6 321 AESind 400 AEI 482 QM - Quaternario Marino 712 These values are in good agreement with velocities obtained by downhole measurements performed at Medolla and Mirandola (Martelli and Romani, 2013). Also Mirabello downhole data (Minarelli et al. , 2016) show SH-wave velocity lower than 800 m/s, which formally defines the “seismic bedrock” by the European Building Code (EN 1998-5 2004) and the Italian Building Code (NTC 2018). Fig. 1 - SH-wave common-shot-gather: clear reflections are observed until 1.8 s twt (about 400 m).