GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 3.2 471 surveys using a non-linear inversion approach (Barryman G. 1990 and reference therein). The ERT3D dataset inversion provided parameterization of resistivity values. The data were merged in a single dataset and implemented in a 3D model that was filtered and processed by software ErtLab_64™ (Geostudi Astier & Multi-Phase Technology) that uses a 3D tetrahedral finite element method. For a 3D visualization of all the inverted dataset, we used software Voxler 4 (3D Well & Volumetric Data Visualization by Golden Software), that is able to generate both planar and vertical views of seismic velocity and resistivity values. Results and interpretation. The first step was the reconstruction of the geological setting, mainly identifying the presence of the seismic basement (Vs=800, according to the NTC2018). For this reason, 2D seismic tomography was performed, returning the presence of basement at about 15 m of depth. For a more detailed interpretation, the geognostic survey standing 3 meters from the pile, is plotted above the 2D seismic tomography suggesting the presence of fractured marly limestone from about 10 m of depth in the 2DSRT central sector, as shown in figure2. Then, SRT3D was performed. More specifically, according to recent studies (Ferrari et al., 1991; Di Tommaso et al., 2010 and references therein), the Vp through the concrete ranges from 3000 to over 4600 m/s, depending on the concrete quality; if the concrete is characterized by defects (discontinuity, spaces, earth inclusion, etc.), a longer wave crossing time occurs with a consequent decrease in the propagation velocity. In this study, the 3d seismic tomography, returning Vp maximum value ​of 7500 m/s, highlights the cylindrical pile shape, suggesting a maximum depth of about 6 m from the ground level, confirming the viaduct design project provided by the client. For a better visualization, the Vp values ​higher than 5000 m/s, certainly related to the concrete, are saturated with magenta colour (Fig.2, Fig.3). Fig. 2 - SRT2D and geognostic survey in Voxler4 environment; a) SRT3D and ρ volume plotted above SRT2D in P wave; b) SRT3D and ρ volume plotted above SRT2D in Sh wave.