GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 3.2 561 produce results with large uncertainty intervals of parameters, and then must be necessarily constrained by detailed stratigraphic information.An application of HVSR inversion is presented in order to verify the effectiveness of this technique for purposes of reconstruction of the buried geological setting where stratigraphic constraints are available. The study area lies in the south-eastern zone of the Palermo plain, known as “Bandita”. It falls within the geological context of the Monti di Palermo (Abate et al. , 1978), resulting from the superposition of structural-stratigraphic units arising from the Miocene compressive deformation of limestone and dolomite shallow-marine deposits (Panormide Platform), and Mesozoic limestone and siliciclastic deep sea deposits (Imerese Basin), with overlying Oligo- Miocene terrigenous covers (Numidian Flysch). Specifically, the reconstruction of the stratigraphy in the studied area has been obtained by a seismic-stratigraphic three-dimensional modelling. This allowed for geophysical and geognostic data derived from surveys carried out during this study, as well as previously by the municipality of Palermo. Lithostratigraphic characterization by geognostic drillings. The reconstruction of the lithostratigraphic successions was based on previous knowledge arising from 93 available stratigraphic logs, randomly distributed in the zone, with maximum depth of 30 m below ground level. The analysis of this information allowed to define the quite articulate geological and lithostratigraphic setting of the area. This is characterized by the extensive presence of calcarenite-sand complex of the lower-middle Pleistocene, (Calcarenite di Palermo). This in the western sector rests on gray-blue clays of the Lower Pleistocene and in the eastern sector discordantly on the Numidian Flysch. By analyzing the collected stratigraphy, a reclassification of lithological samples was made, considering five main groups: sands and limestones (K+S), silts and sands (As), sands and sandy gravels (Ls), backfill (Dtr), marl and clay (FN). The pre-Pleistocene sedimentary basement, characterized by Numidian Flysch is not evenly distributed throughout the area. The roof of the flysch deposits is found at depths varying from 15 to 30 m. Particularly in the eastern part the depth is considerably greater near the coast than the inland areas. In the western part instead, Fig. 1 – Example of inversion of HVSR curve, constrained by a lithological drilling: (a) theoretic and experimental HVSR curve; (b) three component velocity spectra; (c) interpretative geophysical model; (d) lithostratigraphic data.