GNGTS 2021 - Atti del 39° Convegno Nazionale

429 GNGTS 2021 S essione 3.2 CHARACTERISATION OF THE NEAR SURFACE USING SEISMIC DATA WITH LITTLE A PRIORI INFORMATION: TWO CASE STUDIES F. Da Col, F. Accaino, G. Böhm., F. Meneghini, A. Schleifer National Institute of Oceanography and Applied Geophysics - OGS Introduction Seismic methods are a common tool to characterize the shallow subsurface. Specifically, surface-wave analysis and first-break tomography are widely used to compute useful geotechnical and engineering parameters, such as the V S30 . While surface-wave analysis is the most commonly used method, its integration with other processing methods, such as S-wave seismics, is gaining increasing popularity (e.g. Cardarelli et al., 2014). In this work we want to highlight the importance of such integration and present two case studies from two very different environments. In the first, the seismic data was acquired along the roads of a village, where the geology is known to be complex as it is composed mostly of fractured and thrusted flysch. In the second, the data was acquired on a sandy beach, where coarser grained sediments carried by a nearby river overlay clay muds from a lagoon. In both cases, the only a priori information available came from geological maps and geomorphological studies as no previous geophysical investigation was carried out, nor any wells were available. Data acquisition We show the results of the processing of two datasets, in which P-,SV-, SH- and surface- waves were acquired separately. The first was acquired along the roads of the village of Kaštela (Split area, Croatia, see Figure 1a). Previous geological studies showed that the area lies in a compressional environment, with eocenic flysch, several hundreds of metres thick, overlaying limestone. Due to the compression, the flysch presents strong fracturing and thrusting, with mostly subvertical bedding. We acquired three lines, with a receiver spacing of 2 m and a source spacing of 4 m. As a source, we used a vibrator capable of producing both P- and S-waves, re-orienting the vibrating plate to produce horizontally and vertically polarized shear-waves. The sweep frequencies ranged from 20 to 160 Hz, with a sweep length of 15s. Horizontal receivers were deployed to record S-waves, re-orienting them together with the source to record the SV- and SH- wavefields. To record the surface-waves, we deployed 24 4.5 Hz geophones spaced every 4 m and used as a source a dropping mass of 100 kg. Figure 1. a) Location of the town of Kastela on the Dalmatian coast and location of the three seismic lines in the village b) Location of the beach of Bibione in the northern Adriatic and location of the survey line on the beach.