GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 3.2 GNGTS 2024 Fig. 1 – a) Locaton of the 3 seismic lines and the area used for the tomographic model (black rectangle) with the aerial image of the Arquata del Tronto area in transparency. The black dots represent the locatons of the wells drilled in the area (S1, S2, S3 and S4). The coordinates of the lower right vertex are given in the Monte Mario/Italy Zone 2 reference system B) 3D view of the line positons with respect to the topography of the area. 3. 3D velocity model All 3 seismic lines were taken into account in the reconstructon of the 3D velocity model (Figg. 2, 3). All coordinates (referred to Monte Mario/Italy zone 2, in the Gauss-Boaga projecton) were referenced to an X=0 Y=0 origin, which corresponds to the point X=2380.35 km and Y=4736.85 km (Fig. 1). The Cat3D sofware developed by OGS was used for the inversion of the arrival tmes. The tomographic method used for these data is based on a ray tracing algorithm (Böhm et al., 1999) for the direct model and the iteratve method SIRT (Simultaneous Iteratve Reconstructon Technique) (Stewart, 1991) for the travel tme inversion. In additon, a model optmizaton technique based on the staggered grid method (Vesnaver and Böhm, 2000) was applied, which increases the resoluton of the fnal inversion model by summing multple tomographic solutons obtained from a well- conditoned (low null space) base grid shifed in the X and Y directons, while preserving the reliability of the tomographic soluton proper to the base grid. In general, the acquired seismic data showed the presence of coherent noise (air waves, ground roll, difractons), which in some cases was relevant, especially in the acquisitons with S waves source. This is partly due to the presence of loose sediments or flling material below the surface (especially in the area of the old village) and the presence of walls, ditches and scarps close to the survey lines, which sometmes led to interference in the data caused by refected and/or refracted lateral arrivals from these elements.