GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.2 611 which often limits the use of the technique for environmental purposes, is that they usually provide only 1D model distribution of Vs in depth. Recently, 2D methodologies allowing the extraction of SW dispersion curves with lateral resolution have been developed. Miller et al. (1999) and Xia et al. (2000) firstly extended the surface-wave analysis to pseudo-2D S-wave velocity distribution by which integrated initially the multi-channel analysis of seismic waves method with common midpoint data acquisition. Luo et al. (2008) proposed pseudo-2D S-wave velocity distribution basing on multichannel records by inverting high-frequency surface-wave dispersion curves calculated through cross-correlation combined with a phase- shift scanning method. The study of the phase-offset curves, frequency-by-frequency, takes the name of multi- offset phase analysis (MOPA) methods (Strobbia and Foti, 2006; Vignoli and Cassiani, 2010). Vignoli et al. (2011) calculated the derivative of the phase with respect to offset frequency-by- frequency and then extracted the dispersion curve window-by-window, with a lateral resolution that depends on the length of the window considered. This way, in principle, the lateral resolution of MOPA dispersion curve extraction strategy can reach the geophone spacing. We applied here the MOPA surface wave analysis in the complex former industrial (ex- esso) shore site near Trieste (Italy). The port area represents a unique test site. It was chosen to stress out the potential of the proposed methodology, cause it host an abrupt buried lateral heterogeneity due to anthropic interventions. In particular the test site hosts a buried quay, at a previous time adopted as ships pier and nowadays totally covered by continental and marine sediments, presenting clear subsoil lateral discontinuities. The site geology was previously explored with several boreholes stratigraphy, seismic surveys and electrical tomography (ERT) surveys, in the framework of re-use project interesting the area (Romeo et al., 2015). We acquired N.2 surface waves surveys along the known heterogeneities, in order to test the MOPA capabilities to retrieve correctly the 2D characteristic of the site. The results were compared with ERT data and boreholes information. In addition, the subsoil models obtained by the ERT survey, MOPA analysis and the geological information were used for advanced synthetic seismograms computation. Our results proved as advanced surface waves analysis are able to detect 2D subsoil discontinuities, suggesting as this approach should become a standard for surface wave analysis. Acknowledgements We thank the ‘Autorità Portuale di Trieste’ for the use of the geological information and the authorization to publish the data. M. Hashemijokar worked during his research period at the Dept. of Geosciences of University of Padova in the framework of the international visiting PhD candidate program. References Romeo R., L. Petronio, A. Affatato, A. Barbagallo, R. Belletti, G. Boehm, J. Boaga, G. Cassiani, P. Paganini, M.T. Perri, D. Sorgo, M. Ros. GeoPhysIcal InvestigatIons in a dismissed IndustrIal sIte: ex-esso (Trieste, Italy). 2015. Proceedings of Gruppo Nazionale Geofisica Terra Solida 2015, Trieste 2015. Louie, J. N., 2001, Faster, better: shear-wave velocity to 100 meters depth from refraction microtremor arrays: Bulletin Seismological Society of America, 91, 347–364. Socco, L.V. and C. Strobbia, 2004, Surface Wave Methods for near-surface characterization: a tutorial: Near Surface Geophysics, 2, 165-185. Miller, R., Xia, J., Park, C., Ivanov, J., and Williams, E., 1999, Using MASW to map bedrock in Olathe, Kansas; Technical Program with biographies, SEG, 69th Annual Meeting, Houston, Texas, 874-877 Luo, Y. H., J. H. Xia, R. D. Miller, Y. X. Xu, J. P. Liu, and Q. S. Liu, 2008, Rayleigh-wave dispersive energy imaging using a high-resolution linear Radon transform: Pure and Applied Geophysics, 165, 903-922. Xia J., Millera R., Park C., Hunter J., Harris, Ivanova J. Comparing shear-wave velocity profiles inverted from multichannel surface wave with borehole measurements . 2002 ,Soil Dynamics and Earthquake Engineering Volume 22, Issue 3, Strobbia C., and Foti S., 2006,Multi-offset phase analysis of surface wave data (MOPA). J. Appl. Geophys, 59, 300- 313. Doi:10.1016/j.jappgeo. Vignoli G. and G. Cassiani,2010, Identification of lateral discontinuities via multi-offset phase analysis of surface wave data, Geophysical Prospecting, 58 Vignoli G., Strobbia C., Cassiani G. and Vermeer P., 2011, Statistical Multi-Offset Phase Analysis (sMOPA) for surface wave processing in laterally varying media, Geophysics, 76, 1 -11, doi:10.1190/1.3542076