GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 39 Fig. 2, in the common range of frequency, retrieving the K true and overcoming the difficult orientated noise source condition. The use of two linear arrays allows the identification of the true propagation direction, and the correction for the error that a single array would produce. For the Badia Polesine case these passive results were supported by the controlled source test and by the geotechnical local informations (personal communication of Dr. F. Marinoni). For the presented real case we tested with success also the response of the L-shaped array with use of classical beamforming techniques thus identifying true velocity and direction of events, independently on the source distribution been more or less spatially polarized. Conclusion. ReMi is based on the hypothesis of a uniform wavefield, and only in this case the results can be considered representative of the local propagation properties. The properties of the spectrum should be analyzed and tested to verify the validity of the basic hypothesis site by site. To overcome the limitations, an alternative and quick acquisition solution consist in L–shaped arrays practice. The proposed LeMi combination of two linear evenly spaced arrays presents the advantages of practical fieldwork, allowing the acquisition of simple multichannel data on the same spread. The use of LeMi approach allows: i) the detection of predominant orientated noise source; ii) the identification of the true wavenumber, overcoming the error that a single array would produce. It must be underlined that combining 2 arrays simultaneously practically limits the number of available receivers and consequently the f-K resolution. Narrow directional source admits theoretically LeMi treatment even if record of the 2 L-shaped branches are not simultaneously but the issue can become critical with a more complex source distribution (different than uniform or narrow directional noise sources). If LeMi data are based on the simultaneous analysis of the data of the two arrays, the proposed acquisition scheme allows in any case more advanced 2D treatment approaches as beamformer. These should be adopted in the cases where there are evidences of more variable source distribution. In conclusion the passive LeMi methods, characterized by easy deployment and treatment, can considerable improve the correct estimation of seismic velocity for site characterization fronting narrow oriented and variable source orientation. This avoids the overestimation of the subsoil velocities, typical of linear passive array as ReMi, in the current case of oriented noise source. This can play a relevant role in all the seismic hazard characterization studies, since the overestimation of parameter as Vs can lead to totally misleading seismic response estimation. Acknowledgments. Authors thank the geologist F. Marinoni for the Badia Polesine site geological and geotechnical information. Jacopo Boaga work is supported by the European FP-7 project CLIMB:’ Climate Induced Changes on the Hydrology of Mediterranean Basins’. References Aki, K. ,1957, Space and time spectra of stationary stochastic waves, with special reference to microtremors, Bull. Earthq. Res. Inst., 35, 415–457 Beavers J. , 2002, A review of seismic hazard description in US design codes and procedures Progress in Structural Engineering and Materials Volume 4, Issue 1 Boaga J, Cassiani G., Strobbia C, Vignoli G. ,2013, Mode mis-identification in Rayleigh waves: ellipticity as a cause and a cure, Geophysics, v. 78 ,3,. Boaga J, Vignoli G, Cassiani G. ,2011, Shear wave profiles from surface wave inversion: the impact of uncertainty onto seismic site response analysis. Journal of Geophysics and Engineering 8 ,2011, 162–174 doi:10.1088/1742- 2132/8/2/004 Boaga J., Vaccari F., Panza F.G. ,2010,, Shear wave structural models of Venice Plain, Italy,from Time Cross- Correlation of seismic Noise. Engineering Geology n.116, 2010 Bonnefoy-Claudet, S., Cotton, F., Bard, P.-Y., ,2006,. The nature of noise wave!eld and its applications for site effects studies — a literature review. Earth-Science Reviews 79 ,3–4,, 205–227. Capon, J., 1969. High-resolution frequency–wavenumber spectrum analysis. IEEE 1408–1419. Chaillat S., Marc Bonnet and Jean-François Semblat ,2009, A new fast multi-domain BEM to model seismic wave propagation and amplification in 3D geological structures Geophys. J. Int., vol. 177. Eurocode 8, EN 1998-1 ,2003, – General rules, seismic actions and rules for buildings. CEN European Committee for Bruxelles, Belgium