GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 3.2 619 The results show a good match between 13 and 19 Hz (Fig. 3): at lower frequencies the two modes are not sufficiently separated, having very similar velocities, while at higher frequencies a third energy spot appears between them, making the analysis, based on two modes only, obviously inadequate. Uncertainties are also small and acceptable. This example shows how the Multi-Mode MOPA can correctly perform when the applicability conditions are met, which in this specific case verifies in a quite narrow frequency range. Noise is affecting the results accuracy, especially regarding higher frequencies, and this is correctly accounted for by the computed uncertainties. Conclusions. A new approach, which makes it possible to apply the Multi-Offset Phase Analysis (MOPA) to two-modes situations, was here presented: we called it Multi-Mode Multi- Offset Phase Analysis (MMMOPA). The method is based on the analysis of both phase and amplitude spectra, which represents a main step forward with respect to the classic MOPA. MMMOPAwas tested on both synthetic and real datasets, with success. This work only aims at presenting the core of the method, with a set of strict applicability conditions: the analysis is focused on laterally homogeneous media, where two (and only two) modes are interfering. We also focused our attention on the single shot point. Future developments of MMMOPA will be oriented towards a generalization of the method, starting from a 2D approach. In order to do so data redundancy of multi-shot schemes should be exploited to compensate the sparse character of the method, where measures are done in conjunction with amplitude maxima only. References Ballard R. F.; 1964: Determination of soil shear moduli at depths by in-situ vibratory Techniques . Miscellaneous Paper no. 4-691, Waterways Experiment Station, Vicksburg, Mississippi. Boaga J., Vignoli G., Deiana R. and Cassiani G.; 2014: The influence of subsoil structure and acquisition parameters on surface wave mode contamination. Journal of Environmental and Engineering Geophysics, 19 , 87-99, doi: 10.2113/JEEG19.2.87. Garofalo F., Foti S., Hollender F., Bard P. Y., Cornou C., Cox B. R., Ohrnberger M., Sicilia D., Asten M., Di Giulio G., Forbriger T., Guillier B., Hayashi K., Martin A., Matsushima S., Mercerat D., Poggi V. and Yamanaka H.; 2016: InterPACIFIC project: Comparison of invasive and non-invasive methods for seismic site characterization. Part I: Intra comparison of surface waves methods . Soil Dynamics and Earthquake Engineering, 82 , 222-240, doi: 10.1016/j.soildyn.2015.12.010. Jones R. B.; 1958: In-situ measurement of the dynamic properties of soil by vibration Methods . Géotechnique, 8 , no. 1, 1-21, doi: 10.1680/geot.1958.8.1.1. Jones R. B.; 1962: Surface wave technique for measuring the elastic properties and thickness of Roads . Theoretical development: British Journal of Applied Physics, 13 , no.1, 21-29. Kausel E.; 1989: Punch: program for the dynamic analysis of layered soils, version 3.0 . Massachusetts Institute of Technology, Boston, Mass. Nazarian S. and Stokoe II K. H.; 1984: In situ shear wave velocity from spectral analysis of surface waves. Proceeding of the 8th Conference on Earthquake Engineering, 3 , 31-38. Park C. B., Miller R. D. and Xia J.; 1999: Multichannel analysis of surface waves. Geophysics, 64 , no. 3, 800-808. Xia J., Miller R. D. and Park C. B.; 1999: Estimation of near-surface shear-wave velocity by inversion of Rayleigh waves : Geophysics, 64 , no. 3, 691-700. Strobbia C. and Foti S.; 2006: Multi-offset phase analysis of surface wave data (MOPA). Journal of Applied Geophysics, 59 , 300-313, doi: 10.1016/j.jappgeo.2005.10.009.