GNGTS 2015 - Atti del 34° Convegno Nazionale

Conclusions. A strategy for the use of the phase lags between pair of signals recorded by either an array or by an arbitrary 2-D distribution of receivers and produced by multiple sources was presented. This strategy could in principle be used to build, in future, an inversion process. However, in this work we rather presented a Direct Interpretation of the Phase Lags (DIPL) to tackle both the 2-D and 3-D investigation of the subsurface. We showed that the use of equations 1-4, coupled with the comparison to the frequency-velocity transform allows retaining the local property of the data and at the same time takes into account both the fundamental and higher modes of the surface wave’s propagation, thus enabling the construction of reliable 2-D Vs pseudo-sections capable of detecting and evaluating lateral heterogeneities below a MASW- like profile. Further, we extended this approach to three dimensions and we discussed the capabilities and limitations of both 2-D and 3-D approaches. Finally, examples of application to field data were given; where the 2-D case concerns data collected over a regular (i.e. practically 1-D) subsurface while the 3-D data were collected to characterize a very shallow portion of the subsurface under the foundation of a residential building. In both cases we found the results in agreement with both cross-hole and CPT investigations purposely carried-out at the same sites, and whose results cannot be shown here because of the limits imposed on the number figures. References Aki, K. and Richards, P. G. (2002): Quantitative seismology, Theory and methods (second edition), University Sciences Books, Susalito, California, pp. 700. Bignardi, S., Abu Zeid, N., D’Attoli, M., Morelli, G., Occhi, M., Russo, M., Santarato, G. (2015/a): Near foundation soil stiffening evaluation after resins injection by a novel 3D interpretation of surface waves data: Near Surface Geoscience 2015 - 21st European Meeting of Environmental and Engineering Geophysics (EAGE) . September 6-10, 2015, Turin. Italy. DOI: 10.3997/2214-4609.201413799. Bignardi, S., Abu Zeid, N., Santarato, G. (2015/b): Direct interpretation of phase lags of MASW data: An example for evaluation of jet grouting for soil stiffening enhancement against soil liquefaction: SEG Technical Program Expanded Abstracts 2015 : pp. 2218-2223. DOI: 10.1190/segam2015-5925998.1. Bignardi, S., Santarato, G. and Abu Zeid, N. (2014): Thickness variations in layered subsurface models - effects on simulated MASW: 76th EAGE Conference & Exhibition 2014 Experience the Energy , Amsterdam, The Netherlands. Bignardi, S., Fedele, F., Yezzi, A. J., Rix, G. J. and Santarato, G., (2012): Geometric seismic wave inversion by the boundary element method: Bulletin of the Seismological Society of America , 102 , 802-811. Fig. 3 – Results of the Phase Lag Direct interpretation for a 3-D, near foundation subsurface. The footprint of the building is highlighted in red. a) Vs distribution beneath a building retrieved using a 2-D array of 24 receivers (showed as green dots) and 21 sources (showed as red dots). The picture highlights two anomalies due to the sewer system (gray squares) and a low velocity zone around the corner (x=7,y=7), that was the target of the investigation. b) Corresponding sensitivity pattern. Only the zones with a sufficient spectral resolution, in terms of wavenumber coverage, are shown. The high values around the target corner mean that the result in this portion of the subsurface is reliable while reliability lowers toward the back of the house and at depths higher than three meters. GNGTS 2015 S essione 3.2 87