Shear-wave velocity profiling at sites with high stiffness contrasts: a comparison between invasive and non-invasive methods

Three sites of the Italian Strong Motion Network (RAN) have been selected for detailed S-wave profiling, using both borehole and surface wave seismic methods. At these sites, the presence of stiffness contrasts within the Soil column is found to influence the surface wave propagation profoundly. Advanced aspects in surface wave inversion such as resolution, accuracy and higher-mode interpretation must be properly taken into account to obtain realistic results from the surface wave dispersion observations. The possibility of mode misidentification and the loss of resolution with depth in surface wave interpretation are explored using synthetic modelling together with active and passive seismic data sets. With high stiffness contrasts, the possibility of mode jumps and higher mode dominance over specific frequency ranges is very probable. This is true also for normally dispersive sites, where the shear velocity increases with depth, though higher mode dominance is recognized as more common in the case of a shear-wave velocity inversion within the soil column and the sensitivity of the dispersion curves with respect to those layers beneath the low-velocity zone may be significantly reduced. Pitfalls in the inversion resulting from mode misidentification can be avoided by investigating the effective phase velocity distribution, using active data sets and full waveform seismic modelling. When an unambiguous modal identification is achieved, the results obtained by surface wave inversion are very satisfactorily consistent with borehole data.