GNGTS 2013 - Atti del 32° Convegno Nazionale

limited spectral resolution of finite array lengths, this ambiguity can be expected even with larger velocity differences between the two osculating modes. In order to investigate this situation we test synthetic cases with and without a large velocity contrast. The synthetic datasets (full-waveform seismic records) are generated using the SEM2DPACK software (Ampuero, 2008). The excitation source is, in all cases, a Ricker wavelet with 20 Hz frequency peak, located at the ground surface and corresponding to a vertical motion. The simulated recorded data are considered reliable up to 50 Hz. At the bottom and at the lateral boundaries we applied Stacey (1988) absorbing conditions. In order to assess purely the effects of the geological conditions which generate mode contamination, we simulated an optimal surface wave acquisition without spatial aliasing and spectral resolution issues, using a 640 m long acquisition array with 1 m receiver spacing. We consider 3 simple cases of the same subsoil model with a soft layer resting on top of a stiffer half space; we applied an increasing impedance contrast with the aim of generating the well known case of mode mis-identification as in Fig. 1. As in this simple modal synthetic case, a large velocity contrast shows an ‘osculation point’, which is the frequency around the two modes have a ‘kissing’ point. For frequencies smaller than the osculation frequency most energy is located on the first higher mode rather than on the fundamental mode, while for frequencies larger than the osculation frequency, energy remains on the fundamental mode. The behaviour of the high impedance contrast case can lead to serious misinterpretation of the dispersion curve, with very large overestimation of the bedrock shear wave velocity. Note that this erroneous velocity estimation is acting against precaution, that can have detrimental impacts on seismic amplification assessment. Inverting the apparent curve assuming that the curve belongs to a fundamental mode will lead to a large overestimation of the half-space velocity. This in turn can have a severe impact on seismic response analysis (e.g. Boaga et al. , 2011, 2012a). In practical cases, in presence of coherent and incoherent noise, with the loss of spectral resolution due to the limited array length and the geophone spacing, the identification of the Fig. 1 – Modal curves and ellipticity of the fundamental mode for three models with varying velocity contrasts: a 6m layer with shear wave velocity equal to 200 m/s layer resting on a half space having shear wave velocity equal to 300 m/s (blue), 400 m/s (black) and 500 m/s (red) respectively. 23 GNGTS 2013 S essione 3.1