The importance of a detailed knowledge of the shear-wave velocity structure in the very shallow depth was recently stressed in strong ground motion studies. In this article, we investigate the 1D velocity and attenuation structure of the very shallow coverage of a sediment-filled valley by using surface waves from a small explosive source, recorded along a 65-m-long linear array. The group and phase velocity dispersion curves are obtained from the analysis of the multi-channel recordings and inverted for the velocity structure. The velocity model obtained in this way is then used as a new starting model for a waveform inversion process, in which both compressional- and shear-wave velocities are iteratively adjusted, obtaining a significative improvement of the fit between data and synthetics.
The Q structure is investigated by using the technique described in Malagnini et al. (1995). The velocity is kept fixed, and the reference frequency for the computation of causal Q is put at 10 Hz, due to the high-frequency content of the recorded waveforms. The resulting Q values in the upper layer (QP = QS = 9 in the first 20 m in depth) are higher than those previously observed at 1 Hz in the same site (QP = QS = 2 in the first 22 m in depth), suggesting, at least in the investigated case, a frequency-dependent attenuation structure for soft-soil coverages.