SHEAR-WAVE VELOCITY STRUCTURE OF SEDIMENTS FROM THE INVERSION OF EXPLOSION-INDUCED RAYLEIGH-WAVES - COMPARISON WITH CROSS-HOLE MEASUREMENTS

Rayleigh waves generated by a 10-kg dynamite explosion were recorded a long a dense, linear array of 47 seismic stations. The array had a tot al length of 1.4 km, and it was deployed on the sediments of the Po Ri ver floodplain near the city of Ferrara (Northern Italy). The recorded signals were dominated by fundamental-mode Rayleigh waves. Several te chniques, including multiple-filter analysis, phase velocity stacking, and single-station cross-correlation, were used to obtain group and p hase velocity dispersion from subsets for the data set, Synthetic seis mograms were computed for one velocity structure, showing a good match to the recorded waveforms. Late Rayleigh-wave codas were characterize d by persistent, monochromatic ringing, interpreted as due to surface waves scattered by weak lateral heterogeneities. Scattering within the alluvium of the great Po River Valley likely occurred across a reticu lar system of ancient, secondary river beds. Compared to the rest of t he floodplain, these preferential directions of local drainage are cha racterized by different sedimentary environments. The availability of in situ, cross-hole measurements of shear-wave velocities allowed the comparison with our results from surface observations, demonstrating t hat the analysis of explosion-induced Rayleigh waves can be used for t he geophysical characterization of flat-layered structures at a local scale for seismic engineering purposes.