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

摘要

Rayleigh waves generated by a 10-kg dynamite explosion were recorded along a dense, linear array of 47 seismic stations. The array had a total length of 1.4 km, and it was deployed on the sediments of the Po River floodplain near the city of Ferrara (Northern Italy). The recorded signals were dominated by fundamental-mode Rayleigh waves. Several techniques, including multiple-filter analysis, phase velocity stacking, and single-station cross-correlation, were used to obtain group and phase velocity dispersion from subsets for the data set, Synthetic seismograms were computed for one velocity structure, showing a good match to the recorded waveforms. Late Rayleigh-wave codas were characterized 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 reticular system of ancient, secondary river beds. Compared to the rest of the floodplain, these preferential directions of local drainage are characterized 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 that the analysis of explosion-induced Rayleigh waves can be used for the geophysical characterization of flat-layered structures at a local scale for seismic engineering purposes. [References: 13]