On the feasibility of surface waves approximation to interpret ambient vibrations wave field

Random vibrations monitoring and analysis represents a cheap and effective tool for the seismic characterization of the subsoil. A standard approach to the physical interpretation of ambient vibrations measurements is based on the hypothesis that surface waves dominates the relevant wave field. To evaluate the reliability of this hypothesis, a general physical model for ambient vibrations generated by anthropic activities has been. In this model, the complete wave field generated by a random distribution of independent point loads at the surface of a weakly dissipative layered Earth has been considered. Horizontal to Vertical Spectral Ratios computed from the complete wave field solution have been compared with those deduced in the surface waves hypothesis. This comparison suggests that this approximation supplies reliable only results in the frequency range above the resonance frequency of the local subsoil. At the resonance frequency, a reasonable agreement is obtained when a source-free area exists around the receiver with a radius of the order of few tens of meters.