Multimodal Inversion of Surface Wave Data at Sites with Shallow Bedrock

In this paper results of a characterization campaign, conducted by means of surface wave tests, are presented. Tests are conducted on sites with shallow bedrock and the presence of stiff seismic interfaces. The sites are the location of some stations of the Italian Accelerometer Network in the Liguria Region (Italy) and the experimental tests are part of a the development of the Italian strong motion database. Due to the peculiar shear wave velocity profile of the sites, a multimodal inversion is required because the effective dispersion curve is strongly affected by the presence of higher modes of propagation. Three examples are presented, in order to show that a multimodal inversion is of paramount importance in order to increase investigation depth, avoid errors in the retrieved soil profiles due to mode misidentification and improve the resolution on shallow layers, particularly in situations in which the effect of higher modes could be relevant (i.e. velocity inversions or strong seismic contrasts). The used inversion algorithm is a stochastic algorithm, which demonstrates its ability of properly taking into account higher modes of propagation allowing for a correct definition of seismic velocities and interfaces depth in different stratigraphic conditions. Combined refraction interpretation performed on the same seismic dataset provides an useful comparison to assess the reliability of the results.