Cross-Correlation Analysis of Surface Waves for Group and Phase Velocity Mapping along a 2D Seismic Profile

Surface-Wave Analysis is widely adopted for estimating S-wave velocity of near-surface, a crucial parameter appearing in geotechnical standards. In this paper two different approaches are implemented and compared for the creation of pseudo-2D dispersion images of phase and group velocity through the analysis of multichannel cross-correlation (CC) of surface waves at a Test site. The surface wave travel time tomography approach can produce precise phase and group velocity maps, since the eikonal equation is used for the inversion of the automatic estimated phase and group delays. However, when sharp lateral variations are exist near surface, an average 1D reference phase velocity model is not adequate to properly estimate them, due to cycle skipping problems at high frequencies. The implementation of the common mid-point cross correlation approach results to smoother mapping of the velocity distribution, though sensitive to near surface variations, both on group and phase velocity. In that way, a more reliable 2D reference phase velocity model can be assigned to the first approach. Moreover, the 3D velocity dispersion imaging assists in defining specific problematic areas as well as frequency bands of possible significant higher mode interference, in order to be excluded from the 2D Vs inversion procedure.