Joint estimation of S-wave velocity and damping ratio of the near-surface from active Rayleigh wave surveys processed with a wavefield decomposition approach

The use of surface wave measurements to derive an S-wave velocity profile of the subsurface has become a widely applied procedure; however, their potential use to reconstruct the S-wave material damping properties of the subsoil is generally overlooked, due to the difficulties in obtaining consistent surface wave amplitude information from field data and translating them into robust estimates of the dissipative properties of the near-surface. In this work, we adapt a wavefield decomposition technique for the processing of elastic surface wave data to the extraction of the complete set of properties of Rayleigh waves generated by a controlled source and propagating in dissipative geomaterials. Retrieved information includes multimodal phase velocity and ellipticity as well as the frequency-dependent attenuation coefficient. We exploit the key advantages of wavefield decomposition processing (joint interpretation of multicomponent recordings, coupled estimation of wave propagation parameters, modelling of multiple superimposing modes) to maximize the robustness of the retrieval of Rayleigh wave properties, especially of the dissipative ones. For the subsequent interpretation of Rayleigh wave dispersion, ellipticity and attenuation data we implement a joint Monte Carlo inversion yielding a coupled estimate of S-wave velocity and damping ratio profile for the subsurface; we incorporate a series of geophysical constraints to narrow down the searched parameter space to realistic soil models. We apply this processing and inversion scheme to a bespoke synthetic data set and to a field survey for the characterization of a strong motion station; in both cases, we succeed in retrieving Rayleigh wave multimodal dispersion, ellipticity and attenuation curves. From the inversion of data from the simulated seismogram we are able to reconstruct the properties of the synthetic model. As for the real case, we determine an S-wave velocity and damping ratio model for the soil column below the station, through which we are able to model the inelastic earthquake local response observed at the site. Basing on the results obtained for the real case, we argue that one of the advantages brought by our processing method-the possibility to process active Rayleigh wave data acquired by a 2-D array illuminated by different source positions-may play a key role in allowing to retrieve dissipative properties of the near-surface closer to the material damping of the soil materials, and less influenced by scattering determined by possible discontinuities in the subsurface.