Improving the analysis and inversion of multimode Rayleigh-wave dispersion by using group-delay time information observed on arrays of high-frequency sensors

Near-surface shear-velocity structure can be inferred from multimode dispersion data. Several methods have been developed to isolate the different modes from seismic signals observed on linear arrays of sensors. Most techniques analyze the wavefield through a frequency-wavenumber (f-k) transform, paying little attention to group-delay-time information. Moreover, classical analyses are generally restricted to fundamental-mode dispersion, limiting the resolution power at depth. We have overcome the limitations of classical f-k analysis by using a wavefield representation in the group-velocity/phase-velocity (U-c) domain. We have then set up a nonlinear inversion procedure, easily tractable on a common field computer, to constrain the 1D vertical profile of shear velocities. Applications to synthetic data and to a set of actual records show that U-c diagrams greatly help to separate dispersion information between different modes, even when they are not detectable on usual f-k diagrams. Tests on synthetic and actual data confirm that the inversion procedure quickly converges to the expected model.