Inversion stability analysis of multimode Rayleigh-wave dispersion curves using low-velocity-layer models

Surface wave inversion is increasingly applied to estimate near-surface shear (S)-wave velocities for geological structures. Our study addresses the sensitivity and stability of the Rayleigh-wave multimode dispersion-curve inversion for earth models containing a low-velocity layer. Due to the dependence of Rayleigh-waves on the S-wave velocity over a range of depth, the inverted Vs generally have uncertainties that vary with depth and structure. An evolutionary algorithm was used to provide a population of final models from inversion of multimode Rayleigh-wave dispersion curves. We analysed the uncertainties of inversion results for three irregular Vs structures, i.e., three models with a low S-wave velocity layer. The results show that 1) because the low-velocity layer traps the energy of Rayleigh-waves and makes the wave travel within it, Rayleigh-wave phase velocities are insensitive to variations in layers beneath a low-velocity layer. This characteristic can influence the inversion stability for these layers' parameters. 2) The high degrees of uncertainties of inverted Vs for these layers still remain although the higher mode Rayleigh-wave data are included in the inversion procedure. It can be concluded that Vs estimates for layers beneath a low-velocity layer are with a low degree of confidence and need to be treated with extra caution.