Inversion of Rayleigh waves using a genetic algorithm in the presence of a low-velocity layer

The shear-wave velocity profile can be obtained by the velocity of Rayleigh waves through the back-calculation based on dispersion curves. However, the dispersion curves obtained in practical application are always discontinuous and correspond to different mode branches due to mode jumping, especially in the presence of low-velocity layer. Mode misidentification may be encountered in inversion based on these jumped dispersion curves. Mode analysis demonstrates that the mode jumping is caused by a different surface displacement distribution with frequency for each mode. This indicates that the surface displacement distribution of the modes should be taken into account for the case of a low-velocity layer. Shear-wave velocity profiles are inversed based on the (possibly discontinuous) dispersion curves of fundamental and/or higher modes using a genetic algorithm (GA). In addition to the dispersion characteristics, the surface displacement distribution is also taken into account for the case of a low-velocity layer; as a result, mode misidentification is avoided.