Constraint on low-velocity layer using higher mode Rayleigh waves in the shallow structure research

Due to high sensitivity to S-wave velocity, Rayleigh-wave dispersion curves of the fundamental and higher modes are usually used to invert near-surface S-wave velocities in engineering geophysical exploration. For the model containing a low-velocity layer, the dispersion curves of the fundamental and higher modes show two typical characteristics. One typical characteristic is that the crossover would be observed between different modes, and the fundamental mode shows obvious indication of low-velocity characteristics in interested frequency ranges. For the other kind of model with low-velocity layers, the dispersion curves have no visual crossing phenomenon in the frequency range of interest, and the low-velocity characteristics may not be observed in the measured dispersion curves. For the latter model containing a low-velocity layer, which is often encountered in practice, investigations on the inversion of multi-mode Rayleigh waves are conducted in this paper based on seismic reflection data. The studies show that if the observed fundamental-mode dispersion curve does not include the frequency band sensitive to the depth of the low-velocity layer, the inversion based on the fundamental-mode alone may not be able to recover the low-velocity characteristics of the model. But the low-velocity layer can be reconstructed accurately by inversion considering both the fundamental and higher mode Rayleigh waves even the observed fundamental mode dispersion curve has no obvious indication of low-velocity characteristics.