Improving the accurate assessment of a shear-wave velocity reversal profile using joint inversion of the effective Rayleigh wave and multimode Love wave dispersion curves

Joint inversion codes of the effective Rayleigh wave and multimode Love wave dispersion curves based on modified GA have been developed. We focus on the attempt to improve the estimated shear-wave velocity profile for a reversal subsurface structure. Two different profiles representing the high-velocity layer (HVL) and low-velocity layer (LVL) cases are used. Effective Rayleigh wave and multimode Love wave dispersion curves are synthesized from these profiles using full P-S V waveform and full SH-waveform reflectivity, respectively. The dispersion curves are then inverted using a modified genetic algorithm with two different inversion approaches, namely single inversion of Love/Rayleigh dispersion curves and joint inversion of Love and Rayleigh dispersion curves. To asses the accuracy of each inversion approach, differences between the true and inverted shear-wave velocity profile are quantified in terms of shear-wave velocity error, E-s. Our numerical modelling showed that the shear-wave velocity errors of the joint inversion approach are relatively smaller than the single inversion approach. These errors indicate that the accuracy of shear-wave velocity reversal can be improved by jointly inverting Rayleigh wave and Love wave dispersion curves. All of these approaches have been applied on the real data acquired at a site in Niigata prefecture, Japan. In this field example, our inversion results show good agreement between the calculated and experimental dispersion curves and can well detect LVL targets.