Dispersion-curve imaging nonuniqueness studies from multi-channel analysis of surface waves (MASW) using synthetic seismic data

We use the multi-channel analysis of surface waves (MASW) method to analyze synthetic seismic data calculated from models that produce very similar high-velocity dispersion curve gradients at low frequencies. The MASW dispersioncurve images of the Rayleigh wave were obtained from data calculated using simple layered models with high (2-layer) and gradual (10-layer) shear-wave velocity (Vs) vertical gradients. The tests demonstrated that, for most of the frequency range, the corresponding dispersion-curve images are near identical with a high-velocity-gradient along low frequencies (HVGLF). The HVGLF trend obtained from the 2-layer model is a result of fundamental-mode Rayleigh-wave transitioning into the first higher mode, a ‘mode kissing.’ The HVGLF trend obtained from the 10-layer model is a result of only the fundamental-mode Rayleigh wave. It was observed that such nonuniqueness can lead to erroneous interpretation of dispersion events and consequent Vs inversion results. Further analysis showed that dispersion curve image patterns differ at relatively very low and very high frequency ranges. As a result, it was concluded that it might be possible to obtain evidence of how to interpret HVGLF dispersion curve images by observing specific patterns that fall outside the main HVGLF dispersion-curve feature at lower and higher frequencies.