BEHAVIOR OF APPARENT DISPERSION CURVE AND ITS IMPLICATION TO MASW TESTING

Despite their efficiency and usefulness in geotechnical engineering, the un-standardized surface wave methods are still limited in certain aspects. Among these methods, the MASW (multi-station analysis of surface wave) is the more popular one nowadays. The MASW is mainly based on picking the fundamental-mode dispersion curve for shear-wave velocity inversion. However, the fundamental mode may not always dominate. This study investigates specifically under what condition and how higher modes may affect the apparent dispersion curves, i.e., apparent from dispersion analysis of seismic recordings. Four simplified yet distinct types of shear-wave velocity profiles were used to examine the experimental dispersion characteristics in relation to their theoretical modal dispersion curves. When there is an embedded layer which is softer than the surface layer, the dominant mode jumps to higher modes at wavelength greater than the bottom depth of the soft layer. As the soft layer moves deeper, the information contained in the fundamental-mode inversion is reduced. The transition between different modes may be smooth and the apparent dispersion curve may appear as a single mode. When multiple modes do separate, there is no fixed pattern as to which higher mode would dominate the sequence of mode jumping. Therefore, definite assignment of mode numbers in multi-mode inversion is unlikely. This study not only reveals this potential pitfall, but also shows remarkable difference in dispersion images and patterns of dominant higher modes in response to different velocity profiles. A modified simplified inversion can correctly detect the embedded soft layer and the type of velocity profile, but the accuracy is not always good. More rigorous inversion based on the apparent dispersion curve or dispersion spectrum is imperative to further improve the reliability of the MASW method.