Assessing the Benefits of Using Multiple Modes in Surface Wave Inversion

The use of multiple-mode dispersion data in surface wave inversion to derive shear wave velocity (vs) profiles has been recognized as a means to improve the accuracy of the inversion result. However, the error associated with non-uniqueness in multiple-mode inversion has not been quantified, so this research was performed to assess this type of error. In this research, inversion of surface wave dispersion data was performed using either one or two modes. Two different vs profiles were assumed, and “experimental” multiple-mode dispersion data were synthesized from these profiles. The dispersion data were then inverted using the Levenberg-Marquardt method. The results demonstrate that, as expected, inclusion of a second mode does not improve the uniqueness of the inversion for regular profiles where stiffness increases with depth. However, inclusion of a second mode significantly improves the uniqueness of the inversion for irregular profiles, where stiffness decreases with depth. For irregular profiles, the match between the true and inverted vs profiles is greatly improved by including a second mode in the inversion. The results also demonstrate that regardless of the nature of the profile, the uniqueness of the inversion improves when the starting profile more closely matches the true profile.