Seismic Site Classification from Surface Wave Data to $V_{s,30}$ without Inversion

Surface wave methods have been extensively employed to estimate time-averaged shear-wave velocity ($V_s$) to $30\mathrm{m}$ ($V_{s,30}$) for seismic site classification. However, the inversion of surface wave dispersion data for $V_s$ profiling is inherently computationally intensive. To overcome the challenge involved in surface-wave inversion, a new method is proposed to directly estimate $V_{s,30}$ from surface wave fundamental-mode dispersion data without inversion. The new method is based on the concept that the phase velocity of surface waves ($V_R$) at a given frequency ($f$) is proportional to the average shear-wave velocity of soils within one-half of the wavelength ($\lambda =V_R/f$). The proposed method starts from calculating the $V_s$ of the top layer as it can be directly estimated from the phase velocity of surface waves whose half wavelengths are smaller than the thickness of the top layer. Likewise, the $V_s$ of the second layer can be deduced from the top layer $V_s$ along with the phase velocity of surface waves whose half wavelengths are within the thickness range of the second layer. Through such a repetition of sequence-deduction, the $V_s\mathrm{s}$ of the other layers can be calculated. This method is verified with five case studies that cover typical geotechnical sites. Results indicate that the proposed method can obtain a similar $V_{s,30}$ with the same site classification for seismic design in comparison with invasive tests and inversion techniques.