Surface-wave free-field inversion based on dispersion property in layered media

Surface-wave free-field inversion is one of the important steps in realizing the seismic inputs to the soil-structure interaction (SSI) systems and further revealing the real seismic response behaviors. Currently available inversion methods regard all ground motions as the body-wave components alone and invert only the body-wave free fields. To make up for the lack of neglecting surface waves in inversion implementations, a surface-wave free-field inversion method based on the dispersion property is proposed in this study to provide more accurate surface-wave excitation for the seismic analysis of SSI systems resting on layered media. With the problem that multiple modes of the dispersive surface waves are coupled to each other and difficult to separate, the proposed method employs the average energy flow density to characterize the participation volume of dispersive modes and then calculates the mode participation factors using the frequency-domain dynamic stiffness matrix. The calculated mode participation factors are used to decouple the dispersive modes and assign the ground surface waves to each single mode, allowing for the inversion of single-mode surface-wave free fields. Finally, all single-mode surface-wave free fields are superimposed to form the multi-mode wavefields through the mode superposition theory. The findings from the analysis of accuracy and applicability of the proposed inversion method demonstrate its capability to effectively recover the actual underground surface-wave free fields, and shed light on the notable impact of dispersive surface waves on the seismic responses of SSI systems, thus providing quantitative guidance for the seismic design and risk assessment of SSI systems.