Estimation of shallow subsurface shear-wave velocity by inverting fundamental and higher-mode Rayleigh waves

Shear (S)-wave velocity measurement of a shallow subsurface with a velocity reversal (e.g., roadbed soils) is challenging. The present study first evaluates the effects of the inclusion of different levels of noise in the fundamental mode phase velocities on the inverted models. Advantages of incorporating higher modes (up to the third mode) into the inversion process are then investigated. Our modeling results demonstrate that the ability to reveal low-velocity layers is degraded with increasing noise. An inversion performed with only fundamental mode data with a high degree of error may yield an unrealistic model. They also indicate that higher modes are relatively more sensitive to changes in S-wave velocity than is the fundamental mode. A better-behaved model can be obtained when the second mode data are inverted simultaneously with the fundamental mode data. A closer match with the true model can be achieved by simultaneously inverting three modes of phase velocities. We verify our modeling results with a real-world example from a roadbed site in Henan, China.