Higher modes of Rayleigh wave extraction from ambient noise and application to structure inversion: a case study in Weiyuan County of Sichuan Province

Cross-correlation of ambient noise is an effective approximation of Green's function, which contains the information of seismic wave propagation path between the pair of stations. Based on the theory, the technique to extract Rayleigh wave dispersion from cross-correlation function of ambient noise, and application to velocity structure inversion are widely used in regional and global regions. However, most of above studies only utilize the fundamental mode of Rayleigh waves and ignore the higher modes. There may be a large uncertainty in the inversed structure only with fundamental mode. Moreover, compared with the fundamental mode sensitive to the shallow structure, the higher modes can constrain the deeper structure. Combination of fundamental and higher-mode dispersion in structure inversion can make full use of surface wave of ambient noise, increase the constraints on the underground structure and reduce the non-uniqueness of the inversion. Hence, it is necessary to develop a method to extract stable higher modes dispersion and for joint inversion of fundamental and higher modes dispersion.

In this paper, we first introduce the vector wavenumber transform method (VWTM) to extract higher modes of Rayleigh wave from ambient noise and show the theoretical derivation. Then, we collect the seismograms recorded by 50 short-period seismometers in Weiyuan County of Sichuan Province between the years 2015 and 2016. After that, we extract higher modes of Rayleigh wave from the ambient noise with VWTM and combine the fundamental and higher-mode dispersion to inverse the underground structure. At last, we discuss the validity and limitation of the VWTM and the inversed underground structure.