A study of the lateral heterogeneity with the ellipticity of Rayleigh waves derived from microtremors

We examine the potential of frequency-dependent Rayleigh wave ellipticity, derived from microtremors, for the investigation of heterogeneous subsurface structure. Based on numerical simulation, we analyse the effects of interference waves in microtremors, primarily the various propagation directions of the Rayleigh waves, linear polarization waves and white noise, on the ellipticity frequency-dependent estimation of the Rayleigh waves. A data processing scheme to separate the Rayleigh waves from the interference waves is proposed and verified by synthetic data. We performed a field experiment in the mountainous areas of Southwest China to show that the ellipticity frequency dependence of Rayleigh waves in the period range of 0.05-5 s can be estimated from the microtremor records with the proposed data processing scheme. In addition, the method is feasible for investigating lateral heterogeneity within the top several hundred metres in the mountain regions. The study also reveals that the features of the ellipticity anomaly of a local heterogeneity are related to the propagation directions of the Rayleigh waves, and to reduce the ambiguity of the anomaly, the propagation direction of the waves picked for the ellipticity estimation should be consistent with (along or opposite to) that of the survey line. Then, to eliminate the effects of the phase differences due to the propagation direction, or time, the ellipticity for each location should be estimated by a single event rather than multiple events from the derived Rayleigh wave arrivals.