Two-station analysis of passive surface waves with continuous wavelet transform and plane-wave-based beamforming

Abstract

Using the cross-correlation or cross-coherence method to extract the traveltime information of surface waves between two stations from the ambient noise seismic data has become a popular method, which is widely utilized in shallow, regional, and global-scale geophysical surveys. Different from the common-used narrow-band filter (NBF), we adopt the continuous wavelet transform (CWT) as an alternative method to measure the phase velocity from the interstation cross-correlation function (CCF). On account of the distribution of urban noisy sources it is hard to satisfy the requirement of random, stable, uncorrelated, and uniform, which usually makes the measured phase-velocity uncertain. For those surveys with an ultra-short recording time of noise, the adverse effect would be more severe. Hence, we propose to adopt the plane-wave-based beamforming (PWBB) to scan the incoming direction of surface waves based on the hypothesis of plane-wave propagation. To suppress the incoherent components during the stack of CCFs, the phase-weighted stack (PWS) technique is also included in the proposed workflow. Several synthetic tests have verified the good performance of the new method. Especially when only a few seismic sensors are arranged in the research area, the advantages of our method would be more obvious. Then, the workflow is applied to the analysis of the field noise data, which were collected at Fengman hydropower station, Jilin province, China. The beamforming result indicates that the main component of the signals is produced by the effects of river scouring under the reservoir gate. Finally, the S-wave velocity tomogram of the research area and its geological interpretation are given. All data tests demonstrate that the proposed CWT with PWBB can achieve the stable dispersion measurement from the recorded seismic data of two station.