On Beamforming of DAS Ambient Noise Recorded in an Urban Environment and Rayleigh‐To‐Love Wave Ratio Estimation

Abstract

Ambient noise sources in urban environments are generally not perfectly uniformly distributed or travel exactly along the axial direction of the receiver array. The horizontally deployed distributed acoustic sensing (DAS) fiber‐optic cable is able to record both Rayleigh and Love waves. Therefore, resolving the source propagation direction and the content of Rayleigh and Love waves from the DAS ambient noise data is essential to resolve a reliable near‐surface shear‐wave velocity model. However, a standard geophone‐based beamforming procedure may not apply to DAS ambient noise data due to the complex measurements of DAS. To address this issue, we investigate the validity of beamforming results by analyzing the simulated seismic ambient noise recorded by a DAS array close to an L‐shape under different source conditions. Synthetic examples demonstrate the importance of correcting amplitude imbalances and polarity reversals in the DAS ambient noise data to ensure a reliable beamforming analysis. In addition, we estimate the Rayleigh‐to‐Love wave (R/L) ratio from the DAS ambient noise data with the resolved source direction, the known array geometry, and the measured amplitude information. Finally, we apply the method to two 2‐minute DAS ambient noise data recorded by the Stanford DAS Array‐1. The results suggest that the analyzed data were mainly generated from the local traffic and dominated by Love waves.