Directional Sensitivity of DAS and Its Effect on Rayleigh-Wave Tomography: A Case Study in Oxnard, California
Distributed acoustic sensing (DAS) provides dense arrays ideal for seismic tomography. However, DAS only records average axial strain change along the cable, which can complicate the interpretation of surface-wave observations. With a rectangular DAS array located in the City of Oxnard, California, we compare phase velocity dispersion at the same location illuminated by differently oriented virtual sources. The dispersion curves are consistent for colinear and noncolinear virtual sources, suggesting that surface-wave observations in most of the cross-correlations are dominated by Rayleigh waves. Our measurements confirm that colinear channel pairs provide higher Rayleigh-wave signal-to-noise ratio (SNR). For cross-correlations of noncolinear channel pairs, the travel time of each connecting ray path can still be obtained despite the lower SNR of Rayleigh wave signals. The inverted Rayleigh-wave dispersion map reveals an ancient river channel consistent with the local geologic map. Our results demonstrate the potential of DAS-based 2D surface-wave tomography without special treatment of directional sensitivity in areas where one type of wave is dominating or can be identified.
The authors are grateful to the City of Oxnard, California, for access to their fiber‐optic network and the support provided by Mike Shaffer, the City of Oxnard's GIS manager. The iDAS v2 system is supported by National Science Foundation (NSF) Grant Number MRI‐1920334. Z. Z. acknowledges support from the Moore Foundation, NSF Faculty Early Career Development Program (CAREER) Award Number 1848166, and U.S. Geological Survey (USGS) Grant Number G22AP00067. J. F. and Z. Z. thank Weiqiang Zhu, Wenbo Wu, and Yida Li for helpful discussions. The authors also appreciate constructive comments and suggestions from two anonymous reviewers.