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Love Wave Ambient-Noise Imaging of Urban Subsurface Velocity Structures: Exploiting the Potential of Horizontally Orthogonal DAS Array

Description: 

Recent advances in Distributed Acoustic Sensing (DAS) technique significantly benefit urban seismology by transforming pre-installed telecommunication fiber-optic cables into dense arrays of seismic sensors. Interferometry of DAS-recorded seismic ambient noise wavefields has been widely applied to analyze surface wave dispersion and thus image seismic velocity structures of the shallow subsurface (~ several hundreds meters) in urban areas. Many previous studies only focused on Rayleigh waves within linear fiber segments to avoid the ambiguity of mixed surface wave modes induced by complex local fiber orientations. However, as more and more two-dimensional dense grids of fiber-optic cables in cities are exploited by DAS technique, ambient noise interferometry that takes advantage of complex DAS array geometry can provide new opportunities for seismic monitoring of subsurface structures in urban environments. In this work, we perform dispersion analysis of Love waves extracted from ambient noise interferometry using orthogonal branches of fiber-optic cables. Theoretical angular responses of DAS ambient noise cross-correlation, together with numerical experiments, help identify sensor pairs expected to record stronger Love waves than Rayleigh waves. We test our methodology on DAS data recorded under San Jose, California. We successfully obtain three sets of Love wave dispersion maps, including both phase and group velocities, with various channel pair orientations. Phase-matched filtering is applied to clean the waveform and thus smooth the results. Consistency of Love wave dispersions among these three sets indicate the robustness of our analysis. We finally perform inversion of Love wave dispersion relations to obtain depth-dependent subsurface velocity structures of the top ~ 100 m. Our Love wave inversion result is consistent with the model presented by Hayashi & Burns (2020), which is obtained from Rayleigh wave dispersion and H/V spectral ratio. This indicates the potential of surface wave analysis on fiber-optic cables with complex geometry, which can further advance the seismic monitoring of urban areas.

Session: Understanding Earth Systems with Fiber-optic Cables

Type: Oral

Date: 4/19/2023

Presentation Time: 11:30 AM (local time)

Presenting Author: Qing Ji

Student Presenter: Yes

Invited Presentation: 

Authors