Distributed acoustic sensing using dark fiber for imaging the expressway underground structures

Distributed Acoustic Sensing (DAS) represents an innovative seismic observation technology based on fiber optic sensing methodology, which enables the transformation of existing urban fiber optic cables into ultra-dense seismic arrays. As a passive seismic imaging technique, ambient noise tomography has been extensively applied in urban subsurface structure exploration. In this study, we utilized the backup communication fiber (dark fiber) from the communication cable buried in the median strip along an expressway near Nanjing to construct a seismic observation array comprising 3,912 channels with a total length of 8 kilometers. The research analyzed the characteristics of ambient noise signals in different environmental settings along the expressway and performed ambient noise tomography on a 2-kilometer section in the middle of the array. The obtained S-wave velocity profile shows remarkable consistency with borehole data, demonstrating the feasibility of this technology for shallow structure imaging along expressways. This study aims to establish a robust and practical workflow for long-term safety monitoring of subsurface structures beneath expressways.