10:45 AM - 11:00 AM
[STT42-05] Monitoring of shallow subsurface using DAS records of surface waves continuously generated by a bridge
Keywords:DAS, Surface Wave Analysis, Passive Source
Surface wave analysis using Distributed Acoustic Sensing (DAS) is an economically efficient method that leverages existing optical fiber cables, making it suitable for long-term observations. Most extant communication cables are typically interred underneath roads and tend to capture copious and sustained acoustic emissions emanating from human activities, such as motor vehicles, making them ideal seismic sources. While they are often utilized as ambient noises in seismic interferometry analyses (e.g., Dou et al., 2017), they are seldom employed directly as passive seismic sources, except in the case of transient sources, such as quarry blasting (Fang et al., 2020), or vehicular traffic (Yuan et al., 2020). In this study, it was found that a bridge vibrated by moving vehicles continuously excite surface waves, which propagate over a maximum distance of about 5 km. Consequently, we explore the bridge as a fixed position, continuously vibrating, passive seismic source, and present the results of the analysis of these surface waves captured by DAS.
In this study, we utilized a fiber-optic cable buried beneath a certain section of National Route 47, which extends from Furukawa to Naruko Onsen in Osaki City, Miyagi Prefecture. The measurement period spanned from March 28 to April 21, 2022. The channel spacing was 6.4 m, the gauge length was 9.6 m, the total number of channels was 4855, covering a length of about 31 kilometers, and the sampling frequency was 200 Hz.
Continuous large vibrations were recorded on the channels corresponding to some bridges, and surface waves were observed to propagate steadily for up to approximately 5 km from both ends of the Shin-Iwadeyama Bridge in particular. Using records from channels within approximately 3 km to the east of the Shin-Iwadeyama Bridge, we calculated dispersion spectra of surface waves every minute using Multichannel Analysis of Surface Waves (MASW) [Park et al., 1999]. In order to obtain stable dispersion spectra, we needed to average 60 dispersion spectra per hour with one minute intervals. This process was carried out continuously for 24 days of the entire Distributed Acoustic Sensing (DAS) recordings, while overlapping 55 samples (55 minutes of data) to obtain consecutive images.
The obtained dispersion spectra showed stable peaks of the fundamental mode of Rayleigh waves from 2 Hz to 10 Hz and higher modes from 3 Hz to 16 Hz. The continuous monitoring of shallow seismic velocity structures on an hourly time scale for an extended period of time is a significant advantage of this method. However, its application is limited to the proximity of large bridges with relatively high traffic volume. Although the observation period did not witness any significant earthquakes that could have affected the ground, these findings highlight the potential utility of this technique for long-term monitoring of subsurface structure especially in tectonically active regions.
Acknowledgments:
We would like to thank the Ministry of Land, Infrastructure, Transport and Tourism for allowing us to use their optical fibers and the staff at the Sendai National Highway Office for their cooperation.
In this study, we utilized a fiber-optic cable buried beneath a certain section of National Route 47, which extends from Furukawa to Naruko Onsen in Osaki City, Miyagi Prefecture. The measurement period spanned from March 28 to April 21, 2022. The channel spacing was 6.4 m, the gauge length was 9.6 m, the total number of channels was 4855, covering a length of about 31 kilometers, and the sampling frequency was 200 Hz.
Continuous large vibrations were recorded on the channels corresponding to some bridges, and surface waves were observed to propagate steadily for up to approximately 5 km from both ends of the Shin-Iwadeyama Bridge in particular. Using records from channels within approximately 3 km to the east of the Shin-Iwadeyama Bridge, we calculated dispersion spectra of surface waves every minute using Multichannel Analysis of Surface Waves (MASW) [Park et al., 1999]. In order to obtain stable dispersion spectra, we needed to average 60 dispersion spectra per hour with one minute intervals. This process was carried out continuously for 24 days of the entire Distributed Acoustic Sensing (DAS) recordings, while overlapping 55 samples (55 minutes of data) to obtain consecutive images.
The obtained dispersion spectra showed stable peaks of the fundamental mode of Rayleigh waves from 2 Hz to 10 Hz and higher modes from 3 Hz to 16 Hz. The continuous monitoring of shallow seismic velocity structures on an hourly time scale for an extended period of time is a significant advantage of this method. However, its application is limited to the proximity of large bridges with relatively high traffic volume. Although the observation period did not witness any significant earthquakes that could have affected the ground, these findings highlight the potential utility of this technique for long-term monitoring of subsurface structure especially in tectonically active regions.
Acknowledgments:
We would like to thank the Ministry of Land, Infrastructure, Transport and Tourism for allowing us to use their optical fibers and the staff at the Sendai National Highway Office for their cooperation.