3:45 PM - 4:00 PM
[STT42-08] Development of a Ground Monitoring Method Using Optical Fiber DAS and Microtremor Survey (No.3)
Keywords:optical fiber, Distributed Acoustic Sensing, microtremor survey, ground
At a test site in Tsukuba City, we conducted ground motion measurements using fiber-optic DAS. Measurements were performed using three different interrogators and five types of fiber-optic cables, including four single-mode and one multi-mode cable. The ground motion measurement results from hammering excitation and ambient microtremors showed no significant differences in amplitude spectra or phase characteristics across all cable and interrogator combinations. Furthermore, a comparison with strain velocity calculated from microtremor records indicated that the amplitude and phase spectra matched well in the frequency range above approximately 2 Hz. These results demonstrate that while the applicable frequency range is approximately 2 Hz and above, fiber-optic DAS measurements can achieve the same level of accuracy as conventional microtremor sensors in capturing the velocity structure of near-surface layers.
Ground motion measurements were conducted using fiber-optic cables along National Route 6 (survey line length: 56.6 km) and National Route 50 (survey line length: 62.8 km). In fiscal year 2023, measurements were performed using a wide dynamic range acquisition mode (Mode 8, ONYX, Sintela), while in fiscal year 2024, a measurement mode with enhanced sensitivity to low frequencies (Mode 9, ONYX peta, Sintela) was employed. The use of Mode 9 enabled the measurement of microtremor strain velocity over distances exceeding 40 km, which was not possible with Mode 8. By applying seismic interferometry to the observed strain velocity data, we estimated the phase velocity of surface waves and inferred the S-wave velocity structure down to a depth of approximately 80 m. The estimated phase velocities were generally consistent with existing microtremor survey data and also aligned well with the AVS30 values from J-SHIS. These results demonstrated that fiber-optic DAS measurements allow for high-density, high-precision estimation of shallow subsurface structures. Furthermore, seismic interferometry using vertical velocity waveforms recorded by microtremor sensors installed near the national highways, in conjunction with strain velocity data from fiber-optic DAS, enabled the clear estimation of surface wave phase velocities. This finding suggests that the methodology can be extended beyond fiber-optic routes to two-dimensional measurements.
Seismic motion measurements were conducted using borehole observations at Hi-net stations (Moka) with depths of 1648 m, as well as fiber-optic cable measurements in Sagami Bay along a survey line of approximately 65 km. The results demonstrated the applicability of this method under various conditions, confirming its potential for applications such as detailed estimation of borehole velocity structures, subsurface investigations of the seafloor, and monitoring the installation conditions of submarine fiber-optic cables.
To enable long-term and real-time monitoring, we developed a prototype system incorporating edge processing for on-site operation at measurement sites. A key challenge in fiber-optic DAS measurements is the massive data size. To address this, the edge processing system automatically reduces data volume and extracts key features, thereby minimizing data transfer to data centers while allowing for effective ground condition assessment. To evaluate the practicality of this system, a experiment was conducted at a test site using an interrogator (Mode9, ONYX peta, Sintela) under operational conditions. Two data input methods were tested: a file-based approach with minute-level intervals and a TCP socket-based approach with second-level intervals. The evaluation was performed on measurement data acquired over a fiber length of approximately 1000 m with a sampling frequency of 500 Hz. The results confirmed that the edge processing system operated in real time and continuously. Additionally, we explored ground analysis methods utilizing fiber-optic DAS, including two-dimensional F-K analysis, and assessed its effectiveness for high-density, real-time ground analysis.
Acknowledgments: This research was conducted as part of the Innovative Science and Technology Initiative for Security, Grant Number JPJ004596 of ATLA, Japan. We would like to express our gratitude to the Hitachi River and National Highway Office of the Kanto Regional Development Bureau, MLIT, for their cooperation in conducting observations along national highways.
Ground motion measurements were conducted using fiber-optic cables along National Route 6 (survey line length: 56.6 km) and National Route 50 (survey line length: 62.8 km). In fiscal year 2023, measurements were performed using a wide dynamic range acquisition mode (Mode 8, ONYX, Sintela), while in fiscal year 2024, a measurement mode with enhanced sensitivity to low frequencies (Mode 9, ONYX peta, Sintela) was employed. The use of Mode 9 enabled the measurement of microtremor strain velocity over distances exceeding 40 km, which was not possible with Mode 8. By applying seismic interferometry to the observed strain velocity data, we estimated the phase velocity of surface waves and inferred the S-wave velocity structure down to a depth of approximately 80 m. The estimated phase velocities were generally consistent with existing microtremor survey data and also aligned well with the AVS30 values from J-SHIS. These results demonstrated that fiber-optic DAS measurements allow for high-density, high-precision estimation of shallow subsurface structures. Furthermore, seismic interferometry using vertical velocity waveforms recorded by microtremor sensors installed near the national highways, in conjunction with strain velocity data from fiber-optic DAS, enabled the clear estimation of surface wave phase velocities. This finding suggests that the methodology can be extended beyond fiber-optic routes to two-dimensional measurements.
Seismic motion measurements were conducted using borehole observations at Hi-net stations (Moka) with depths of 1648 m, as well as fiber-optic cable measurements in Sagami Bay along a survey line of approximately 65 km. The results demonstrated the applicability of this method under various conditions, confirming its potential for applications such as detailed estimation of borehole velocity structures, subsurface investigations of the seafloor, and monitoring the installation conditions of submarine fiber-optic cables.
To enable long-term and real-time monitoring, we developed a prototype system incorporating edge processing for on-site operation at measurement sites. A key challenge in fiber-optic DAS measurements is the massive data size. To address this, the edge processing system automatically reduces data volume and extracts key features, thereby minimizing data transfer to data centers while allowing for effective ground condition assessment. To evaluate the practicality of this system, a experiment was conducted at a test site using an interrogator (Mode9, ONYX peta, Sintela) under operational conditions. Two data input methods were tested: a file-based approach with minute-level intervals and a TCP socket-based approach with second-level intervals. The evaluation was performed on measurement data acquired over a fiber length of approximately 1000 m with a sampling frequency of 500 Hz. The results confirmed that the edge processing system operated in real time and continuously. Additionally, we explored ground analysis methods utilizing fiber-optic DAS, including two-dimensional F-K analysis, and assessed its effectiveness for high-density, real-time ground analysis.
Acknowledgments: This research was conducted as part of the Innovative Science and Technology Initiative for Security, Grant Number JPJ004596 of ATLA, Japan. We would like to express our gratitude to the Hitachi River and National Highway Office of the Kanto Regional Development Bureau, MLIT, for their cooperation in conducting observations along national highways.