10:45 AM - 12:15 PM
[SSS12-P15] Development of the borehole optical fiber strainmeter for Long-Term Borehole Monitoring System at the Nankai trough
Keywords:Nankai trough, Long-term Borehole monitoring system (LTBMS), Optical fiber sensing technology, Borehole optical fiber strainmeter
In the Nankai Trough subuction zone, three long-term borehole monitoring systems (LTBMS: C0002, C0010, and C0006) have been installed in the Kumanonada region as a part of IODP NantroSIZE project to monitor seismic activity, pore pressure change, volumetric strain change, temperature in the borehole. Among the data set, the pore pressure data has revealed that slow slip events occur repeatedly in the shallow part of the plate subduction boundary region (Araki et al., 2019). However, when these strain changes are converted into the amount of strain that can be detected on the seafloor, the strain changes detected on the seafloor are below the detection limit, it is difficult to monitor in the broad area using the DONET real-time monitoring system. Therefore, the JAMSTEC are planning to deploy a new LTBMS capable of observing slow slip with ultra-high sensitivity in the Kii Suido area (DONET2 area) in the Oct. 2023.
The new LTBMS has pore presure gauge and a fiber optical strainmeter which has been observed on the seafloor in the Nankai Trough in recent years (Araki et al., 2019, 2020). The optical fiber strainmeter can observe changes in the length of a 200m optical fiber with a resolution of 1 nm. For the borehole optical fiber strainmeter, 200 m optical fiber is wound around a mandrel and cemented inside the borehole, which enables highly sensitive observation of the volumetric strain in the borehole. And the fiber-optic strainmeter can perform wideband high-precision seismic observations including microearthquakes, strong ground motions and slow earthquakes (VLFE, shallow low frequency tremors) by sampling at 100Hz. Furthermore an optical fiber will be installed inside the borehole for observations by optical fiber sensing using leading technologies such as DAS and TW-COTDR.
In order to develop a borehole optical fiber straineter, we have carried out temperature characteristic evaluations and vibration tests. The optical fiber straineter uses two types of fiber, ZBL single mode fiber and PS1250 fiber, which have different temperature characteristics. Since we separate the temperature change and the strain change inside the borehole by using the temperature characteristic difference between the two optical fibers, it is necessary to understand the characteristics in the laboratory in advance. Also a vortex induced vibration caused by the strong kuroshio current in the Nankai trough can damage the LTBMS during the installation. Therefore we need to confirm that a sensing flame of the LTBMS and optical fiber set has anti-vibration mechanism and that the installation can be performed reliably. A demonstration test sensor with the same specifications as the actual LTBMS sensor was made for the evaluation. We carried out a vibration test using the demonstration test sensor, and it was confirmed that the LTBMS sensor has enough strength even though the VIV occurs during the installation. The demonstration test sensor will be installed in a 20m-deep borehole at the Kamioka Mine in March 2023. After installation in the borehole, cement is put in the borehole to reproduce conditions close to the actual site. After that, we plan to evaluate the borehole optical fiber strainmeter.
The new LTBMS will be installed in Oct. 2023 by the D/V Chikyu, and will be connected to DONET submarine calble in Jan. 2024 by the R/V Shinseimaru and become the fourt real-time LTBMS observatory in the Nankai trough region.
The new LTBMS has pore presure gauge and a fiber optical strainmeter which has been observed on the seafloor in the Nankai Trough in recent years (Araki et al., 2019, 2020). The optical fiber strainmeter can observe changes in the length of a 200m optical fiber with a resolution of 1 nm. For the borehole optical fiber strainmeter, 200 m optical fiber is wound around a mandrel and cemented inside the borehole, which enables highly sensitive observation of the volumetric strain in the borehole. And the fiber-optic strainmeter can perform wideband high-precision seismic observations including microearthquakes, strong ground motions and slow earthquakes (VLFE, shallow low frequency tremors) by sampling at 100Hz. Furthermore an optical fiber will be installed inside the borehole for observations by optical fiber sensing using leading technologies such as DAS and TW-COTDR.
In order to develop a borehole optical fiber straineter, we have carried out temperature characteristic evaluations and vibration tests. The optical fiber straineter uses two types of fiber, ZBL single mode fiber and PS1250 fiber, which have different temperature characteristics. Since we separate the temperature change and the strain change inside the borehole by using the temperature characteristic difference between the two optical fibers, it is necessary to understand the characteristics in the laboratory in advance. Also a vortex induced vibration caused by the strong kuroshio current in the Nankai trough can damage the LTBMS during the installation. Therefore we need to confirm that a sensing flame of the LTBMS and optical fiber set has anti-vibration mechanism and that the installation can be performed reliably. A demonstration test sensor with the same specifications as the actual LTBMS sensor was made for the evaluation. We carried out a vibration test using the demonstration test sensor, and it was confirmed that the LTBMS sensor has enough strength even though the VIV occurs during the installation. The demonstration test sensor will be installed in a 20m-deep borehole at the Kamioka Mine in March 2023. After installation in the borehole, cement is put in the borehole to reproduce conditions close to the actual site. After that, we plan to evaluate the borehole optical fiber strainmeter.
The new LTBMS will be installed in Oct. 2023 by the D/V Chikyu, and will be connected to DONET submarine calble in Jan. 2024 by the R/V Shinseimaru and become the fourt real-time LTBMS observatory in the Nankai trough region.