3:30 PM - 5:00 PM
[STT41-P01] Seismic Observation under a high temperature environment in a deep well using phase-shifted optical interferometry
Determination of the field site
We prepared a high temperature-compatible sensor and a high pressure-compatible borehole housing in Theme B2-2 of Integrated Program for Next Generation Volcano Research and Human Resource Development in FY 2020. In 2021, we found that a 3,000-m class observation well of Niigata Institute of Technology returned by Japan Nuclear Energy Safety Organization (JNES) was suitable as a field site for the application of these techniques. Temperatures of this observation well were approximately 105 and 140 °C at 2,000 m and 3,000 m, respectively (Figure 1). Niigata Institute of Technology is a private university located in Kashiwazaki City, Niigata Prefecture. The 3,000-m observation well was drilled to assess the effects of site characteristics of deep rocks, which were identified as an amplification factor of seismic ground motion for a nuclear plant during the 2007 Chūetsu offshore earthquake (Ref. 1). Presently, based on the shape of the observation well, we selected a method that could fix the casing using the steps located at a depth of 2,000 m (Figure 1).
Installation
As our system does not require electricity for the sensor, only optical fibers are installed in the communication line; thus, the cables has a diameter of approximately 6 mm, which is notably smaller than the diameter usually used at this depth. Thus, the installation process was extremely simple. In the applied method, the cable was extended on the ground and then lifted using a pulley hanging from a crane; later, the side of the cable being pulled was fixed and the pulley was lowered to lower the cable into the borehole. The cable was fixed at the borehole opening; subsequently, the cable was extended on the ground again. This method is called shakutori mushi (inchworm measurement). In the conventional deep seismometer, the seismometer casing and cable are heavy; thus, a large winch and a high-capacity generator are required to work and handle the cable. Further, post installation, the optical sensor seismometer only requires the cable to be fixed with a clip at the borehole opening; however, the conventional deep seismometer requires a large fixture to fix the cable on the ground.
Operation status and post installation
Except for planned power outranges and the optical interrogator disorder due to rising temperatures in the observation room during the summer, continuous observations were recorded since mid-February 2022, at temperatures exceeding 100 °C.
Noise level of the sensors
For comparison, sensors were also installed for observing the dirt floor of the observation hut. Figure 2 shows the noise spectra of the ground seismometer and the underground seismometer where observations were recorded for an hour from 02:00, August 6, 2022. The noise level of the underground seismometer was approximately 20 dB lower than the ground seismometer.
Observed earthquake
The Japan Meteorological Agency Hypocenter List shows an earthquake with a hypocenter located 700 m from the observation point. Figure 3 shows the records obtained with the optical sensor.
The results of epicenter determination using data from deep observation well are presented in Nakamiti, et al. (this conference session S-VC35).
Summary
We confirmed that our high temperature-compatible optical sensor can record high-performance observations for a long time even under a temperature of 100 °C or higher. Going forward, we hope to record longer observations at high temperature conditions and develop the sensor for actual volcano observations.
Acknowledgements:
This work was supported by MEXT “Integrated Program for Next Generation Volcano Research and Human Resource Development”.
We sincerely thank the Niigata Institute of Technology for permitting to use the observation well and observation hut.
References:
(ref. 1) JNES Research Achievements, 3rd Nuclear Seismic Safety Research Committee, Document 5–2, July 29, 2011
We prepared a high temperature-compatible sensor and a high pressure-compatible borehole housing in Theme B2-2 of Integrated Program for Next Generation Volcano Research and Human Resource Development in FY 2020. In 2021, we found that a 3,000-m class observation well of Niigata Institute of Technology returned by Japan Nuclear Energy Safety Organization (JNES) was suitable as a field site for the application of these techniques. Temperatures of this observation well were approximately 105 and 140 °C at 2,000 m and 3,000 m, respectively (Figure 1). Niigata Institute of Technology is a private university located in Kashiwazaki City, Niigata Prefecture. The 3,000-m observation well was drilled to assess the effects of site characteristics of deep rocks, which were identified as an amplification factor of seismic ground motion for a nuclear plant during the 2007 Chūetsu offshore earthquake (Ref. 1). Presently, based on the shape of the observation well, we selected a method that could fix the casing using the steps located at a depth of 2,000 m (Figure 1).
Installation
As our system does not require electricity for the sensor, only optical fibers are installed in the communication line; thus, the cables has a diameter of approximately 6 mm, which is notably smaller than the diameter usually used at this depth. Thus, the installation process was extremely simple. In the applied method, the cable was extended on the ground and then lifted using a pulley hanging from a crane; later, the side of the cable being pulled was fixed and the pulley was lowered to lower the cable into the borehole. The cable was fixed at the borehole opening; subsequently, the cable was extended on the ground again. This method is called shakutori mushi (inchworm measurement). In the conventional deep seismometer, the seismometer casing and cable are heavy; thus, a large winch and a high-capacity generator are required to work and handle the cable. Further, post installation, the optical sensor seismometer only requires the cable to be fixed with a clip at the borehole opening; however, the conventional deep seismometer requires a large fixture to fix the cable on the ground.
Operation status and post installation
Except for planned power outranges and the optical interrogator disorder due to rising temperatures in the observation room during the summer, continuous observations were recorded since mid-February 2022, at temperatures exceeding 100 °C.
Noise level of the sensors
For comparison, sensors were also installed for observing the dirt floor of the observation hut. Figure 2 shows the noise spectra of the ground seismometer and the underground seismometer where observations were recorded for an hour from 02:00, August 6, 2022. The noise level of the underground seismometer was approximately 20 dB lower than the ground seismometer.
Observed earthquake
The Japan Meteorological Agency Hypocenter List shows an earthquake with a hypocenter located 700 m from the observation point. Figure 3 shows the records obtained with the optical sensor.
The results of epicenter determination using data from deep observation well are presented in Nakamiti, et al. (this conference session S-VC35).
Summary
We confirmed that our high temperature-compatible optical sensor can record high-performance observations for a long time even under a temperature of 100 °C or higher. Going forward, we hope to record longer observations at high temperature conditions and develop the sensor for actual volcano observations.
Acknowledgements:
This work was supported by MEXT “Integrated Program for Next Generation Volcano Research and Human Resource Development”.
We sincerely thank the Niigata Institute of Technology for permitting to use the observation well and observation hut.
References:
(ref. 1) JNES Research Achievements, 3rd Nuclear Seismic Safety Research Committee, Document 5–2, July 29, 2011