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[STT41-P02] Deep Seismic Observation System using a Three-Component Optical Accelerometer and Fiber-Optic Distributed Acoustic Sensor
Keywords:Optical seismometer, distributed acoustic sensing, deep wells, microearthquakes
Optical Hybrid System with a Three-Component Optical Accelerometer and distributed acoustic sensor
We developed a new observation system by combining a three-component optical accelerometer and an optical fiber DAS, which can operate even under high temperatures and pressures. This system was installed in a 3,000-m class observation well on the Niigata Institute of Technology campus at a depth of 2,000 m. The optical hybrid system is composed entirely of optical circuits and has no electronic components; additionally, it can function in extreme environments, such as those under high temperatures and pressure.
Underlying Mechanism of Optical Accelerometer
The optical accelerometer employs a method wherein laser light is injected into an optical fiber, and the displacement of the oscillator in a spring-mass-damper system attached to the end of the fiber is observed through interferometry. The pendulum is forced to vibrate by the input of ground motion, and the amplitude detected in the frequency range lower than the pendulum’s natural frequency is proportional to the acceleration of the ground motion, regardless of frequency, making it an accelerometer(Ref. 1).
Background of Observations at the Niigata Institute of Technology
This system that combines an optical accelerometer and DAS was deployed into a deep observation well at the Niigata Institute of Technology for recording observations. The well was initially established by the Japan Nuclear Energy Safety Organization in July 2011 to understand the deep-ground seismic wave transmission characteristics. Subsequently, in March 2019, this well was handed to the Niigata Institute of Technology. The temperatures in the well reached approximately 105 and 140 °C at depths of 2000 and 3000 m, respectively (Ref. 2); thus, an optical accelerometer that can record observations at temperatures above 100 °C was required for deep-ground observations.
Installation Since this system utilized only an optical fiber for communication and eliminated the need for electricity in the sensor components, the diameter of the cable required for installation in the deep well could be as small as 6 mm, thus, significantly reducing the size and complexity of the installation equipment and personnel and simplifying the overall installation process.
Deep Seismic Observations At 2000 m depth and 105 °C, the optical accelerometer continuously acquired 1 kHz sampling continuous data since February 21, 2022. Over the 130 days, since the beginning of the observation until July 6, 301 earthquakes were detected. Their epicenters were estimated using the particle trajectory analysis method on the single-point seismograph records (Fig. 1). Microearthquakes of magnitude −1 were detected in the vicinity of a 5-km radius of the deep well. Additionally, using spare fibers, continuous observations of DAS were conducted between July 27 and August 4, thereby capturing several natural earthquakes (Fig. 2).
Noise Level and Frequency Characteristics of the Three-Component Optical Accelerometer
To demonstrate the characteristics of the three-component optical accelerometer, Fig. 3 shows the noise spectrum. The noise spectrum of F-net Kashiwazaki near the observation point has also been shown for comparison. When the frequencies were >0.3 Hz, the three-component optical accelerometer had a lower noise level than HNM and could detect microseisms, but when the frequencies were <0.3 Hz, the noise level was higher than F-net’s records. Thus, improving the performance in relation to these slightly longer-period seismic motions is urgently required.
Conclusion The optical hybrid system consisting of a three-component optical accelerometer and DAS was found to operate stably for one year, even under temperatures exceeding 100 °C. Our findings showed that the system could detect microearthquakes using the optical accelerometer and analyze the in-ground transmission characteristics using DAS. Further observations and development will be conducted to improve the system performance, particularly for slightly longer-period seismic motions.
Acknowledgments
We thank the Niigata Institute of Technology for their invaluable support, including the use of observation wells and observation huts. We are grateful for their cooperation and assistance throughout the project.
References
1) Yoshida et. al., Jpn. J. Appl. Phys., vol. 55 (2016) 022701.
2) Integrated Program for Next Generation Volcano Research and Human Resource Development. Next Generation Volcano Propulsion Project Proposal B2-2 Results Report [In Japanese].
We developed a new observation system by combining a three-component optical accelerometer and an optical fiber DAS, which can operate even under high temperatures and pressures. This system was installed in a 3,000-m class observation well on the Niigata Institute of Technology campus at a depth of 2,000 m. The optical hybrid system is composed entirely of optical circuits and has no electronic components; additionally, it can function in extreme environments, such as those under high temperatures and pressure.
Underlying Mechanism of Optical Accelerometer
The optical accelerometer employs a method wherein laser light is injected into an optical fiber, and the displacement of the oscillator in a spring-mass-damper system attached to the end of the fiber is observed through interferometry. The pendulum is forced to vibrate by the input of ground motion, and the amplitude detected in the frequency range lower than the pendulum’s natural frequency is proportional to the acceleration of the ground motion, regardless of frequency, making it an accelerometer(Ref. 1).
Background of Observations at the Niigata Institute of Technology
This system that combines an optical accelerometer and DAS was deployed into a deep observation well at the Niigata Institute of Technology for recording observations. The well was initially established by the Japan Nuclear Energy Safety Organization in July 2011 to understand the deep-ground seismic wave transmission characteristics. Subsequently, in March 2019, this well was handed to the Niigata Institute of Technology. The temperatures in the well reached approximately 105 and 140 °C at depths of 2000 and 3000 m, respectively (Ref. 2); thus, an optical accelerometer that can record observations at temperatures above 100 °C was required for deep-ground observations.
Installation Since this system utilized only an optical fiber for communication and eliminated the need for electricity in the sensor components, the diameter of the cable required for installation in the deep well could be as small as 6 mm, thus, significantly reducing the size and complexity of the installation equipment and personnel and simplifying the overall installation process.
Deep Seismic Observations At 2000 m depth and 105 °C, the optical accelerometer continuously acquired 1 kHz sampling continuous data since February 21, 2022. Over the 130 days, since the beginning of the observation until July 6, 301 earthquakes were detected. Their epicenters were estimated using the particle trajectory analysis method on the single-point seismograph records (Fig. 1). Microearthquakes of magnitude −1 were detected in the vicinity of a 5-km radius of the deep well. Additionally, using spare fibers, continuous observations of DAS were conducted between July 27 and August 4, thereby capturing several natural earthquakes (Fig. 2).
Noise Level and Frequency Characteristics of the Three-Component Optical Accelerometer
To demonstrate the characteristics of the three-component optical accelerometer, Fig. 3 shows the noise spectrum. The noise spectrum of F-net Kashiwazaki near the observation point has also been shown for comparison. When the frequencies were >0.3 Hz, the three-component optical accelerometer had a lower noise level than HNM and could detect microseisms, but when the frequencies were <0.3 Hz, the noise level was higher than F-net’s records. Thus, improving the performance in relation to these slightly longer-period seismic motions is urgently required.
Conclusion The optical hybrid system consisting of a three-component optical accelerometer and DAS was found to operate stably for one year, even under temperatures exceeding 100 °C. Our findings showed that the system could detect microearthquakes using the optical accelerometer and analyze the in-ground transmission characteristics using DAS. Further observations and development will be conducted to improve the system performance, particularly for slightly longer-period seismic motions.
Acknowledgments
We thank the Niigata Institute of Technology for their invaluable support, including the use of observation wells and observation huts. We are grateful for their cooperation and assistance throughout the project.
References
1) Yoshida et. al., Jpn. J. Appl. Phys., vol. 55 (2016) 022701.
2) Integrated Program for Next Generation Volcano Research and Human Resource Development. Next Generation Volcano Propulsion Project Proposal B2-2 Results Report [In Japanese].