[STT51-03] Performance evaluation of seismic observation system using phase-shifted optical interferometry
Keywords:Phase-shifted optical interferometry, Noise observation, PSD
Introduction
In order to evaluate the performance of a seismic observation system using phase-shifted optical interferometry (the following, optical sensor system), we observed using this system continuously for about one month in the Tono Yamazaki Observation Tunnel of Tohoku University.
Principle of the optical interferometry
This optical sensor system uses a method to observe the displacement of a spring-mass-damper-system sensor by interferometry. Specifically, an optical pulse whose part is shifted by π / 2 is branched, one is reflected by the reference surface, and the other is reflected by the vibration surface of the pendulum, and one is appropriately delayed to cause interference. This measures the displacement of the vibrating surface (Fig. 1). The pendulum vibrates in response to ground motion input and the amplitude detected in the lower frequency domain than the natural frequency of the pendulum is proportional to the acceleration of the ground motion, so it becomes an accelerometer (ref. 1, 2).
Observation
The Yamazaki Station is a granite tunnel, and one of the group seismic observation networks around the Tono seismological observatory. This station is shared by Tono Yamazaki (code TYS, latitude 39.3772N, longitude 141.5932E, altitude 346m) in F-net of NIED. The optical sensor system was installed at the Tono Yamazaki Station on December 17, 2019, and observed continuously until January 16, 2020. The Tono Yamazaki Station is a straight tunnel with a total length of about 50m. There is an observation room immediately after the entrance, three doors on the way, and a sensor room at the back. The optical transmitter / receiver was installed in the observation room, and three sets of three-component optical sensors were installed in the sensor room. The optical cable connecting the optical transmitter / receiver and optical sensors was passed through a gutter. The natural frequency of the sensor is 2 sets at 24 Hz and 1 set at 100 Hz. For comparison, JU410 micrometermeter made by Hakusan Corporation equipped with a servo accelerometer (JA-40GA, made by Japan Aviation Electronics) was installed in the sensor room from December 17 to 19, 2019. In addition, the data of STS-2 of NIED F-net in the sensor room was also compared with our record.
Result
1) Comparison with the servo accelerometer
The noise spectra of optical sensor and JU410 were obtained using the data during quiet nights when no earthquake occurred. Each sampling frequency is 100 Hz. The optical sensor system is bandpass filtered from 0.1Hz to 30Hz. The gain setting of the JU410 is × 5, the maximum measurement range is ± 2G, and a minimum phase filter with a cutoff of 40% is applied. Fig. 2 is a graph of the PSD of JU410 and the optical sensor with a natural frequency of 24 Hz, using a 30-minute recording from 1:00 am to 1:30 am on December 18, 2019. It can be seen that the noise level of the optical sensor is lower than that of the servo accelerometer.
2) Comparison with STS-2
Fig. 3 is a graph of a PSD using data of about 30 minutes from 0:30 am to 1:00 am on January 16, 2020. The record of STS-2 is differentiated and converted to acceleration. From Fig. 3, it can be seen that the noise level of the optical sensor system almost coincides with the noise level of STS-2 in the range of about 0.13 Hz to about 20 Hz. That is, in this frequency band, it is determined that the optical sensor system detects ground motion noise of Tono Yamazaki Station.
Future tasks
Since the optical sensor has no electronic components in the detector, it can be operated in a high-temperature environment, and short-term high-temperature tests have confirmed that it can operate normally up to about 200 ° C. It is necessary to confirm the durability of performance in the future.
Acknowledgments
From Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University has been granted permission to use the Tono Yamazaki Station. Mr. Kono of Tono Seismological Observatory, Tohoku University provided great support at the site. The F-net data of the Tono Yamazaki Station was downloaded from the NIED Hi-net website and used.
References
1) Yoshida et. al., Jpn. J. Appl. Phys., vol.55 (2016) 022701.
2) Tsutsui et al., J. Disaster Res., vol.14 (2019) pp. 592-603.
In order to evaluate the performance of a seismic observation system using phase-shifted optical interferometry (the following, optical sensor system), we observed using this system continuously for about one month in the Tono Yamazaki Observation Tunnel of Tohoku University.
Principle of the optical interferometry
This optical sensor system uses a method to observe the displacement of a spring-mass-damper-system sensor by interferometry. Specifically, an optical pulse whose part is shifted by π / 2 is branched, one is reflected by the reference surface, and the other is reflected by the vibration surface of the pendulum, and one is appropriately delayed to cause interference. This measures the displacement of the vibrating surface (Fig. 1). The pendulum vibrates in response to ground motion input and the amplitude detected in the lower frequency domain than the natural frequency of the pendulum is proportional to the acceleration of the ground motion, so it becomes an accelerometer (ref. 1, 2).
Observation
The Yamazaki Station is a granite tunnel, and one of the group seismic observation networks around the Tono seismological observatory. This station is shared by Tono Yamazaki (code TYS, latitude 39.3772N, longitude 141.5932E, altitude 346m) in F-net of NIED. The optical sensor system was installed at the Tono Yamazaki Station on December 17, 2019, and observed continuously until January 16, 2020. The Tono Yamazaki Station is a straight tunnel with a total length of about 50m. There is an observation room immediately after the entrance, three doors on the way, and a sensor room at the back. The optical transmitter / receiver was installed in the observation room, and three sets of three-component optical sensors were installed in the sensor room. The optical cable connecting the optical transmitter / receiver and optical sensors was passed through a gutter. The natural frequency of the sensor is 2 sets at 24 Hz and 1 set at 100 Hz. For comparison, JU410 micrometermeter made by Hakusan Corporation equipped with a servo accelerometer (JA-40GA, made by Japan Aviation Electronics) was installed in the sensor room from December 17 to 19, 2019. In addition, the data of STS-2 of NIED F-net in the sensor room was also compared with our record.
Result
1) Comparison with the servo accelerometer
The noise spectra of optical sensor and JU410 were obtained using the data during quiet nights when no earthquake occurred. Each sampling frequency is 100 Hz. The optical sensor system is bandpass filtered from 0.1Hz to 30Hz. The gain setting of the JU410 is × 5, the maximum measurement range is ± 2G, and a minimum phase filter with a cutoff of 40% is applied. Fig. 2 is a graph of the PSD of JU410 and the optical sensor with a natural frequency of 24 Hz, using a 30-minute recording from 1:00 am to 1:30 am on December 18, 2019. It can be seen that the noise level of the optical sensor is lower than that of the servo accelerometer.
2) Comparison with STS-2
Fig. 3 is a graph of a PSD using data of about 30 minutes from 0:30 am to 1:00 am on January 16, 2020. The record of STS-2 is differentiated and converted to acceleration. From Fig. 3, it can be seen that the noise level of the optical sensor system almost coincides with the noise level of STS-2 in the range of about 0.13 Hz to about 20 Hz. That is, in this frequency band, it is determined that the optical sensor system detects ground motion noise of Tono Yamazaki Station.
Future tasks
Since the optical sensor has no electronic components in the detector, it can be operated in a high-temperature environment, and short-term high-temperature tests have confirmed that it can operate normally up to about 200 ° C. It is necessary to confirm the durability of performance in the future.
Acknowledgments
From Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University has been granted permission to use the Tono Yamazaki Station. Mr. Kono of Tono Seismological Observatory, Tohoku University provided great support at the site. The F-net data of the Tono Yamazaki Station was downloaded from the NIED Hi-net website and used.
References
1) Yoshida et. al., Jpn. J. Appl. Phys., vol.55 (2016) 022701.
2) Tsutsui et al., J. Disaster Res., vol.14 (2019) pp. 592-603.