5:15 PM - 6:45 PM
[STT36-P01] Vibration Monitoring in the Off-Tonankai Cable by Distributed Acoustic Sensing (2)
Keywords:DAS, optical fibers
In recent years, many results of seismic observation using the technology of Distributed Acoustic Sensing (DAS) have been published. The main features of DAS are as follows.
(1) One-dimensional distribution measurement over the entire optical fiber is possible with one measurement.
(2) The distance resolution is determined by the light pulse width incident on the optical fiber, and changes in a range narrower than the light pulse width are observed as averaged values, so it is difficult to measure physical values at points.
(3) The scattered light generated by impurities and defects in the optical fiber is weak, and processing such as averaging is performed to reduce noise.
(4) There is a distance resolution determined by the sampling frequency.
(5) Due to the transmission loss of the optical fiber, the signal strength of the scattered light at a farther distance becomes weaker than that near the device. The maximum measurable distance exists.
However, the physical quantity of the data observed by the DAS is a strain rate, different from the physical quantity obtained by the seismometer and is an averaged value in a certain section.
We have been conducting DAS observations since February 2023 using the dark fiber of the Off-Tonankai Optical Submarine Cable, which has been installed along the Nankai Trough off the coast of Omaezaki City, Shizuoka Prefecture. The optical fiber used is of the single mode type. The cable is approximately 200 km long, and is located underground on land up to approximately 1 km from the DAS device, and it is on the ocean floor beyond approximately 1 km. However, in December 2022, there was a problem where the cable was damaged approximately 4 km off the coast of Omaezaki, so we are conducting observations at a distance of approximately 5 km from the DAS equipment.
Since we were able to capture natural earthquakes, we used those records to compare records from each point on the optical fiber. The cause is thought to be the condition of the cable installation, but we found that the amplitude of the same earthquake differs between underground on land and on the ocean floor, and that there are some points that do not have a high correlation with the waveforms of the previous and subsequent points.
(1) One-dimensional distribution measurement over the entire optical fiber is possible with one measurement.
(2) The distance resolution is determined by the light pulse width incident on the optical fiber, and changes in a range narrower than the light pulse width are observed as averaged values, so it is difficult to measure physical values at points.
(3) The scattered light generated by impurities and defects in the optical fiber is weak, and processing such as averaging is performed to reduce noise.
(4) There is a distance resolution determined by the sampling frequency.
(5) Due to the transmission loss of the optical fiber, the signal strength of the scattered light at a farther distance becomes weaker than that near the device. The maximum measurable distance exists.
However, the physical quantity of the data observed by the DAS is a strain rate, different from the physical quantity obtained by the seismometer and is an averaged value in a certain section.
We have been conducting DAS observations since February 2023 using the dark fiber of the Off-Tonankai Optical Submarine Cable, which has been installed along the Nankai Trough off the coast of Omaezaki City, Shizuoka Prefecture. The optical fiber used is of the single mode type. The cable is approximately 200 km long, and is located underground on land up to approximately 1 km from the DAS device, and it is on the ocean floor beyond approximately 1 km. However, in December 2022, there was a problem where the cable was damaged approximately 4 km off the coast of Omaezaki, so we are conducting observations at a distance of approximately 5 km from the DAS equipment.
Since we were able to capture natural earthquakes, we used those records to compare records from each point on the optical fiber. The cause is thought to be the condition of the cable installation, but we found that the amplitude of the same earthquake differs between underground on land and on the ocean floor, and that there are some points that do not have a high correlation with the waveforms of the previous and subsequent points.