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[SSS12-P10] Amplitude of artificial vibration captured by DAS (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 test-observed whether the vibration intensity obtained by DAS is linear with respect to the magnitude of seismic motion by laying an optical fiber in a tunnel about 1 km in distance in Hamamatsu City, Shizuoka Prefecture, Japan. Artificial vibration was applied to the optical fiber, the sensor part of the DAS, using a shaker. Overall, as the vibration of the shaker increased, the magnitude of the vibration captured by the DAS tended to increase. However, when the magnitude of the vibration added by the shaker is kept constant and observed for a long period of time, the amplitude of the DAS is not always constant, and its changes are not periodic or regular in time.
(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 test-observed whether the vibration intensity obtained by DAS is linear with respect to the magnitude of seismic motion by laying an optical fiber in a tunnel about 1 km in distance in Hamamatsu City, Shizuoka Prefecture, Japan. Artificial vibration was applied to the optical fiber, the sensor part of the DAS, using a shaker. Overall, as the vibration of the shaker increased, the magnitude of the vibration captured by the DAS tended to increase. However, when the magnitude of the vibration added by the shaker is kept constant and observed for a long period of time, the amplitude of the DAS is not always constant, and its changes are not periodic or regular in time.