Presentation Information
[S02-07]High performance seismic observation by distributed acoustic sensing using seafloor cable observation system off Sanriku
*Masanao Shinohara1, Zuyuan He2, Shinichi Tanaka1, Hironori Otsuka1 (1. Earthquake Research Institute, the University of Tokyo, 2. State Key Laboratory of Photonics and Communications, Shanghai Jiao Tong University)
Recently Distributed Acoustic Sensing (DAS) technology becomes available for geophysical observation. DAS measurement utilizes a coherent laser pulse. Travel time of light and gauge length correspond to distance of measurement point and spatial resolution, respectively. Spatial sampling of the observation can be set to a few meters, and a maximum length of a DAS measurement reaches more than 100 km. Therefore, seismic observation using DAS technology on seafloor becomes popular at the present. However, conventional DAS measurements for seismic observations have disadvantages compared to seismic observations using a seismometer which is based on a principle of a pendulum. The instrumental noise level of conventional DAS measurements is higher than that of a seismometer. Therefore, recording of very small earthquakes were sometimes limited. In addition, the instrumental noise level of conventional DAS measurements becomes higher when the measurement period is longer. The large noise in the long period band affects seismic observation for teleseismic events and slow earthquakes. Recently a high-performance DAS interrogator has been developed with low instrumental noise level. There is a possibility that the low-noise level DAS interrogator pioneers seismic observation especially in marine region where ambient seismological noise level is low.
In 1996, a seafloor seismic tsunami observation system using an optical fiber cable was deployed off Sanriku by Earthquake Research Institute, the University of Tokyo. Six spare (dark) optical fibers are included in the system for future extension, and are suitable for DAS measurement. The length of the dark fiber reached 120 km without a repeater or amplifier. We have performed several temporary DAS measurements using the Sanriku system from February 2019 to collect seismic data. Although many earthquakes could be recorded, very small micro-earthquakes were sometimes difficult to record due to large instrumental noises of the conventional interrogators. The number of recorded teleseismic events seem to be limited and only large teleseismic events were recorded. The DAS records obtained by conventional DAS interrogators can be compared to data from the accelerometer attached on the same cable system. Conventional DAS records converted to acceleration have the noise levels comparable to those from the small-size accelerometer which has a relatively large instrumental noise among the seismic sensors using a pendulum.
In March 2025, we carried out a seismic DAS observation with the off-Sanriku cable observation system using a high-performance DAS interrogator ixDAS-3000A (AllianStream Photonics), which implements a pulse compression scheme to enhance the SNR and a rotate-vector-summation algorithm to mitigate the signal fading, thus realizing very low instrumental noise level. Because six dark fibers are contained in the seafloor cable, we made another DAS observation at the same time by a conventional DAS interrogator connecting to a separate fiber to compare the records. We set the same recording parameters to both interrogators. Total lengths of the measurement was 100 km with spatial sampling interval of 2 m. The gauge lengths were set to 10 - 100 meters. The high-performance DAS interrogator could record microseisms up to 100 km in the records whose gauge length was 100 m. According to spectrum analysis, ambient seismic noise levels seemed to be at least 20 dB lower in all recording frequency for any gauge length than those obtained by the conventional DAS interrogator. For periods around 10 s, ambient seismic noise recorded using a gauge length of 100 m was significant low. The low noise level in periods around 10 s is believed to be a large advantage to observe teleseismic events and slow earthquakes.
Fig. Spatial variation of ambient noise spectra on the seafloor obtained by the ixDAS-3000A DAS interrogator (AllianStream Photonics) in March 2025. The horizontal axis is the distance from the coast, and the vertical axis shows frequency. Note that noises around 10 s (0.1 Hz) are low, which is commonly observed on the deep seafloor using a high-performance broadband seismometer.
In 1996, a seafloor seismic tsunami observation system using an optical fiber cable was deployed off Sanriku by Earthquake Research Institute, the University of Tokyo. Six spare (dark) optical fibers are included in the system for future extension, and are suitable for DAS measurement. The length of the dark fiber reached 120 km without a repeater or amplifier. We have performed several temporary DAS measurements using the Sanriku system from February 2019 to collect seismic data. Although many earthquakes could be recorded, very small micro-earthquakes were sometimes difficult to record due to large instrumental noises of the conventional interrogators. The number of recorded teleseismic events seem to be limited and only large teleseismic events were recorded. The DAS records obtained by conventional DAS interrogators can be compared to data from the accelerometer attached on the same cable system. Conventional DAS records converted to acceleration have the noise levels comparable to those from the small-size accelerometer which has a relatively large instrumental noise among the seismic sensors using a pendulum.
In March 2025, we carried out a seismic DAS observation with the off-Sanriku cable observation system using a high-performance DAS interrogator ixDAS-3000A (AllianStream Photonics), which implements a pulse compression scheme to enhance the SNR and a rotate-vector-summation algorithm to mitigate the signal fading, thus realizing very low instrumental noise level. Because six dark fibers are contained in the seafloor cable, we made another DAS observation at the same time by a conventional DAS interrogator connecting to a separate fiber to compare the records. We set the same recording parameters to both interrogators. Total lengths of the measurement was 100 km with spatial sampling interval of 2 m. The gauge lengths were set to 10 - 100 meters. The high-performance DAS interrogator could record microseisms up to 100 km in the records whose gauge length was 100 m. According to spectrum analysis, ambient seismic noise levels seemed to be at least 20 dB lower in all recording frequency for any gauge length than those obtained by the conventional DAS interrogator. For periods around 10 s, ambient seismic noise recorded using a gauge length of 100 m was significant low. The low noise level in periods around 10 s is believed to be a large advantage to observe teleseismic events and slow earthquakes.
Fig. Spatial variation of ambient noise spectra on the seafloor obtained by the ixDAS-3000A DAS interrogator (AllianStream Photonics) in March 2025. The horizontal axis is the distance from the coast, and the vertical axis shows frequency. Note that noises around 10 s (0.1 Hz) are low, which is commonly observed on the deep seafloor using a high-performance broadband seismometer.
