9:30 AM - 9:45 AM
[STT36-03] Seismic observation using the seafloor optical fiber cable with distributed acoustic sensing in the Japan Sea
Keywords:Distributed acoustic sensing, Seafloor optical cable, The Japan Sea, Aftershock observation
Seafloor cabled observation system was installed as the first system of cable observation system near Awashima, Niigata prefecture in Japan Sea in 2010. The cable system has a total length of 25 km and 4 stations with 5 km interval. The system was equipped with Information Communication Technology for data delivery and control of system for the first time. A single armored optical submarine cable were buried 1 m below the seafloor to avoid a conflict with fishing activity. The data were stored continuously from the installation. The optical fiber seafloor cable has 8 fibers. Three pairs are used for data transmission and two single fibers are employed for the clock module. The data transmission channel is duplicated and one of the data transmission channels is turned in the furthest station from the landing station for ring configuration. Therefore two fibers reach the furthest station without repeater.
Recently optical fiber sensing technology such as Distributed Acoustic Sensing (DAS) is developed and is available for seismic and geodetic observation. Especially DAS is attractive for seismic observation, because spatially high-density data can be obtained for a long distance. The DAS observations have been carried out using fibers in existing seafloor observation cable systems in Japan, and it is found that the DAS data by seafloor cables are useful for the monitoring. A DAS measurement is one of optical fiber sensing technologies. A coherent laser pulse with short duration is transmitted continuously to a single mode optical fiber, and backscattered light is observed. When a small deformation of a fiber occurs by a vibration near fiber, a pattern change of the backscattered light is observed. Travel time of light and gauge length correspond to distance of measurement point and spatial resolution, respectively. Spatial sampling of the observation is a few meters in the shortest case. From these characteristics, a DAS measurement enables a dense seismic observation as a linear array. A length of the array which has a short interval of sensor corresponds to a length of a deployed optical fiber. According to the present technology, the maximum length of a DAS measurement reaches more than 100 km. Therefore, seismic observation using DAS technology on seafloor becomes popular at the present.
Using a fiber of the cable observation system in the Japan Sea, we made the first observation of a DAS measurement in April 2023. The fiber used for the DAS measurement in the Japan Sea system was available. Before the DAS measurement, we confirmed good condition of the fibers using Optical Time Domain Reflectometer. An interrogator for the DAS measurement was installed in the landing station temporarily, and data were recorded for 22 km length with a spatial interval of 1 m and sampling frequency of 200 Hz. The gauge length was set to 16 m. The data were collected continuously for approximately four days. As a result, micro-earthquakes occurring near the cable system was recorded. From records of the microearthquake, DAS data obtained from the seafloor cable system in Japan Sea are estimated to have a good quality. In September 2023, a DAS observation with a short-term of about one-day was also carried out using the cable system. The second observation was also system evaluation after exchange of optical connectors in the landing station. The parameters of recording for the observation were identical as the first observation. Spatial interval of sampling and gauge length were also set to 1 m and 16 m, respectively. The 2024 Noto-Hanto earthquake occurred on January 1st and many aftershocks follows. To record aftershocks, we started a DAS observation using the cable observation system in the Japan Sea in early February. Reflecting an environment of the DAS observation, many aftershocks were recorded by the DAS observation.
Recently optical fiber sensing technology such as Distributed Acoustic Sensing (DAS) is developed and is available for seismic and geodetic observation. Especially DAS is attractive for seismic observation, because spatially high-density data can be obtained for a long distance. The DAS observations have been carried out using fibers in existing seafloor observation cable systems in Japan, and it is found that the DAS data by seafloor cables are useful for the monitoring. A DAS measurement is one of optical fiber sensing technologies. A coherent laser pulse with short duration is transmitted continuously to a single mode optical fiber, and backscattered light is observed. When a small deformation of a fiber occurs by a vibration near fiber, a pattern change of the backscattered light is observed. Travel time of light and gauge length correspond to distance of measurement point and spatial resolution, respectively. Spatial sampling of the observation is a few meters in the shortest case. From these characteristics, a DAS measurement enables a dense seismic observation as a linear array. A length of the array which has a short interval of sensor corresponds to a length of a deployed optical fiber. According to the present technology, the maximum length of a DAS measurement reaches more than 100 km. Therefore, seismic observation using DAS technology on seafloor becomes popular at the present.
Using a fiber of the cable observation system in the Japan Sea, we made the first observation of a DAS measurement in April 2023. The fiber used for the DAS measurement in the Japan Sea system was available. Before the DAS measurement, we confirmed good condition of the fibers using Optical Time Domain Reflectometer. An interrogator for the DAS measurement was installed in the landing station temporarily, and data were recorded for 22 km length with a spatial interval of 1 m and sampling frequency of 200 Hz. The gauge length was set to 16 m. The data were collected continuously for approximately four days. As a result, micro-earthquakes occurring near the cable system was recorded. From records of the microearthquake, DAS data obtained from the seafloor cable system in Japan Sea are estimated to have a good quality. In September 2023, a DAS observation with a short-term of about one-day was also carried out using the cable system. The second observation was also system evaluation after exchange of optical connectors in the landing station. The parameters of recording for the observation were identical as the first observation. Spatial interval of sampling and gauge length were also set to 1 m and 16 m, respectively. The 2024 Noto-Hanto earthquake occurred on January 1st and many aftershocks follows. To record aftershocks, we started a DAS observation using the cable observation system in the Japan Sea in early February. Reflecting an environment of the DAS observation, many aftershocks were recorded by the DAS observation.