5:15 PM - 6:45 PM
[STT36-P04] Development of seafloor fiber-optic sensing cable for distributed strain, pressure, temperature observation.
Keywords:distributed fiber optic sensing, seafloor observation, Sagami Bay
Distributed fibre-optic sensing using submarine fibre-optic cables is considered useful for seismic and geodetic observations in the seafloor. DAS observations with off Muroto submarine cable demonstrated to analyze slow earthquakes (Baba et al., 2023), but its utilization to further slower event is still limited because the effect of seafloor temperature fluctuations is more pronounced (Ide et al., 2021). With the aim of separating temperature fluctuations from seafloor strain sensing, we developed a submarine optical fiber sensing cable with OCC Ltd. that can be extended to the seabed by ROV. We conducted test installation of the cable in Sagami Bay, connecting it to off Hatsushima Island seafloor station. The deployed cable was observed from shore station by DAS and TW-COTDR distributed optical sensing interrogators.
The developed submarine fiber-optic sensing extension cable has three types of optical fiber sensing units with different sensitivity to temperature, strain and pressure in a 10 km long cable, enabling strain, temperature and pressure to be separated along each point of the cable. The cable can be deployed in the seafloor, using the ROV cable deployment system used in the construction DONET.
As a sea trial of this submarine fiber-optic sensing extension cable, a test extension was conducted in February 2023 from the off Hatsushima deep-sea station in Sagami Bay to the the bottom area of the Sagami Trough.
During the test deployment, an AP Sensing DAS interrogator was installed at the shore station to observe the fibre-optic cable being deployed, enabling the position of the deployed cable on the fibre-optic sensing system to be determined. After installation, submarine water thermometers were installed in the vicinity of the submarine cable, with the aim of verifying the temperature sensitivity of the optical fiber. Subsequently, observations were carried out not only with the DAS instrument, but also with the Neubrex TW-COTDR instrument, which is considered suitable for observations of longer-period phenomena. Observations were intermittent, but continuous observation records of more than two weeks were obtained by DAS and TW-COTDR.
The data obtained confirmed that earthquake-induced ground motions were observed in the fiber-optic strain and pressure observation units, but that no earthquake-induced ground motions were recorded in the water temperature observation unit. In addition, the strain observation unit observed an increase in noise periodically with a semi-daily cycle due to the influence of water currents in the seafloor. This effect of the water flow was not so evident in the pressure observation unit, and the results suggest that the pressure observation unit observes seismic motion through water pressure fluctuations rather than strain fluctuations. A comparison of the records of seismic motion obtained in the strain observation unit with the mainline cable deployed from Hatsushima island showed that the optical fiber-sensing extension cable has a smaller sensitivity to seismic motion. This is mainly due to the poor coupling between the extension cable and sediments, as the cable is not buried in and is deployed with slack. In addition, the sensitivity of ocean tide pressure change by the pressure-unit was small, and the results suggest that reliable observation of tides is difficult with less stable fiber optic interrogators such as commercial DASs.
Although there is room for improvement in the installation method, the ability to observe the seafloor without the influence of temperature is a major advantage, and we hope to deploy a further improved submarine optical fiber sensing extension cable to the Nankai Trough seafloor and other areas, and use it for dense seismic and geodetic observation in the seafloor.
The developed submarine fiber-optic sensing extension cable has three types of optical fiber sensing units with different sensitivity to temperature, strain and pressure in a 10 km long cable, enabling strain, temperature and pressure to be separated along each point of the cable. The cable can be deployed in the seafloor, using the ROV cable deployment system used in the construction DONET.
As a sea trial of this submarine fiber-optic sensing extension cable, a test extension was conducted in February 2023 from the off Hatsushima deep-sea station in Sagami Bay to the the bottom area of the Sagami Trough.
During the test deployment, an AP Sensing DAS interrogator was installed at the shore station to observe the fibre-optic cable being deployed, enabling the position of the deployed cable on the fibre-optic sensing system to be determined. After installation, submarine water thermometers were installed in the vicinity of the submarine cable, with the aim of verifying the temperature sensitivity of the optical fiber. Subsequently, observations were carried out not only with the DAS instrument, but also with the Neubrex TW-COTDR instrument, which is considered suitable for observations of longer-period phenomena. Observations were intermittent, but continuous observation records of more than two weeks were obtained by DAS and TW-COTDR.
The data obtained confirmed that earthquake-induced ground motions were observed in the fiber-optic strain and pressure observation units, but that no earthquake-induced ground motions were recorded in the water temperature observation unit. In addition, the strain observation unit observed an increase in noise periodically with a semi-daily cycle due to the influence of water currents in the seafloor. This effect of the water flow was not so evident in the pressure observation unit, and the results suggest that the pressure observation unit observes seismic motion through water pressure fluctuations rather than strain fluctuations. A comparison of the records of seismic motion obtained in the strain observation unit with the mainline cable deployed from Hatsushima island showed that the optical fiber-sensing extension cable has a smaller sensitivity to seismic motion. This is mainly due to the poor coupling between the extension cable and sediments, as the cable is not buried in and is deployed with slack. In addition, the sensitivity of ocean tide pressure change by the pressure-unit was small, and the results suggest that reliable observation of tides is difficult with less stable fiber optic interrogators such as commercial DASs.
Although there is room for improvement in the installation method, the ability to observe the seafloor without the influence of temperature is a major advantage, and we hope to deploy a further improved submarine optical fiber sensing extension cable to the Nankai Trough seafloor and other areas, and use it for dense seismic and geodetic observation in the seafloor.