2:00 PM - 2:15 PM
[STT42-02] Potential and Challenges of Global Seafloor Optical Fiber Sensing
Keywords:Distributed fiber optic sensing, Submarine Cable, global
The ocean covers two-thirds of Earth's surface, yet deep-sea observations remain limited. While seafloor cable observation networks have been developed in coastal areas of Japan, the US, Canada, and Taiwan, large-scale deep-ocean observations are rare, with a few examples like the US OOI and Canada’s Neptune. Current deep-sea observations rely on ship-based campaigns in oceanography and sparse broadband seismic arrays in seismology, making long-term monitoring of critical phenomena—such as plate deformation, earthquakes, and deep-water circulation—extremely challenging. Moreover, the spatial density of these observations is low, hindering accurate monitoring of long-term changes.
To overcome these limitations, we propose leveraging globally deployed seafloor optical fiber cables as sensors. Broadband DAS observations using a 120 km seafloor cable off Cape Muroto (ref) have demonstrated that Rayleigh backscattered waves from optical fibers can be evaluated using stable onshore laser reference light, achieving resolutions comparable to broadband seismometers. Additionally, TW-COTDR with wavelength-calibrated laser reference light has proven stable for weeks, enabling long-period strain measurements on seafloor optical fibers.
Standard transoceanic optical fiber cables transmit data over distances exceeding 100 km using optical amplifiers (repeaters), which prevent Rayleigh backscatter from returning beyond repeater sections. However, for cable condition monitoring, a feedback system called HLLB allows Rayleigh backscatter from beyond repeaters to be received via separate optical fibers. This system enables COTDR measurements, making it theoretically possible to analyze strain over long distances—up to 10,000 km in trans-Pacific cables—if DAS or TW-COTDR techniques are applied.
In fact, studies such as Mazur et al. (2024) have reported DAS observations on similar repeatered systems, while Marra et al. (2022) analyzed strain in an Atlantic submarine cable using HLLB. This suggests that similar measurements on domestic 1,000 km cables or transoceanic 10,000 km cables are feasible in the near future. If such long-distance sensing is realized, it would provide unprecedented data not only for the largely unmonitored deep ocean but also for tectonic dynamics at plate boundaries, as many submarine cables traverse these critical regions.
However, a key challenge is ensuring that the detected signals have well-defined physical interpretations. For example, Araki et al. (2024) showed that long-period strain observed in the Muroto offshore cable can mainly be explained by seafloor temperature variations, as verified by comparison with seafloor thermometers. Tonegawa and Araki (2024) demonstrated that DAS can detect tsunami-induced pressure changes, but further studies comparing DAS signals with direct seafloor pressure gauge data are needed to understand the underlying mechanisms. Additionally, while submarine optical fibers are generally loosely encased within cables, multiple studies have successfully detected seismic motion, including long-period signals, suggesting that the measurement system itself requires a precise physical and quantitative explanation.
Seafloor cable fiber optic sensing captures not only strain but also pressure and temperature changes, necessitating techniques to disentangle these effects for geophysical applications. Araki et al. (2024) have developed a specialized submarine sensing cable capable of separately measuring temperature, pressure, and strain, and have conducted validation experiments at sea. Other studies suggest similar possibilities for downhole fiber optic observations in terrestrial boreholes and seafloor drilling sites (Araki et al., 2024).
In fiber optic sensing technology, Raman scattering has been used for temperature analysis (DTS), and combining Rayleigh (TW-COTDR) and Brillouin (BOTDR) scattering has allowed for the separation of strain and temperature with limited accuracy. While current resolutions remain insufficient for many geophysical applications, optical sensing technology has room for improvement. Enhancing these analytical techniques, particularly for existing commercial cables, remains a critical research challenge.
To overcome these limitations, we propose leveraging globally deployed seafloor optical fiber cables as sensors. Broadband DAS observations using a 120 km seafloor cable off Cape Muroto (ref) have demonstrated that Rayleigh backscattered waves from optical fibers can be evaluated using stable onshore laser reference light, achieving resolutions comparable to broadband seismometers. Additionally, TW-COTDR with wavelength-calibrated laser reference light has proven stable for weeks, enabling long-period strain measurements on seafloor optical fibers.
Standard transoceanic optical fiber cables transmit data over distances exceeding 100 km using optical amplifiers (repeaters), which prevent Rayleigh backscatter from returning beyond repeater sections. However, for cable condition monitoring, a feedback system called HLLB allows Rayleigh backscatter from beyond repeaters to be received via separate optical fibers. This system enables COTDR measurements, making it theoretically possible to analyze strain over long distances—up to 10,000 km in trans-Pacific cables—if DAS or TW-COTDR techniques are applied.
In fact, studies such as Mazur et al. (2024) have reported DAS observations on similar repeatered systems, while Marra et al. (2022) analyzed strain in an Atlantic submarine cable using HLLB. This suggests that similar measurements on domestic 1,000 km cables or transoceanic 10,000 km cables are feasible in the near future. If such long-distance sensing is realized, it would provide unprecedented data not only for the largely unmonitored deep ocean but also for tectonic dynamics at plate boundaries, as many submarine cables traverse these critical regions.
However, a key challenge is ensuring that the detected signals have well-defined physical interpretations. For example, Araki et al. (2024) showed that long-period strain observed in the Muroto offshore cable can mainly be explained by seafloor temperature variations, as verified by comparison with seafloor thermometers. Tonegawa and Araki (2024) demonstrated that DAS can detect tsunami-induced pressure changes, but further studies comparing DAS signals with direct seafloor pressure gauge data are needed to understand the underlying mechanisms. Additionally, while submarine optical fibers are generally loosely encased within cables, multiple studies have successfully detected seismic motion, including long-period signals, suggesting that the measurement system itself requires a precise physical and quantitative explanation.
Seafloor cable fiber optic sensing captures not only strain but also pressure and temperature changes, necessitating techniques to disentangle these effects for geophysical applications. Araki et al. (2024) have developed a specialized submarine sensing cable capable of separately measuring temperature, pressure, and strain, and have conducted validation experiments at sea. Other studies suggest similar possibilities for downhole fiber optic observations in terrestrial boreholes and seafloor drilling sites (Araki et al., 2024).
In fiber optic sensing technology, Raman scattering has been used for temperature analysis (DTS), and combining Rayleigh (TW-COTDR) and Brillouin (BOTDR) scattering has allowed for the separation of strain and temperature with limited accuracy. While current resolutions remain insufficient for many geophysical applications, optical sensing technology has room for improvement. Enhancing these analytical techniques, particularly for existing commercial cables, remains a critical research challenge.