Distributed acoustic sensing (DAS) technology regards the same optical fiber as both a sensing and transmission medium, enabling real-time capture of vibrations and acoustic signals in the vicinity of optical fibers. This technology features robust environmental adaptability, high sensitivity, and exceptional resistance to electromagnetic interference. DAS can transform optical cables spanning tens of kilometers into evenly distributed arrays for vibration and acoustic sensing, thus achieving spatial sampling resolutions of less than 1 m. Consequently, it has caught significant attention in fields such as oil and gas exploration and marine geophysics. However, the maximum sensing range of DAS is constrained by optical source bandwidth and stability, with the current maximum sensing range limited to approximately 100 km. Therefore, employing DAS for intercontinental undersea long-distance communication cables poses significant challenges. We introduce a DAS seabed in-situ monitoring system integrated with an underwater experiment platform, coupled with relevant offshore trials. This deployment extends DAS capabilities to more remote and deeper-sea areas. The analysis of offshore trial results validates the system feasibility. In the future, consideration can be given to combining submersible technologies for laying sensing optical cables on the seabed, which holds substantial significance for advancing marine geophysical research.

We employ pressure chamber technology to construct a deep-sea DAS system capable of withstanding pressure at depths of up to 10000 m and conduct rigorous pressure testing in the laboratory, subjecting the system to pressure of up to 110 MPa. Subsequently, the system is deployed on a deep-sea in-situ experimental platform equipped with autonomous underwater mobility capabilities and is placed at a depth of 1423 m for in-situ testing. Throughout the sea trial, the base station undergoes various operational states including submersion, landing, moving, and raising, with distinct vibration data generated by each of these states. By analyzing the data collected by the DAS, we can discern the base station's temporal and spectral characteristics in different operational conditions. These research findings confirm the feasibility of the design and deployment of deep-sea DAS systems in the extreme deep-sea environment.

When the base station is stationary, DAS can capture the background noise of the sea, with an average noise level of 4.64×10^-4 rad/Hz, which closely aligns with laboratory measurements. The operational states of the base station can be primarily categorized as follows. ① When the base station submerges, there is a significant energy transfer upon contact with the seafloor, resulting in strong vibrations. Subsequently, the remaining modules of the base station enter the water one by one to generate a series of relatively weaker vibrations. ② When the base station lands on the seafloor, by comparing two consecutive impact signals, the energy of the second signal is observed to attenuate significantly compared to the first one, which indicates successful soft landing of the base station. ③ Before the base station is raised, the payload should be released. DAS records the produced vibration signals when the base station releases its payload. It is noticed that when the base station is in weight balance, the payload release primarily generates high-frequency signals above 30 Hz, but when it is not in balance, the signal energy is mainly distributed below 20 Hz. ④ When the base station moves underwater using servo propellers for direction control, compared to the stationary state, the rotation of the propellers and the movement of the base station introduce stronger interference below 60 Hz, and these interference signals' harmonics are also present in the high-frequency range.

DAS technology is a novel seismic monitoring technique emerging in recent years, and features high spatial density, long-range capabilities, and dynamic measurements. Meanwhile, it offers sensing distances of up to several tens of kilometers with spatial resolution of a few meters. This technology is known for its simplicity, low development and maintenance costs, and ability to provide real-time data transmission, and it has the potential to significantly reduce observation costs, thus becoming a promising choice for deployment in critical marine regions. We present the deep-sea validation testing of the deep-sea DAS systems. During a 21-day in-situ sea trial at a depth of 1423 m, over 600 GB experimental data are recorded. Analysis of the vibration events generated during the base station's submersion, settling on the seabed, payload release, and movement processes confirms its capability for deep-sea operations and vibration signal detection. Additionally, it validates the operational reliability and stability in the deep-sea environment, laying a strong foundation for future trials at depths of up to 10000 m.