Ship-information monitoring is vital to marine ecological protection, fishery resource management, and sea-area safety maintenance. The current mainstream ship-monitoring methods include automatic identification systems, optical cameras, infrared thermal imaging, radar monitoring, and remote-sensing technologies. However, these technical methods present the disadvantages of active monitoring, being affected by light, limited monitoring range, susceptibility to electromagnetic-wave interference, and high cost. By contrast, distributed acoustic sensing (DAS) offers unique advantages. It can use the existing submarine communication optical cables, offers passive monitoring, resists electromagnetic interference, supports large-scale networking and long-distance monitoring, and costs lower than remote-sensing technology. Diane et al. used a submarine communication cable to obtain the direction and speed of a ship. Liu et al. used a sensitized optical cable suspended in water to obtain ship voiceprint information and proposed an array-orientation method based on adaptive phase-difference correction. These studies achieved significant advancements in ship-speed estimation, trajectory tracking, and voiceprint recognition. However, DAS has not been used to analyze ship hydrodynamic pressure field (SHPF) in ship monitoring, and the inherent parameters of ships (such as length) have not been obtained. This study proposes DAS combined with a submarine photoelectric composite cable to synchronously monitor the hydrodynamic pressure field and acoustic field of an overtopped ship. Additionally, ship information is derived by combining the signal characteristics of SHPF, acoustic-field information, a signal-enhancement algorithm, and Doppler-frequency shifts.

In this study, a phase-sensitive optical time-domain reflectometer (Φ-OTDR) combined with a submarine photoelectric composite cable was used to synchronously monitor the SHPF and acoustic field of an overtopped ship to perceive ship information. First, based on potential flow theory and the spatial response characteristics of an optical fiber, a response model of the optical fiber to SHPF was established, and the ship-overtopping time and duration were obtained based on the characteristics of the hydrodynamic pressure field. Subsequently, a signal-enhancement algorithm was used to improve the signal-to-noise ratio of the ship’s acoustic signal, and high-definition spectrum features were successfully extracted. Next, a Doppler-frequency-shift distribution model was established along the axial direction of the optical cable. Based on the least-squares fitting method, the ship speed was inverted using the experimental spectrum information, and the ship length was estimated by combining the ship-overtopping duration. This method successfully combines an existing submarine photoelectric composite cable and a Φ-OTDR to quantitatively acquire ship speed and length, thus providing a new approach for ensuring marine information security.

The proposed method realizes SHPF monitoring, acquires the high-definition spectrum characteristics of ship spectra, and quantitatively acquires ship speed and length. The SHPF curves obtained from theoretical simulation and experiment are consistent (Fig. 9). The center of the hydrodynamic pressure curve of the ship exhibits a symmetrical distribution with a prominent negative pressure peak. Based on the characteristics of the SHPF, the ship-overtopping time and duration can be estimated. Using the obtained high-definition ship-spectrum feature map (Fig. 8), a signal with a center frequency of 21.29 Hz was selected for further processing. The ship speed was obtained via inversion using the least-squares method. The absolute error is only 0.09 m/s, and the relative error is 2.22%. The absolute error of the ship length estimated using the ship-overtopping duration is 4 m, and the relative error is 3.88%.

In this study, DAS and existing submarine photoelectric composite cables were used to simultaneously monitor the hydrodynamic-pressure-field and acoustic-field signals of ships. This approach is an improvement over the previous method, where DAS detects only a single physical field (acoustic field) signal of ships. Thus, it allows more ship parameters to be obtained from SHPF and acoustic-field signals. A response model of an optical fiber to an SHPF was established, and the accuracy of the theory was verified experimentally. The ship-overtopping duration was estimated using the zero-crossing negative pressure peak of the hydrodynamic pressure field. A model depicting the Doppler-frequency shift along the optical cable was established, and high-definition ship spectrum characteristics were obtained using a signal-enhancement algorithm. The ship speed was inverted using the least-squares method, and the ship length was obtained by combining the ship-overtopping duration. In this study, DAS was successfully used to synchronously detect SHPF and acoustic-field signals as well as to quantitatively acquire ship speed and length, thus expanding the information-perception ability of DAS in ship monitoring.