Distributed Acoustic Sensing for Ship Detection and Identification

Distributed Acoustic Sensing (DAS) is a method of measuring the strain caused by acoustic waves along an optical fibre. Although mainly used for structural monitoring or geophysical applications, DAS is gaining credibility in coastal surveillance and maritime traffic intelligence. This study helps to understand the potential of DAS for such applications. The first part focuses on ship detection. Based on the analysed data, it is shown that depending on the level of ship radiated noise, ship either go undetected at very low speeds (2 knots), or be detected at a distance of 1 km to 1.2 km from the cable with a cable section depth of 400 to 450 m. The onset of swell can be observed, punctually increasing cavitation as the propeller passes under the swell trough, increasing the detection range to 1.8 km. DAS only detects longitudinal strain, resulting in a “cone of blindness” at the nadir of the vessel, where the sound is not detected. For all visible ships, the angle of this cone is approximately 29° at the point of intersection, and increases with the distance. The directivity of DAS also leads to time-space acoustic signatures appearing asymmetrical in space at specific angles between the ship's route and the optic fibre. The second part focuses on the identification of a bulk carrier. Using the propulsion specifications of ships, such as the type of propulsion, the number of engines, the number of cylinders and the number of blades on the propeller, it is possible to associate most the of peaks in the spectrum of ship noise with firing rates or blade harmonics. The influence of swell is also observed on the acoustic signature. Finally, while the ship is detectable for a period of 8 minutes, it is only identifiable for a period of 3 minutes and 45 seconds.