Spectral Characterisation of Undewater Radiated Noise Using Distributed Acoustic Sensing

Distributed Acoustic Sensing (DAS) provides dense, distributed measurements along subsea fibre-optic cables and has demonstrated potential for passive detection, e.g., of surface vessels, whales, and similar objects. Beyond detection and localisation, the spectral content of the acoustic signals contains information about the object, such as propulsion, machinery, and operational state for ships. In hydrophones, such information is often extracted using spectral and modulation-based techniques, including Low-Frequency Analysis and Recording (LOFAR) and Detection of Envelope Modulation on Noise (DEMON). However, the direct transfer of these methods to DAS is non-trivial due to differences in frequencies, coupling conditions, and the limited acoustic bandwidth typically available in DAS systems.

In this study, we investigate how ship radiated noise can be spectrally characterised using DAS by adapting analysis concepts originally developed for hydrophone-based passive acoustics. We focus on frequency bands accessible to DAS low-frequency tonal components are observable. A preprocessing pipeline is applied to enhance signal-to-noise ratio and suppress non-acoustic contributions. Within this framework, DEMON-inspired envelope analysis is explored in a form compatible with DAS bandwidth constraints, alongside direct low-frequency spectral methods.

To place the extracted spectral features in a physical and operational context, the DAS-derived signatures are compared with ship information from AIS data and auxillary ship information. This comparison enables assessment of how observed spectral characteristics relate to known ship properties and operating conditions, and clarifies which elements of classical underwater radiated noise theory are transferable to DAS observations.

By focusing on spectral characterisation rather than localisation performance, this work aims to bridge the gap between traditional hydrophone-based ship acoustics and emerging DAS-based maritime monitoring, providing a methodological foundation for interpreting DAS detections in terms of vessel class and behaviour.