Passive Acoustic Characterization of Marine Bedload Transport Based on Interparticle Collision Dynamics

Marine sediments are very important for keeping the coast stable and protecting the shoreline naturally. However, anthropogenic activities can greatly change how sediment moves, making their accurate monitoring essential. In marine settings, understanding bedload sediment transport can be challenging due to conventional methods reliant on visual observations or direct sediment sampling tend to be intrusive, spatially constrained, and inadequate for long-term or continuous monitoring. In this situation, passive underwater acoustics is a promising non-intrusive option that can provide continuous monitoring with high temporal resolution. This study investigates the acoustic signatures related to marine bedload transport, focusing particularly on the sounds generated by interparticle collisions of mobile sediments. A series of controlled laboratory experiments are performed utilising simplified experimental arrangements in which artificial sediments (spherical glass beads) are mobilised under oscillatory motion that simulates wave-induced seabed forcing. We use glass beads of different sizes to create idealised bedload conditions, and we use an oscillating plate to control the movement of the particles. Hydrophones placed close to the sediment bed record acoustic pressure signals. The recorded acoustic signals are analyzed in both the time and frequency domains. Individual particle impacts are characterised by short transient acoustic events, and spectral analyses show clear peak frequencies that are linked to sediment motion. The results indicate that the peak frequency of the acoustic spectrum is predominantly determined by particle diameter and is additionally influenced by the amplitude and frequency of the applied oscillatory motion. These observations align with theoretical models, such as those suggested by Thorne (1985), that explain the generation of pressure waves during underwater particle collisions. To further explore the mechanisms of sound generation, experiments are conducted with both smooth and rough beds below the beads layers. The analysis reveals the existence of sediment-specific acoustic signatures, facilitating the differentiation of particle sizes according to their spectral characteristics. This study illustrates the significant potential of passive acoustic methods for the remote monitoring of marine bedload transport. The study offers novel insights into sound generation mechanisms linked to sediment motion across various particle sizes, motion amplitudes, and bed configurations, utilising a combination of laboratory experiments, theoretical frameworks, and comprehensive spectral analysis, with direct implications for intricate coastal and offshore environments.