Underwater sound levels of transiting crew transfer vessels

Underwater sound recordings from Helgoland, Germany, were analyzed to detect passages of crew transfer vessels (CTVs). From these opportunistic observations, the source levels of 13 individual vessels were derived using the smoothed semi-coherent image method, including frequency-dependent absorption. Statistical analysis, using both generalized additive models and random forest models, showed that vessel-specific differences are the primary source of variability in source levels. While speed, length, and propulsion type all influence source levels, their effects vary across vessels and frequency bands, with no single factor dominating overall. The results indicate that, despite their relatively small size, CTVs have radiated noise levels similar to larger cargo vessels. The low variability in source levels across vessels suggests that a single source level spectrum for transiting CTVs could be a viable input for future noise modelling efforts.