J A SA. ARTICLE 
| U) CrossMark 
elek fer updates 
Underwater sound levels of transiting crew transfer vessels 
Fritjof Basan, ® © Christ A. F. de Jong,” © Christian Krüger,' and Jens-Georg Fischer‘ 
Federal Maritime and Hydrographic Agency (BSH), Hamburg, Germany 
"Netherlands Organization for Applied Scientific Research (TNO), The Hague, The Netherlands 
a8 
md 
ABSTRACT: 
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 lev- 
als, 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 
CTVSs could be a viable input for future noise modelling efforts. 
© 2026 Authorfs). All article content, except where otherwise noted, is licensed under a Creative Commons 
Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1121/10.0043324 
(Received 25 September 2025; revised 21 January 2026; accepted 16 March 2026; published online 17 April 2026) 
IEditor: James F. Lynch] Pages: 3406—3415 
i. INTRODUCTION 
Anthropogenic underwater noise has been recognized 
as a key environmental stressor for marine life, affecting the 
communication, foraging, and navigation of marine organ- 
isms (Duarte et al., 2021; Erbe et al., 2018; Hawkins and 
Popper, 2017). In the North Sea, the rapid expansion of off- 
shore wind farms has led to increased vessel traffic for both 
construction and maintenance activities. Although the noise 
impacts of construction activities, especially pile driving, 
are well documented as a major disturbance to marine mam- 
mals (Brandt ef al., 2011) and are subject to strict regulatory 
measures (Juretzek ef al., 2021), continuous noise emissions 
from service vessels, including crew transfer vessels 
(CTVs), remain less well understood. Depending on dis- 
tance to shore and operational requirements, wind farm 
maintenance is carried out by service operation vessels 
‘SOVs) or CTVs (McMorland et al., 2022). CTVs, which 
are typically high-speed catamarans with azimuth pod (AP) 
or propeller propulsion, play a central role in servicing off- 
shore wind farms in the German Exclusive Economic Zone, 
particularly around Helgoland. Previous studies suggest that 
small, high-speed vessels can generate significant noise 
emissions, particularly in higher-frequency bands relevant 
to marine mammal communication (Hermannsen et al., 
2025). However, comprehensive measurements of CTV 
source levels (SLs) and their dependences on vessel-specific 
parameters are scarce. To address this knowledge gap, we 
analyzed passive acoustic recordings from two autonomous 
recorders deployed north of Helgoland over a 4-month 
period. Using the smoothed semi-coherent image (SSCI) 
YEmail: fritjof.basan@bsh.de 
3406 J. Acoust. Soc. Am. 159 (4), Aprı 2026 
method (Yubero et al., 2025), extended with a standard 
frequency-dependent absorption term to ensure accurate 
high-frequency propagation, we calculated SLs and deep 
water radiated noise levels (RNLs) for 13 individual CTVsSs. 
To provide a consistent reference scale for contextualizing 
the measured RNLs, we compare the resulting spectra to the 
RNL curve of a typical large commercial vessel (a 200m 
bulker travelling at 14Kkn), computed following 
MacGillivray and de Jong (2021). 
Based on the JOMOPANS-ECHO (J-E) model 
(MacGillivray and de Jong, 2021), vessel SLs are 
expected to primarily depend on vessel class, speed, and 
length. Prediction of the SL of a CTV is uncertain, 
because the database that was used for the development of 
the J-E model does not include CTVs or similar vessels. 
This study provides empirical data to improve underwater 
noise models for offshore wind farm service vessels. As 
part of the EU Interreg North Sea Region DEMASK 
Project (2025), our objective is to refine noise prediction 
methodologies by facilitating the implementation of CTV 
service traffic in existing models. By improving the under- 
standing of CTV noise emissions, this research contributes 
to regulatory frameworks and marine spatial planning in 
the North Sea. 
Il. METHODS 
After describing the measurement location and data col- 
lection process (Secs. II A and IIB), we detail the identifica- 
tion of CTV passages in the acoustic data in Sec. IIC. The 
subsequent sections explain the calculation of SLs and 
RNLs using the SSCI method, including frequency- 
dependent absorption correction (Sec. IID), followed by 
©Author(s) 2026. ©_S