JASA 
https:/doi.org/10.1121/10.0043324 
Statistical analysis using generalized additive models 
‘GAMSs) and random forest (RF) models (Sec. IIE). 
A. Measurement setup 
Selecting an appropriate measurement location was cru- 
cial to maximize the number of recorded CTV passages. To 
identify a suitable site, automatic identification system 
(AIS) data from the JOMOPANS Project (2020) were ana- 
lyzed, focusing on tracks of vessels classified as “high-speed 
orafts.” 
Helgoland serves as a key departure point for CTVs, 
servicing four offshore wind farms to its north: Amrumbank 
West, Kaskasi II, Nordsee Ost, and Meerwind Süd/Ost. 
Siven its strategic position and high vessel traffic, a mea- 
surement site was chosen within the northern approach fair- 
way to Helgoland. 
To minimize the risk of equipment loss due to fishing 
activity, additional AIS data from the JOMOPANS Project 
were reviewed to avoid high-density fishing areas. The 
selected measurement location is shown in Fig. 1. Two 
hydrophones were deployed at this site, positioned 60m 
apart to ensure data redundancy. Recordings were collected 
:ontinuously from June 24 to October 08, 2024. The black 
lines in Fig. 1 indicate the AIS-tracked CTV passages 
recorded during this period. 
The recorders were installed in custom-made cages that 
were held vertically in the water column with floating 
buoys. Both recorders were measuring for 55min every 
hour, ensuring sufficient time for the recorder to properly 
save the audio files in WAV format. The sensitivity of the 
tecorders was tested prior to deployment using a custom- 
made acoustic calibrator at 125 Hz (IEC, 2019). The sample 
rate was set to 96 kHz, enabling an analysis of decidecade 
bands up to 40kHz. Table I provides an overview of the 
8' 
° 54°N 
3 15.00’ 
5 
127 
Longitude 
8°E 
FIG. 1. Tracks of crew transfer vessels around Helgoland (black) and other 
marine traffic (orange) between June and October 2024. 
J. Acoust. Soc. Am. 159 (4), April 2026 
TABLE I. Overview of measurement location and used equipment. 
Parameter Recorder 1 Recorder 2 
Latitude 54.23°N 54.23°N 
Longitude 7.82°E 7.82°E 
Water depth 24.8 m 24.5 m 
Recorder type Sound Trap ST600 Sound Trap ST600 
Sampling rate 96 kHz 96 kHz 
Duty cycle 55 min/1h 55 min/1h 
Sensitivity —177.7dB re 1 V/uPa —177.9dB re 1 V/uPa 
location of the measurement, the equipment used, and the 
settings chosen. 
B. Data collection and preprocessing 
The raw data files were quality controlled following the 
recommended practice from Ward et al. (2021). Then all 
recordings were processed to compute 1-s sound pressure 
levels (SPLs) for all decidecade frequency bands between 
LOHz and 40kHz using the tool BSOUNDH, developed by 
Fraunhofer IDMT (Oldenburg, Germany) during the BSH 
SOUND Mapping Project (2021). The tool uses an inte- 
grated filter bank and was tested to conform with the com- 
mon standards (IEC, 2014, 2016). 
C. Identification of CTV passages 
AIS data, sourced from the European Maritime Safety 
Agency (EMSA), were analyzed to identify vessel passages 
where the distance of the closest point of approach (DCPA) 
to the hydrophone was less than 1 km (mean DCPA 429m, 
standard deviation 229 m). Only passages without other ves- 
sels within 2 x DCPA were considered, resulting in the 
identification of 3243 passages, of which 2526 were CTV 
passages. To correlate these passages with the acoustic data, 
peaks in the broadband SPL time series were identified 
within +2 min of the time of the closest point of approach 
(TCPA) for all passages. Only peaks with a minimum prom- 
inence of 6dB above their local acoustic background were 
considered, where prominence is defined as the level differ- 
ence between a peak and the lowest surrounding local mini- 
mum. All 810 preselected peaks were then manually 
validated using a custom MATLAB interface (see Fig. 2). The 
closest point of approach (CPA) for each passage was deter- 
mined by examining the narrowband spectrogram up to 
1000 Hz. Specifically, the minimum of the characteristic 
U-shaped interference pattern—known as the Lloyd’s mirror 
effect, which results from the interference between direct 
and surface-reflected sound paths—was manually identified 
(Urick, 1983). However, manual selection of these minima 
was complicated by uncertainties that may arise from 
frequency-dependent effects, the directional characteristics 
of the sound source, or its relative position and motion. 
The shift between manually validated TCPAs and 
TCPAs from the AIS data reveals a constantly increasing 
time drift between the recorders and the AIS data. Both 
recorders exhibit a similar clock drift (Fig. 3). A linear 
Basan et al. 3407