OCEANS MTS/IEEE Brest (France), June 2025, abstract

Monitoring Deep-Sea Soundscape off the TROPIC Seamount Using a Glider

H. Chognot, S.M. Jesus, L.O. Junior
LARSyS, University of Algarve, 8005-139 Faro, Portugal
and
F. Margirier
ALSEAMAR, 13790 Le Rousset, France

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Ref.: OCEANS MTS/IEEE, Brest (France), June 2025

Abstract:
The TRIDENT project is a European initiative, aiming at monitoring the environmental impacts of deep-sea exploration and exploitation activities, including ocean noise as a key parameter [1]. As part of this project, a research cruise (TBS24) took place at the TROPIC Seamount in June 2024 on board of the Mario Ruivo, a research vessel of the Portuguese institute IPMA.
This seamount is located between Canary Archipelago and Cabo Verde at an approximate depth of 1000 m in surrounding water depth of 4500 m. It is both a biodiversity hotspot and a site of interest for mineral exploration due to its rare mineral-rich flanks [2]. During TBS24, a SeaExplorer glider [3] was deployed for seven days, collecting acoustic, as well as non-acoustic data such as CTD, oxygen, chlorophyll concentration, and turbidity, up to a depth of 1000 m across a 40 km radius path around the seamount. This route follows a calibration strategy according to previous modelling efforts [4] and was chosen regarding a location where an acoustic model of the soundscape zone is supposed to have a high variability in the predictions of transmission loss.
The acoustic data, originally sampled at 192000 Hz, was down sampled to 12000 Hz keeping only the frequency band where ship traffic noise, wind noise and deep-sea activities are present [5][6]. To evaluate the ambient noise, power spectral densities (PSD) were calculated using the Welch's method. Then, to obtain results only link with the depth, spectral probability distributions (SPD) of sound pressure level (SPL) were calculated for three ocean layers, up-layer from the surface to 300 m, mid-layer from 300m to 600 m and down-layer from 600 to 1000 m. Then, to match with the soundscape model output, 26 third-octave frequencies from 15 to 6000 Hz of SPL distributions across were selected.
Calculated sound speed profiles produced expected results, with a declining celerity correlated with the thermocline 1(a). Some parts of the acoustic data, were contaminated by glider self-noise, that corresponds to the surface, turning and phases. This noise contamination was avoided by removing the first and last 30m of the glider route. SPD results show a decreasing median from 70 dB to 38 dB across the frequency range, that is in line with the higher sound attenuation at higher frequency 1(b). Also, the sound level distributions remained generally invariant with respect to depth 1(c) and time (not shown). This stability is correlated with constant wind-speed and low ship traffic conditions observed during the deployment period, as confirmed by the Copernicus Marine Services and an AIS data.

These findings illustrate the value of glider acoustic measurements in calibrating deep-sea soundscape models. First, gliders can include hydrophones alongside other sensors, providing an acoustic data link with water column properties. Second, gliders also enable data collection along chosen trajectories, down to 1000 m, allowing model comparisons on a regional-scale. Third, gliders collect data soundlessly, apart from occasional contaminations during manoeuvres, which can be identified and removed. One challenge was to link the moving data with fixed model outputs. The chosen method was to segment the data in location and time to match with the model resolution. When compared to fixed hydrophones, more model points could be studied, but with reduced time coverage. In a context of deep-sea activities monitoring, this method proved to be valuable for obtaining far-field data over extensive regions as a complements to fixed hydrophones, which are more suited for near-field and mid-field monitoring.

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