A study on the evolutionary patterns of the acoustic environment of large-scale truss-type deep-sea aquaculture platforms

Abstract

To investigate the impact of large-scale offshore aquaculture platform operations on the acoustic environment of local waters, this study analyzed the observed evolution of the acoustic environment around a large truss-type platform and its surrounding waters based on a continuous 46-hour synchronous multi-channel observation dataset acquired under typical calm sea conditions. The results indicate that the interference caused by platform operations on the underwater acoustic environment is primarily concentrated in the low-frequency band below 500 Hz, and the underwater near-field exhibits a distinct discrete harmonic skeleton, The power spectral density at the characteristic fundamental frequency of 24.41 Hz reached 122.29 dB re  1 μ Pa 2 / Hz (approximately 16 dB higher than under idle conditions). However, in the frequency range above 2 kHz, the signal completely overlaps with the environmental background; the pixel-level temporal synchronization between the deck air and underwater sound fields at the moment of start-up and shutdown, combined with the strong acoustic impedance mismatch at the air-water interface, quantitatively confirms that the dominant path for structural elastic waves excited by the propulsion system is through solid transmission via the rigid truss pile foundation to underwater radiation; Within the observation window, the distant-station acoustic level exhibited an apparent day–night difference, which is consistent with the diurnal variation of the shallow-water sound-speed profile; a 4-m sound-channel axis was observed during the day, whereas a positive gradient with upward refraction prevailed at night, plausibly enhancing sea-surface scattering and contributing to the observed nocturnal decay of the distant-station characteristic sound; Furthermore, Within the 46-hour observation period, the low-frequency soundscape at the distant station appears to be modulated by semi-diurnal tidal dynamics within the observation window; noise peaks lag behind tidal peak levels by approximately 6 hours, and during neap tides (current velocity < 0.3 m/s), the acoustic background drops below –40 dB re  1 μ Pa 2 / Hz , exhibiting macroscopic non-stationary characteristics driven alternately by tidal variations and ocean currents. These findings are derived from a single 46-h campaign under calm conditions and should be regarded as process-level evidence rather than climatological statistics, providing reference data support and scientific methodological references for acoustic environment monitoring, ecological assessment, and the optimized deployment of aquaculture acoustic equipment on similar deep-sea aquaculture platforms.