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<p>Ocean bottom seismometers (OBS) are essential tools for marine geophysical research, but their deployment in shallow continental shelf environments (&lt; 200&#xa0;m water depth) is severely hindered by intensive fishing activities, particularly bottom trawling. As a result, seismic observation gaps in shallow-water shelf areas directly constrain the capacity to assess seismic hazards associated with offshore active fault zones. This study presents the design, implementation, and field validation of a trawl-resistant seafloor platform that integrates a broadband OBS within a prismatic protective frame featuring eight sloped surfaces. The system was deployed for a 96-day period in the Bohai Sea (water depth 25–30&#xa0;m), an area characterized by high-density trawl fishing operations. The protective housing incorporates 30° sloped surfaces to facilitate the sliding of fishing nets, includes an elastic coupling mechanism to maintain seismic coupling, and includes an acoustic release system with backup recovery capabilities. A theoretical estimate of the coupling system resonant frequency, based on the spring stiffness and the combined mass of the OBS and coupling frame, yields a value of approximately 3–5&#xa0;Hz, which falls within the upper portion of the instrument passband (0.02–40&#xa0;Hz); consequently, unless the mechanical transfer function of the platform‑coupling system is corrected for, residual amplitude and phase distortions cannot be ruled out. Although the initial acoustic release failed due to one-way communication limitations, the system was successfully recovered using side-scan sonar positioning and diver-assisted retrieval. The OBS recorded regional earthquake waveforms, from which four representative events were selected for detailed analysis. Continuous recording yielded a 100% data continuity rate over the 96-day operational period (excluding deployment and recovery intervals) and clearly captured regional earthquake waveforms, including the Rushan M4.2 (epicentral distance 165.8&#xa0;km), Ryukyu Islands M6.7 (1335&#xa0;km), Kyushu M6.0 (1137&#xa0;km), and Gaizhou M3.1 (324&#xa0;km) events. The observed P and S wave arrival times are broadly consistent with predictions from the IASP91 theoretical model, with travel-time residuals less than 7&#xa0;s for the four selected events. The 7&#xa0;s residual for the Ryukyu event (1335&#xa0;km) represents a relative error of ~ 5.3%, whereas the 1.6&#xa0;s residual for the Rushan event (165.8&#xa0;km) corresponds to a larger relative error (~ 5.8%) due to the shorter propagation path; both values fall within acceptable limits. For the Rushan event, the measured P–S time of ~ 20&#xa0;s gives an epicentral distance consistent with the catalog value within ~ 1.3%, confirming the reliability of the location. Power spectral density analysis shows a vertical component noise level of approximately − 135 dB re 1 (m/s)²/Hz at 1&#xa0;Hz, which is approximately 5–10 dB above the Peterson New Low Noise Model (NLNM) velocity reference and consistent with typical shallow-water noise conditions. The results indicate that this trawl-resistant platform can sustain long-term seismic observation in a fishing-intensive shallow-water environment. While the single-site nature of this feasibility test precludes conclusions about broad generalizability, the results provide a proof of concept that motivates multi-site validation across diverse shelf settings.</p>

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Design and single-site feasibility test of a trawl-resistant seafloor platform for broadband seismic observation in fishing-intensive shallow waters: a case study from the Bohai Sea

  • 期刊论文

  • 发表日期：

    2026年7月29日

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  • Xianfeng Li,
  • Qingfeng Hua,
  • Yanliang Pei,
  • Chenguang Liu,
  • Kaiben Yu,
  • Yanpeng Zheng

摘要

Ocean bottom seismometers (OBS) are essential tools for marine geophysical research, but their deployment in shallow continental shelf environments (< 200 m water depth) is severely hindered by intensive fishing activities, particularly bottom trawling. As a result, seismic observation gaps in shallow-water shelf areas directly constrain the capacity to assess seismic hazards associated with offshore active fault zones. This study presents the design, implementation, and field validation of a trawl-resistant seafloor platform that integrates a broadband OBS within a prismatic protective frame featuring eight sloped surfaces. The system was deployed for a 96-day period in the Bohai Sea (water depth 25–30 m), an area characterized by high-density trawl fishing operations. The protective housing incorporates 30° sloped surfaces to facilitate the sliding of fishing nets, includes an elastic coupling mechanism to maintain seismic coupling, and includes an acoustic release system with backup recovery capabilities. A theoretical estimate of the coupling system resonant frequency, based on the spring stiffness and the combined mass of the OBS and coupling frame, yields a value of approximately 3–5 Hz, which falls within the upper portion of the instrument passband (0.02–40 Hz); consequently, unless the mechanical transfer function of the platform‑coupling system is corrected for, residual amplitude and phase distortions cannot be ruled out. Although the initial acoustic release failed due to one-way communication limitations, the system was successfully recovered using side-scan sonar positioning and diver-assisted retrieval. The OBS recorded regional earthquake waveforms, from which four representative events were selected for detailed analysis. Continuous recording yielded a 100% data continuity rate over the 96-day operational period (excluding deployment and recovery intervals) and clearly captured regional earthquake waveforms, including the Rushan M4.2 (epicentral distance 165.8 km), Ryukyu Islands M6.7 (1335 km), Kyushu M6.0 (1137 km), and Gaizhou M3.1 (324 km) events. The observed P and S wave arrival times are broadly consistent with predictions from the IASP91 theoretical model, with travel-time residuals less than 7 s for the four selected events. The 7 s residual for the Ryukyu event (1335 km) represents a relative error of ~ 5.3%, whereas the 1.6 s residual for the Rushan event (165.8 km) corresponds to a larger relative error (~ 5.8%) due to the shorter propagation path; both values fall within acceptable limits. For the Rushan event, the measured P–S time of ~ 20 s gives an epicentral distance consistent with the catalog value within ~ 1.3%, confirming the reliability of the location. Power spectral density analysis shows a vertical component noise level of approximately − 135 dB re 1 (m/s)²/Hz at 1 Hz, which is approximately 5–10 dB above the Peterson New Low Noise Model (NLNM) velocity reference and consistent with typical shallow-water noise conditions. The results indicate that this trawl-resistant platform can sustain long-term seismic observation in a fishing-intensive shallow-water environment. While the single-site nature of this feasibility test precludes conclusions about broad generalizability, the results provide a proof of concept that motivates multi-site validation across diverse shelf settings.