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Where does background noise in the ocean come from?

Where does background noise in the ocean come from?. ORE 654 Guest Lecture Fred Duennebier fred@soest.hawaii.edu October 21, 2011. Outline Ocean Background Acoustic Levels What has been observed? Microseisms Kibblewhite , Wake, Cessaro , Webb, H2O, Bromirski

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Where does background noise in the ocean come from?

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  1. Where does background noise in the ocean come from? ORE 654 Guest Lecture Fred Duennebier fred@soest.hawaii.edu October 21, 2011

  2. Outline Ocean Background Acoustic Levels What has been observed? Microseisms Kibblewhite, Wake, Cessaro, Webb, H2O, Bromirski What’s missing? New Measurements ALOHA Cabled Observatory (ACO) Correlation with Wind Correlation with Waves Seismic Correlation Importance of Spreading Function Conclusions

  3. A: Ship B: Rain C: Humpback D: Blue E: Minke E

  4. play sounds

  5. Webb, 1992 Noise levels near 0.2 Hz are ~18 dB higher in the Pacific than the N. Atlantic and ~42 dB higher than in the ice-covered Arctic.

  6. What’s missing? What would make the data more useful? • LONG time series • deep-ocean • large fetch • concurrent wind, wave, seismic measurements

  7. WHOTS OCEAN BUOY at Station ALOHA, 2010

  8. Waimea Bay Waverider Buoy

  9. ALOHA PROOF ACOUSTIC DATA • 20 – month acoustic time series • 0.02- 10 KHz Bandwidth • 24- bit samples • 24,000 samples/sec • pre-whitening filter • >150 dB dynamic range

  10. ACO Installed and Operating as of June, 2011

  11. How does energy from the WIND get to the ocean floor?

  12. How does energy from the WIND get to the ocean floor? • WAVE GENERATION

  13. How does energy from the WIND get to the ocean floor? • WAVE GENERATION • Longuet-Higgins mechanism

  14. How does energy from the WIND get to the ocean floor? • WAVE GENERATION • Longuet-Higgins mechanism • Lateral transport as seismic waves

  15. Wave Amplitude Wavenumber at peak amplitude Wavenumber 2π/l

  16. frequency at peak wave amplitude Nondimensional frequency Wave frequency Wind speed at 10 m elevation

  17. Longuet-Higgins Pressure at the ocean floor Angular acoustic Frequency Density/sound velocity Overlap Integral Spreading Function Angle from downwind direction HUGHES’ Equation

  18. Why is this important? For wave energy to propagate to the ocean floor requires opposing wave trains of the same frequency. The sound then observed in the deep-ocean will have TWICE THE FREQUENCY of the waves and amplitude proportional to the product of the amplitudes of the opposing waves.

  19. 15 10 5 0.5 0.1 10 1 0.1 10 1 0.1

  20. Using Nondimensional Frequency

  21. Summer

  22. Winter

  23. Sound Propagation by the Longuet-Higgins mechanism requires OPPOSING waves. Most long-period seas have only a very small opposing component, and do not contribute to the sound field.

  24. Kedar et al., 2008

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