Where does background noise in the ocean come from? ORE 654
Guest Lecture Fred Duennebier [email protected] October 21,
2011
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Outline Ocean Background Acoustic Levels What has been
observed? Microseisms Kibblewhite, Wake, Cessaro, Webb, H2O,
Bromirski Whats missing? New Measurements ALOHA Cabled Observatory
(ACO) Correlation with Wind Correlation with Waves Seismic
Correlation Importance of Spreading Function Conclusions
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E A: Ship B: Rain C: Humpback D: Blue E: Minke
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play sounds
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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.
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Whats missing? What would make the data more useful? LONG time
series deep-ocean large fetch concurrent wind, wave, seismic
measurements
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WHOTS OCEAN BUOY at Station ALOHA, 2010
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Waimea Bay Waverider Buoy
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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
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ACO Installed and Operating as of June, 2011
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How does energy from the WIND get to the ocean floor?
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WAVE GENERATION
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How does energy from the WIND get to the ocean floor? WAVE
GENERATION Longuet-Higgins mechanism
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How does energy from the WIND get to the ocean floor? WAVE
GENERATION Longuet-Higgins mechanism Lateral transport as seismic
waves
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Wave Amplitude Wavenumber 2/ Wavenumber at peak amplitude
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frequency at peak wave amplitude Wind speed at 10 m elevation
Nondimensional frequency Wave frequency
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Longuet-Higgins Pressure at the ocean floor Angular acoustic
Frequency Density/sound velocity Overlap Integral Spreading
Function Angle from downwind direction HUGHES Equation
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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.
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0.1 1 10 0.1 1 10 0.1 0.5 5 10 15
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Using Nondimensional Frequency
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Summer
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Winter
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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.