Slide 1

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

Slide 2

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

Slide 3

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

Slide 4

play sounds

Slide 5

Slide 6

Slide 7

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.

Slide 8

Whats missing? What would make the data more useful? LONG time series deep-ocean large fetch concurrent wind, wave, seismic measurements

Slide 9

Slide 10

WHOTS OCEAN BUOY at Station ALOHA, 2010

Slide 11

Waimea Bay Waverider Buoy

Slide 12

Slide 13

Slide 14

Slide 15

Slide 16

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

Slide 17

Slide 18

Slide 19

Slide 20

ACO Installed and Operating as of June, 2011

Slide 21

Slide 22

Slide 23

Slide 24

Slide 25

Slide 26

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

Slide 27

WAVE GENERATION

Slide 28

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

Slide 29

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

Slide 30

Wave Amplitude Wavenumber 2/ Wavenumber at peak amplitude

Slide 31

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

Slide 32

Slide 33

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

Slide 34

Slide 35

Slide 36

Slide 37

Slide 38

Slide 39

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.

Slide 40

Slide 41

0.1 1 10 0.1 1 10 0.1 0.5 5 10 15

Slide 42

Slide 43

Slide 44

Slide 45

Using Nondimensional Frequency

Slide 46

Summer

Slide 47

Winter

Slide 48

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.

Slide 49

Kedar et al., 2008

Slide 50

Slide 51