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Apr 06, 2023 444 likes •557 views

Broadband acoustic scattering from nonlinear internal waves Andone C. Lavery Woods Hole Oceanographic Institution Presented by Timothy K. Stanton Woods Hole Oceanographic Institution Acknowledgements: Dezhang Chu (Northwest Fisheries

SLIDE 1

Broadband acoustic scattering from nonlinear internal waves

Andone C. Lavery Woods Hole Oceanographic Institution Presented by Timothy K. Stanton Woods Hole Oceanographic Institution

Acknowledgements: Dezhang Chu (Northwest Fisheries Sciences Center) Jim Moum and team (Oregon State University) Karen Fisher (Los Alamos National Lab) Doris Leong (Dalhousie University) Paul Heslinga (Falmouth Academy) Funding provided by ONR Ocean Acoustics and WHOI

SLIDE 2

Why use acoustic scattering techniques for investigating physical processes?

Moum et. al., J. Phys. Oceanography 33, 2093-2112, 2003. Depth [m] Distance along ship track [m]

20 40

200 –100 0 100 200

120 kHz

Fish or large zooplankton Small zooplankton/ Microstructure?

SLIDE 3

Sources of scattering and scattering models

FREQUENCY (Hz) Volume Scattering Strength (dB)

Fish (Gas-bearing) Squid (Fluid-like?) Turbulent Microstructure Medusae (Fluid-like) Euphausiids (Fluid-like) Copepods (Fluid-like) Pteropods (Elastic-shelled) Siphonophores (Gas-bearing)

Bandwidth of acoustic system

SLIDE 4

SW06/NLIWI Experiment

Latitude

Longitude Longitude

Long Island N e w J e r s e y

SARS Satellite Image July 23, 2006 Experiment duration: July 31- August 27, 2006

50 100

Complex IW field

SLIDE 5

Broadband acoustic backscattering system

Vertical Down-Looking Horizontal Side-Looking

4 Broadband Transducers: 160 – 270 kHz, 10o (3dB-BW) 220 – 330 kHz , 8o (3dB-BW) 340 – 470 kHz ,12o (3dB-BW) 450 – 600 kHz , 9o (3dB-BW)

SLIDE 6

Microstructure measurements (Jim Moum)

SLIDE 7

Zooplankton net tows

Conclusions:

1. Abundance and biomass dominated by small copepods (fluid-like) 2. Scattering dominated by small pteropods (elastic-shelled) and amphipods (fluid-like)

Depth (m) Local Time

1 2 3 4 5 6 7 8 9

MOC 4: 24 August 2006 MOCNESS: Multiple Opening/Closing Net and Environmental Sampling System MOCNESS track superimposed on 120 kHz echogram

SLIDE 8

Depth (m) Depth (m) Depth (m) Depth (m) Time (GMT) Mika 1: 08-14-2006

Region 2: Zooplankton dominated. Inferred pteropod: diameter = 0.78 mm. Frequency (kHz) Volume Scattering Strength (dB)

1 2 2 2 2 1 1 1

160-270 kHz 220-330 kHz 340-470 kHz 450-600 kHz

Dominated by turbulence Dominated by zooplankton

Region 1: Microstructure dominated. Inferred dissipation: ε = 8x10-6 W/kg. Kelvin-Helmholtz shear instability (region 1) Zooplankton (region 2)

Broadband spectra: example 1

SLIDE 9

Broadband spectra: example 2

8.05 8.1 8.15 8.2 8.25 8.3

Time (GMT) Broadband: 160-270 kHz Broadband: 450-590 kHz Depth (m) Depth (m) Region 2: MIXED:

Microstructure dominated at low frequencies.

Inferred dissipation: ε = 2.5×10-6 W/kg.

Zooplankton dominated at high frequencies.

Region 3: Zooplankton dominated at all frequencies. Inferred: 960 pteropods/m3 of diameter 0.53 mm.

region 1 region 3 region 2