ORE 654 Applications of Ocean Acoustics Lecture 3b Doppler shift and example sound levels

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ORE 654Applications of Ocean AcousticsLecture
3bDoppler shift and example sound levels

• Bruce Howe
• Ocean and Resources Engineering
• School of Ocean and Earth Science and Technology
• University of Hawaii at Manoa
• Fall Semester 2011

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Doppler shift or effect

• Apparent change in signal frequency after
  propagation caused by the relative motion of a
  source and receiver

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Doppler frequency shift

• Consider repetitive pulse every T seconds
• If distance D constant, receive time is D/c and
  apparent frequency remains f0 1/T
• Distance decrease between s and r because of
  relative speed vr, arrival time changes
• Length of time between pulses will
  change/decrease, and apparent frequency changes
  too
• As distance decreases (vr ), frequency
  increases, wavelength decreases

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Doppler and ASW

• Acoustic source passing a fixed receiver
• df goes to zero at x0
• df/f can be large (0.7) for relative speed of 10
  kts (18.5 km/hr) 5 m/s
• Characteristic signature - pick out of clutter,
  track over time
• df/f gtgt radar

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NPAL / ATOC Kauai source

• 260 W
• M-sequence coded signals
• 75 Hz, 35 Hz bandwidth
• 28 ms peak
• 27.28 s period
• 2 hour transmissions, 1 per day

Red segments ARS recordings
79
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DIVES 25 and 56 - examples
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Example time series
1/75 Hz 13.3 ms
10.8 ms
14.7 ms
13.0 ms



zoom PSD
Kauai example
Example PSD
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Coherent processing of M-sequence coded signals
Relative travel time 0.4 s
Relative travel time 27.28 s

• Peaks in each block shift due to changing s/r
  range
• Measured travel time changes
• 3.7 ms per 27.28 s block
• Match glider kinematics
• 0.204 m/s, 136 m horizontally, 33 m vertically,
  in 12 minutes

Relative travel time 0.3 s
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Coherent gain
Relative travel time 0.4 s
Even with glider motion, coherent processing was
possible, with 9.4 dB of gain Doppler
consistent Theoretical gain is 14 dB peaks
still not properly aligned more to do
1 Block
26 Blocks
Relative travel time 0.16 s
Time 12 minutes
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5 cm/s differences
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Doppler - other

• Doppler velocimeters
• Scatters plankton, bubbles (ideally passive
  tracers)
• Basic limits on maximum unambiguous range Rm and
  velocity Vm
• Coherent and incoherent systems
• Moving (ocean) surface induces Doppler shift in
  scattered signal
• Measure velocity of platform relative to fixed
  seafloor Doppler velocity log

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Sound levels

• Remember 1 W 170.8 dB re 1 µPa at 1 m, water

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Sound levels

• In air? 1 W ? dB

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Sound levels

• Remember 1 W 170.8 dB re 1 µPa at 1 m, water
• Remember that air ref is 20 µPa gt20 log(20) 26
  dB (re 1 µPa)
• Remember that impedance difference gt 20 log(1000
  1500 / 1.3 330) 35.8 dB (re ?c water)
• So net difference between air and water is 61.5
  dB
• and 1 W air 109.3 dB re 20 µPa at 1 m, air

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Sound levels
Source Broadband Source Level(underwater dB at 1 m)
Lightning Strike on Water Surface 260
Seafloor Volcanic Eruption 255
Sperm Whale Clicks 163 - 236
Beluga Whale Echolocation Click 206-225(peak-to-peak)
White-beaked Dolphin Echolocation Clicks 194-219(peak-to-peak)
Spinner Dolphin Pulse Bursts 108-115
Bottlenose Dolphin Whistles 125-173
Blue Whale Moans 155 - 188
Humpback Whale Song 144 - 174
Humpback Whale Fluke and Flipper Slap 183 - 192
Snapping Shrimp 183 - 189(peak-to-peak)
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Sound levels
Source Broadband Source Level(underwater dB at 1 m)
Tug and Barge (18 km/hour) 171
Supply Ship (Kigoriak) 181
Large Tanker 186
Icebreaking 193
Airgun array (32 guns) 259 (peak)
AN/SQS-53C(U. S. Navy tactical mid-frequency sonar, center frequencies 2.6 and 3.3 kHz) 235
SURTASS-LFA (100-500 Hz) 215 underwater dB for a single projector, 18 in vertical array
Heard Island Feasibility Test (HIFT)(Center frequency 57 Hz) 206 underwater dB for a single projector, 5 in vertical array
Acoustic Thermometry of Ocean Climate (ATOC)/North Pacific Acoustic Laboratory (NPAL) (Center frequency 75 Hz) 195
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Sounds in air DOSIT web site