Effects of a Suspended Bottom Boundary Layer on Sonar Propagation

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Effects of a Suspended Bottom Boundary Layer on
Sonar Propagation

• Michael Cornelius
• June 2004

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Purpose

• Determine the impact of a suspended bottom
  boundary layer on the reverberation
  characteristics of a simulated mine
• Determine critical value of volume attenuation
  that renders mine object undetectable

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Relevance

• Mine Warfare
• Future investigation of bottom boundary layers on
  acoustic detection

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Mine Warfare
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CASS/GRAB

• Comprehensive Acoustic Simulation System (CASS)
• Gaussian Ray Bundle (GRAB) Eigenray model
• Navy standard model for active and passive range
  dependent acoustic propagation, reverberation and
  signal excess
• Frequency range 600Hz to 100 kHz

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CASS/GRAB Model Description

• The CASS model is the range dependent improvement
  of the Generic Sonar Model (GSM). CASS performs
  signal excess calculations.
• The GRAB model is a subset of the CASS model and
  its main function is to compute eigenrays and
  propagation loss as inputs in the CASS signal
  excess calculations.

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Comprehensive Acoustic Simulation System/Guassian
Ray Bundle (CASS/GRAB)

• In the GRAB model, the travel time, source angle,
  target angle, and phase of the ray bundles are
  equal to those values for the classic ray path.
• The main difference between the GRAB model and a
  classic ray path is that the amplitude of the
  Gaussian ray bundles is global, affecting all
  depths to some degree whereas classic ray path
  amplitudes are local. GRAB calculates amplitude
  globally by distributing the amplitudes according
  to the Gaussian equation

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Klein 5000 Sonar

• Klein specifics 455 KHz
• 5 beams per side
• Resolution 20cm_at_75m
• 36cm_at_150m
• Can be towed at 15kts
• Source Level 240dB

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Klein 5000 Sonar
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Image From Klein 5000
Image X50.462m Y61.672m Silty clay
bottom Object 5m x 3m x 2m Assumed Steel
X 30m Y 28m Bathymetric
Data Resolution- 3m in Y 2.5m
in X
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Image From Klein 5000
Sonar Depth 30.4m Range of Depths 95m-77m
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Sound Velocity Profile
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Bottom Type Geoacoustic Properties
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• Suspended sediment layer changes the volume
  scattering strength,

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CASS/GRAB Input Parameters

• Bottom depth
• Target depth
• Transducer depth
• Wind speed
• Bottom type grain size index
• Frequency min/max
• Self noise

• Source level
• Pulse length
• Target strength/depth
• Transmitter tilt angle
• Surface scattering /reflection model
• Bottom scattering /reflection model

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Difference in Input Files

• 1. Normal Bathymetry- No Synthetic Mine
• 41 files, batch file, PlotCASSReverb_all.m
• Workingwithout
• 2. Altered Bathymetry- Mine inserted
• 1-17 same, 17-22 mine, 22-41 same
• Working
• 3. Altered Bathymetry-Mine inserted
• Bottom Boundary Layer Present
• Workingwithlayer

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Input Type 1Normal Bathymetry-No Mine
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Input Type 2Altered Bathymetry- Mine
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Adding Layer

• Object in 87 meters of water
• Approx. 2 meters high
• Layer inserted at 78 meters

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Suspended Bottom Boundary Layer

• VOLUME SCATTERING STRENGTH TABLE
• M DB//M
• 0.00 -95.00
• 77.00 -95.00
• 78.00 -65.00
• 95.00 -65.00
• EOT

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• Acoustic impact of bottom suspended layer is to
  increase the volume scattering strength (XXXX,
  199x),
• Increase of the volume scattering strength -gt
  increase of the volume reverberation

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Input Type 3 Altered Bathymetry- Mine-Layer
(-30 dB/m)
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Input Type 3Altered Bathymetry- Mine-Layer (-27
dB/m)
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Input Type 3Altered Bathymetry- Mine-Layer(-22
dB/m)
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Conclusions

• A side scan sonar image can be represented
  through reverb characteristics.
• Labor intensive changing of input files.
• Critical values of Volume Scattering Strength for
  this situation were -30 to
• -22 dB/m

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Where to Next?

• Finer resolution and a more complex object could
  produce more useful results.
• Field measurements of layer to limit assumptions.
• Impact of layer on SVP and Volume Attenuation?