Environmental Variability on Acoustic Prediction Using CASS/GRAB

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Environmental Variability on Acoustic Prediction
Using CASS/GRAB

• Nick A. Vares
• June 2002

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Purpose

• Determine the impact of bottom type and wind
  variations due to limited data on bottom moored
  mine detection
• Determine the significance of transducer depth on
  bottom moored mine detection

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Relevance

• Littoral engagement
• Mine warfare
• Diesel submarines
• Unmanned Undersea Vehicles (UUVs)

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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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Mine Hunting Sonar

• Generic VHF forward looking
• CASS/GRAB input file for MIW with signal excess
  output
• Generic bottom moored mine

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AN/SQQ-32 Mine Hunting Sonar System

• The CASS/GRAB Acoustic model input file used in
  this study simulates a VHF forward looking sonar,
  similar to the Acoustic Performance of the
  AN/SQQ-32.
• The AN/SQQ-32 is the key mine hunting component
  of the U.S. Navys Mine Hunting and
  Countermeasure ships.

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Detection Sonar and Classification Sonar Assembly
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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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Bottom Type Geoacoustic Properties
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Bottom Type Variability

• Muddy sand (3.0) and sandy silt (5.0)
• Grain size index variation 1.0 in 0.5
  increments
• 5.14 m/s, - 40, bottom 30 m, transducer 5.18 m

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Yellow Sea Bottom Sediment Chart

• Bottom Sediment types can vary greatly over a
  small area
• Mud
• Sand
• Gravel
• Rock

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Wind Variability

• Muddy sand (3.0) and sandy silt (5.0)
• Tilt angle - 40, bottom 30 m, transducer 5.18 m
• 5.14 2.57 m/s wind

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AN/SQQ-32 Employment

• Variable depth high frequency sonar system
• Sonar can be place at various positions in the
  water column to optimize the detection of either
  moored or bottom mines.

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Transducer 5.18 m vs 25 m

• Tilt angles 40 to 120
• Wind 2.57 to 12.86 m/s
• Coarse sand to silt bottoms
• 30 m water depth

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Conclusions

• Bottom type and wind variability are important
  for sandy silt detections
• Bottom type and wind data variability are on the
  order of a few decibels
• Deep transducers provide higher signal excess for
  most detectable cases

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Recommendations

• Sensor improvements of a few decibels are
  significant for detection
• Money would be better spent on sensor development
• Employment of sensors deeper aids bottom moored
  mine detection