Effects of a Effects of a Suspended Bottom Suspended Bottom Boundary Layer on Boundary Layer on Sonar PropagationSonar Propagation

Michael CorneliusMichael Cornelius

June 2004June 2004

PurposePurpose

Determine the impact of a Determine the impact of a suspended bottom boundary layer suspended bottom boundary layer on the reverberation on the reverberation characteristics of a simulated minecharacteristics of a simulated mine

Determine critical value of volume Determine critical value of volume attenuation that renders mine attenuation that renders mine object undetectableobject undetectable

RelevanceRelevance

Mine WarfareMine Warfare Future investigation of bottom Future investigation of bottom

boundary layers on acoustic boundary layers on acoustic detectiondetection

Mine WarfareMine Warfare

CASS/GRABCASS/GRAB Comprehensive Acoustic Comprehensive Acoustic

Simulation System (CASS)Simulation System (CASS) Gaussian Ray Bundle (GRAB) Gaussian Ray Bundle (GRAB)

Eigenray modelEigenray model Navy standard model for active Navy standard model for active

and passive range dependent and passive range dependent acoustic propagation, acoustic propagation, reverberation and signal excessreverberation and signal excess

Frequency range 600Hz to 100 kHzFrequency range 600Hz to 100 kHz

CASS/GRAB Model CASS/GRAB Model DescriptionDescription

The CASS model is the The CASS model is the range dependent range dependent improvement of the Generic improvement of the Generic Sonar Model (GSM). CASS Sonar Model (GSM). CASS performs signal excess performs signal excess calculations.calculations.

The GRAB model is a subset The GRAB model is a subset of the CASS model and its of the CASS model and its main function is to compute main function is to compute eigenrays and propagation eigenrays and propagation loss as inputs in the CASS loss as inputs in the CASS signal excess calculations.signal excess calculations.

CASSComprehensive Acoustic

System Simulation

Propagation Model 1: FAME

Propagation Model 3: COLOSSUSPropagation Model 4: AMOS equations

Backscatter ModelsReverberationNoise Models

Signal to NoiseSignal Excess

Graphic DisplaysSystem Parameters (Beamforming)

Propagation Model 2: GRAB Gaussian Ray Bundle OAML GRAB v1.0

Environmental Interpolations Environmental Model Interpolations Surface and Bottom Forward Loss Volume Attenuation Sound Speed Algorithms Call GRAB

Comprehensive Acoustic Comprehensive Acoustic Simulation Simulation

System/Guassian Ray System/Guassian Ray Bundle (CASS/GRAB)Bundle (CASS/GRAB)

In the GRAB model, the travel time, source angle, target In the GRAB model, the travel time, source angle, target angle, and phase of the ray bundles are equal to those angle, and phase of the ray bundles are equal to those values for the classic ray path. values for the classic ray path.

The main difference between the GRAB model and a classic The main difference between the GRAB model and a classic ray path is that the amplitude of the Gaussian ray bundles ray path is that the amplitude of the Gaussian ray bundles is global, affecting all depths to some degree whereas is global, affecting all depths to some degree whereas classic ray path amplitudes are local. GRAB calculates classic ray path amplitudes are local. GRAB calculates amplitude globally by distributing the amplitudes according amplitude globally by distributing the amplitudes according to the Gaussian equationto the Gaussian equation

,

,

exp . ( ) /0

22

205

p rz z

r

Klein 5000 SonarKlein 5000 Sonar

Klein specifics: Klein specifics: 455 KHz455 KHz

5 beams per side5 beams per side

Resolution: 20cm@75mResolution: 20cm@75m

36cm@150m36cm@150m

Can be towed at 15ktsCan be towed at 15kts

Source Level 240dBSource Level 240dB

Klein 5000 SonarKlein 5000 Sonar

Image From Klein 5000Image From Klein 5000Image: X=50.462m

Y=61.672mSilty clay bottom

Object: 5m x 3m x 2mAssumed Steel

X= 30m Y= 28m

Bathymetric DataResolution- 3m in Y 2.5m in X

Image From Klein 5000Image From Klein 5000Sonar: Depth 30.4m

Range of Depths:

95m-77m

Sound Velocity ProfileSound Velocity ProfileSound Velocity Profile

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

1508 1510 1512 1514 1516 1518 1520 1522

Sound Speed (m/s)

De

pth

(m

)

Bottom Type Geoacoustic Bottom Type Geoacoustic PropertiesProperties

Suspended sediment layer Suspended sediment layer changes the volume scattering changes the volume scattering strength, …strength, …

CASS/GRAB Input CASS/GRAB Input ParametersParameters

Bottom depthBottom depth Target depthTarget depth Transducer depthTransducer depth Wind speedWind speed Bottom type grain Bottom type grain

size indexsize index Frequency min/maxFrequency min/max Self noiseSelf noise

Source levelSource level Pulse lengthPulse length Target strength/depthTarget strength/depth Transmitter tilt angleTransmitter tilt angle Surface scattering Surface scattering

/reflection model/reflection model Bottom scattering Bottom scattering

/reflection model/reflection model

Difference in Input FilesDifference in Input Files

1. Normal Bathymetry- No Synthetic Mine1. Normal Bathymetry- No Synthetic Mine 41 files, batch file, 41 files, batch file, PlotCASSReverb_all.mPlotCASSReverb_all.m

WorkingwithoutWorkingwithout2. Altered Bathymetry- Mine inserted2. Altered Bathymetry- Mine inserted

1-17 same, 17-22 mine, 22-41 same1-17 same, 17-22 mine, 22-41 same WorkingWorking3. Altered Bathymetry-Mine inserted3. Altered Bathymetry-Mine inserted Bottom Boundary Layer PresentBottom Boundary Layer Present WorkingwithlayerWorkingwithlayer

Input Type 1Input Type 1Normal Bathymetry-No MineNormal Bathymetry-No Mine

Input Type 2Input Type 2Altered Bathymetry- MineAltered Bathymetry- Mine

Adding LayerAdding Layer

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

Suspended Bottom Boundary Suspended Bottom Boundary LayerLayer

VOLUME SCATTERING STRENGTH TABLE

M DB//M 0.00 -95.00 77.00 -95.00 78.00 -65.00 95.00 -65.00 EOT

Acoustic impact of bottom Acoustic impact of bottom suspended layer is to increase the suspended layer is to increase the volume scattering strength (XXXX, volume scattering strength (XXXX, 199x), 199x),

Increase of the volume scattering Increase of the volume scattering strength -> increase of the strength -> increase of the volume reverberationvolume reverberation

Input Type 3 Input Type 3 Altered Bathymetry- Mine-Layer Altered Bathymetry- Mine-Layer

(-30 dB/m)(-30 dB/m)

Input Type 3Input Type 3Altered Bathymetry- Mine-Layer Altered Bathymetry- Mine-Layer

(-27 dB/m)(-27 dB/m)

Input Type 3Input Type 3Altered Bathymetry- Mine-LayerAltered Bathymetry- Mine-Layer

(-22 dB/m)(-22 dB/m)

ConclusionsConclusions

A side scan sonar image can be A side scan sonar image can be represented through reverb represented through reverb characteristics.characteristics.

Labor intensive changing of input Labor intensive changing of input files.files.

Critical values of Volume Scattering Critical values of Volume Scattering Strength for this situation were -30 to Strength for this situation were -30 to

-22 dB/m-22 dB/m

Where to Next?Where to Next?

Finer resolution and a more Finer resolution and a more complex object could produce complex object could produce more useful results.more useful results.

Field measurements of layer to Field measurements of layer to limit assumptions.limit assumptions.

Impact of layer on SVP and Volume Impact of layer on SVP and Volume Attenuation?Attenuation?