1. Effects of a Suspended Bottom Boundary Layer on Sonar Propagation Michael Cornelius June 2004

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

3. Relevance Mine Warfare Mine Warfare Future investigation of bottom boundary layers on acoustic detection Future investigation of bottom boundary layers on acoustic detection

4. Mine Warfare

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

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

7. 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. 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 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

8. Klein 5000 Sonar Klein specifics: 455 KHz 5 beams per side Resolution: 20cm@75m 36cm@150m 36cm@150m Can be towed at 15kts Source Level 240dB

9. Klein 5000 Sonar

10. Image From Klein 5000 Image: X=50.462m Y=61.672m 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

11. Image From Klein 5000 Sonar: Depth 30.4m Range of Depths: 95m-77m

12. Sound Velocity Profile

13. Bottom Type Geoacoustic Properties

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

15. CASS/GRAB Input Parameters Bottom depth Bottom depth Target depth Target depth Transducer depth Transducer depth Wind speed Wind speed Bottom type grain size index Bottom type grain size index Frequency min/max Frequency min/max Self noise Self noise Source level Pulse length Target strength/depth Transmitter tilt angle Surface scattering /reflection model Bottom scattering /reflection model

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

17. Input Type 1 Normal Bathymetry-No Mine

18. Input Type 2 Altered Bathymetry- Mine

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

20. 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

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

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

23. Input Type 3 Altered Bathymetry- Mine-Layer (-27 dB/m)

24. Input Type 3 Altered Bathymetry- Mine-Layer (-22 dB/m)

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

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