1. Acoustics Research Group, Department of Electrical & Computer Engineering, University of Canterbury, New Zealand Acoustics Research Group Towards an understanding of how a SAS images buried targets. Prof. Peter Gough (presenter) Dr. Michael Hayes

2. Acoustics Research Group Presentation outline. Basic principles of Synthetic Aperture Sonar Experimental results. –Some evidence of seafloor penetration past the critical angle One possible reason –Rough seafloor scattering into the sediment A simple dipping sonar to measure backscattering cross section with angle

3. Acoustics Research Group Synthetic Aperture Operation

4. Acoustics Research Group Sea trial geometry 25 m 5m 22 0 Rock Sand/mud/gravel

5. Acoustics Research Group Full resolution image 90-110kHz Experimental data

6. Acoustics Research Group Full resolution image: 20-40kHz Experimental data

7. Acoustics Research Group Rough Seafloor with RMS = 1cm. Simulated data

8. Acoustics Research Group Power Spectral Density along ky axis

9. Acoustics Research Group Power Spectral Density along kx axis

10. Acoustics Research Group Rough seafloor with RMS= 1cm Axes equal Vertical axis expanded

11. Acoustics Research Group Rippled Seafloor with RMS = 1cm Simulated data

12. Acoustics Research Group Power Spectral Density along ky axis

13. Acoustics Research Group Power Spectral Density along kx axis

14. Acoustics Research Group Rippled seafloor RMS = 1cm Axes equal Vertical axis expanded

15. Acoustics Research Group Using an auxiliary dipping sonar to measure backscattering cross section as a function of incidence angle Spherical transducer Distance of transducer to seafloor from 1m to 0.1m Waveform 3cycle pulse @ 50kHz to 150kHz Transmitted pulse length ≈ 5 wavelengths Total seafloor insonified ≈ 3m diameter circle

16. Acoustics Research Group First echoes from nadir

17. Acoustics Research Group Echoes at a later delay time

18. Acoustics Research Group Insonified annulus Provided the pulse length is << 1m and the slant range is <1m, the insonified annular area is a linear function of delay time A = 2* π *pulse length* (v *t /2) where v is the speed of sound, t is the delay time and (v*t/2) is the slant range at time t

19. Acoustics Research Group Ground range as a function of time

20. Acoustics Research Group Flat surface echo: 50cm standoff

21. Acoustics Research Group Rough surface echo: 50cm standoff

22. Acoustics Research Group Rippled surface echo: 50cm standoff

23. Acoustics Research Group Waterfall display of echoes as sonar gets further from the seafloor (all compensated for spreading losses and increasing area coverage): rough surface

24. Acoustics Research Group Angular spread vs delay time @ 0.5m

25. Acoustics Research Group Back-scattering cross-section in dB as a function of angle: flat surface

26. Acoustics Research Group Back-scattering cross-section in dB as a function of angle: rough surface

27. Acoustics Research Group Back-scattering cross-section in dB as a function of angle : rippled surface

28. Acoustics Research Group Theoretical Backscattering strength using the HF Kirchhoff approximation: for four values of RMS slope on medium sand

29. Acoustics Research Group Narrow beam backscatter at 200kHz: “Smooth” sand at 1.72m standoff

30. Acoustics Research Group Detail of echo: “smooth” sand at 1.72m offset

31. Acoustics Research Group Tank experimental data: silty sand

32. Acoustics Research Group Conclusions Understanding how SAS can image targets buried in sand and sediments needs a better understanding of the seafloor backscattering cross section especially at larger angles of incidence A simple auxiliary dipping sonar can estimate –the average backscattering cross section as a function of incidence angle and with careful calibration –the acoustic impedance of the seafloor, perhaps even –the presence and spatial frequency of sand ripples but clearly not their orientation