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

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

Acoustics Research Group Synthetic Aperture Operation

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

Acoustics Research Group Full resolution image kHz Experimental data

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

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

Acoustics Research Group Power Spectral Density along ky axis

Acoustics Research Group Power Spectral Density along kx axis

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

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

Acoustics Research Group Power Spectral Density along ky axis

Acoustics Research Group Power Spectral Density along kx axis

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

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 50kHz to 150kHz Transmitted pulse length ≈ 5 wavelengths Total seafloor insonified ≈ 3m diameter circle

Acoustics Research Group First echoes from nadir

Acoustics Research Group Echoes at a later delay time

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

Acoustics Research Group Ground range as a function of time

Acoustics Research Group Flat surface echo: 50cm standoff

Acoustics Research Group Rough surface echo: 50cm standoff

Acoustics Research Group Rippled surface echo: 50cm standoff

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

Acoustics Research Group Angular spread vs delay 0.5m

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

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

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

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

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

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

Acoustics Research Group Tank experimental data: silty sand

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