Abstract
The previous chapter (Chapter 2) presented basic acoustic theory, from the generation of a sound wave to its scattering on/in the seabed and its reception by the sonar platform. Acoustic pressures are used to quantify the response of the seabed at the particular frequency used. These backscatter measurements are combined across-track to create a swath, and successive swaths are combined to form an image of backscatter variations. If the sonar is calibrated, these variations can provide generic measurements, called backscatter strengths, comparable with results from other surveys (with the same sonar at the same settings, or not). These processes are detailed in Section 3.2. Sidescan sonar imagery can be usefully compared and merged with bathymetry measurements. The newest generation of sidescan sonars can now produce imagery and bathymetry at the same time, using interferometry (explained in Section 3.3). These measurements of the seafloor topography are at the same resolution and co-registered with the measurements of the acoustic response of the seafloor. The large majority of sidescan sonars currently in use do not have interferometric capabilities though, so bathymetry is derived from other sources, like multibeam sonars. These measurements usually do not have the same ground resolution (e.g., 100m multibeam bathymetry and 6 m sonar imagery), and rarely benefit from the same exact geographic registration requiring the use of techniques like rubbersheeting (presented in Chapter 4). The problem is compounded by the fact that new multibeam systems also provide some forms of imagery, calculated by analysing each beam or only portions centered on the irst seabed return.
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3.6 Further Reading
About the transformation of raw acoustic measurements into sidescan sonar imagery
Burdic, W.S. (1990). Underwater Acoustic System Analysis, Second Edition. Prentice Hall, Englewood Cliffs, NJ, 466 pp.
Medwin, H.; and C.S. Clay (1998). Fundamentals of Acoustical Oceanography. Academic Press, London, 712 pp.
About sidescan sonar bathymetry processing
Denbigh, P.N. (1989). Swath bathymetry: Principles of operation and an analysis of errors. IEEE Journal of Oceanic Engineering, 14(4), 289–298, October.
Jin, G.; and D. Tang (1996). Uncertainties of differential phase estimation associated with interferometric sonars. IEEE J. Oceanic Eng., 21(1), 53–63.
Lurton, X. (2000). Swath bathymetry using phase difference: Theoretical analysis of acoustical measurement precision. IEEE J. Oceanic Eng., 25(3), 351–363.
Bird, J.S.; and G.K. Mullins (2005). Analysis of swath bathymetry sonar accuracy. IEEE J. Oceanic Engineering, 30(2), 372–390.
About multibeam bathymetry
de Moustier, C. (1988). State of the art in swath bathymetric survey systems. Int. Hydrogr. Rev., 65(2), 25–54.
Lurton, X. (2002). An Introduction to Underwater Acoustics. Springer/Praxis, Heidelberg, Germany/Chichester, U.K., 380 pp.
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© 2009 Praxis Publishing Ltd, Chichester, UK
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Blondel, P. (2009). Imagery and bathymetry. In: The Handbook of Sidescan Sonar. Springer Praxis Books. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-49886-5_3
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DOI: https://doi.org/10.1007/978-3-540-49886-5_3
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