Image-processing of sidescan sonar images using seabed topography data from a multibeam sonar

What is it about?

Sidescan sonar images, like side-looking airborne radar images, have geometrical distortions arising from the data being recorded against signal travel time not horizontal range. This distortion can be removed, as shown here, using bathymetry data of appropriate resolution by working out how the "slant range" corresponds with the horizontal range. The amplitudes of the data are also distorted in a number of ways. For example, the sonar's beam patterns lead to a striping along-track. Some of these distortions are reduced by the conversion of the sonar data into seabed backscattering strengths (effectively a ratio of the backscattered signal intensity to the intensity of the signal incident on the seabed, normalised by area).

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Why is it important?

Converting sidescan sonar data to quantitative backscattering strengths, if done accurately, would allow the data to be interpreted by comparison with measurements made in acoustic experiments on known seabeds and with predictions of acoustic models. This has not quite been achieved with GLORIA data because of a remaining unresolved uncertainty in the system's amplifier gains (see Mitchell and Somers, IEEE J Ocean Engineering, 1989), but it is nevertheless still possible to study the variability in the data quantitatively as shown here and later papers (e.g., see Mitchell, J Geophys Res 1993).