SIDESCAN SONAR IMAGE-PROCESSING TECHNIQUES

A four-step processing sequence is described to produce image mosaics from the various segments of a sidescanned acoustic imaging survey of a given seafloor area. Starting with data consisting for each ping of acoustic backscatter levels versus horizontal range across-track, medi an prefiltering is used first to reduce the influence of outliers on s ubsequent linear processes. Artifacts that are clearly unrelated to th e backscattering properties of the seafloor are then isolated on a pin g by ping basis through a spectral analysis that relies on a decomposi tion using Chebyshev polynomials to filter the low spatial frequency c omponents of the image. Contrast enhancement is then achieved through an original implementation of the classical gray level histogram equal ization technique by balancing local versus global histogram contribut ions. Finally, pixels are mapped on a geographic grid taking due accou nt of the geometry of the measurement and of the spacing between pings to minimize along-track smearing of features. Examples of results obt ained with these processing techniques are given for SeaMARC II data r ecorded during a complete survey of Fieberling Guyot (32-degrees.5N, 1 28-degrees-W).