Shade correction of side-scan sonar imagery by histogram transformation

Systematic intensity variation with range across a sidescan sonar record, or “shading”, degrades visualisation, and is particularly distracting when repeated in each record forming a sonar mosaic. Shade variation is difficult to predict, not only because of uncertainties in the instrument function, but also because backscatter variation with incidence angle, and hence range, depends on seabed material. The paper describes a method of shade correction in which the correction is computed from the statistics of the record itself. This is first segmented into a number of classes of seabed material using a statistical classification program, and then column histograms for each different type of material are equalized with each other by a table look-up procedure. The paper discusses the relevance of good segmentation to geometric as well as shade correction for the sonar mosaicing task