Side-looking sonar records or images are often a valuable addition to conventional marine geophysical surveys. Recently, side-looking sonar images have been computer processed using software originally developed for planetary and terrestrial imaging missions. These general purpose programs were used to perform geometric corrections for slant-range and ships speed as well as shading corrections for illumination gradients. Although these corrections are important steps in preparing the images for interpretation, they are typically insufficient to register the images to a cartographic base. This type of correction is a prerequisite in order to correlate side-looking sonar images with other data (e.g. bathymetry, geophysical data, samples, etc.). Current NASA/JPL efforts to computer process side-looking sonar images focus on using navigation data to rectify and register GLORIA side-looking sonar images against a cartographic base and display the results in one of 7 standard map projections. Images from adjacent, overlapping tracklines are then computer mosaicked to provide a regional or synoptic display of the seafloor terrain.
Side-looking sonar records or images are often a valuable addition to conventional marine geophysical surveys. These sonar images portray the seafloor in a plan-view and in a manner similar to aerial photographs of the continents. Furthermore, a single sonar image can provide more information regarding seafloor morphology than dozens of bathymetric profiles. Long range side-looking sonar images, such as those collected by the Geologic Long Range Inclined Asdic (GLORIA) of the British Institute of Oceanographic Sciences (IOS), render a synoptic or regional picture of the seafloor.
However, the potential value of side-looking sonar images is frequently unrealized since the output from sonar scan line recorders (either chart paper or photographic film) cannot be conveniently registered to a cartographic map base. There are several causes for this. Side-looking sonar images possess many distortions inherent in the image scanning technique. Also, unless tracklines are perfectly straight, the resulting image printed will poorly represent the "footprint" of the scan lines on the seafloor. Moreover, chart paper and even photographic prints are not amenable to the resectioning and rectification needed to remove these distortions and fit the image to a standard map projection. When available as a map projection, side-looking sonar images can be straightforwardly and more conveniently compared to other map based data (for example, topography, sediment type and grain size, bottom currents, etc.). Side-looking sonar image distortions also account for the irreconcilable mismatches encountered in joining adjacent image swaths to create sonar mosaics.
In an effort to study solutions to these problems, the IOS has collaborated with the National Aeronautics and Space Administration and the Jet Propulsion Laboratory (NASA/JPL) to digitally rectify and mosaic GLORIA images. Computer image processing techniques were chosen because of the flexibility, rigorous control, and repeatability required for rectification and mosaicking. Also, the processing techniques and facilities used already existed since they were originally created to support NASA unmanned planetary exploration programs. Because most of these image processing functions are general purpose, they were readily adapted to side-looking sonar images.
