The conventional presentation of the data from a side scan sonar suffers from several types of distortion. Until these distortions are removed, sonar images from two or more overlapping passes through an area can not be combined to form a "mosaic." The major distortions arise from the original acquisition of the data in an arbitrary s-t coordinate system, where s represents the slant range from the vehicle to a point on the bottom, normal to the vehicle track, while t represents time of day, and thus vehicle position along its track. Since the slant range from the vehicle to a bottom point depends on both the horizontal and vertical distance to that point, and the vehicle track is not a straight line traversed with uniform velocity, the original data do not represent an undistorted picture of the ocean bottom. Digital processing of the sonar data together with precise navigation data acquired at the same time has been used to remove these distortions and produce properly rectified images.
Over the last fifteen years there has been an increasing appreciation of the usefulness of side scan sonars to picture the sea floor for basic science, commercial exploitation, and operational (e.g. sea .floor search) purposes. Many of us who have been involved in such use for a long time have learned to read and interpret the raw analog pictures which such systems produce in their simplest display configurations. As more complex problems are attacked, however, it becomes increasingly useful to transform these pictures into other formats, particularly to make it easier to comprehend small details or patterns encompassing areas greater than a single swath width.
In our case we were spurred to action on this score by the data which we assembled, with NSF support, in the eastern Mediterranean Sea during the summer of 1978. The area in which we were working was one which exemplifies a style of bottom topography called "cobblestone" because of the lumpy appearance of the conventional echo sounder records collected in depths from 2 km to 4 km in that region. Using our deeply towed survey vehicle and associated acoustic transponder navigation, we mapped three patches about 10 by 10 km in different locations, with full side scan coverage, at depths ranging from 2 km to 3 km. These showed intricate patterns of ridges and troughs on kilometer scales, and made it clear that a major element of our data reduction would have to be the production of a side scan sonar mosaic. It is this activity which this paper treats.
Figure 1 shows the actual shape on the ocean bottom of areas which are recorded as squares on the standard simple plot of elapsed time along track versus acoustic travel time across track. The purpose of side scan sonar processing is to return these square areas to their original shape. In order to do this, three major kinds of adjustment are required.
