Side scan sonar apparatus has been commercially available and in use for the past decade to provide a detailed mapping of underwater seabed topography. Typically, the apparatus comprises a sonar or acoustical transducing source and a transducing receiver, both being embodied in an underwater hull (often a submergible vehicle called a tow fish) which is maintained at a selected height above the seabed. The received signals are converted to a format suitable for printing, and the resulting data is applied to a printer mechanism to provide a printed record. The resulting record is not a planar "picture" of the seabed topography, that is, a picture such as might be produced by an underwater camera, but is a nonlinear mapping of the seabed topography onto a planar surface. The nonlinear mapping is generally read or "interpreted" by a trained technician who is familiar with side scan sonars and the nonlinear maps they produce.
The correlation between the nonlinear mapping produced by the side scan sonar and the actual seabed topography depends upon several factors, including tow fish height, vessel velocity, and system range. Thus, an accurate interpretation of the present side scan sonar maps require not only an accurate interpretation of the received sonar data but also an appreciation of the scale factors which are incorporated into the printed data. The nonlinearly and scale factor effects, some of which are selected by the operator using front panel controls, thus "distort" the maps provided by the present side scan sonar systems and makes them difficult to use and substantially impossible to combine or mosaic with each other to provide an acceptable integrated map of an area covered by plural scans. Furthermore, present side scan maps can also be inaccurate because the printed data display reflects both the height and the forward velocity of the sonar source, data which may be unavailable when the map is studied.
In another aspect of side scan sonar systems, the received acoustic data has a relatively large dynamic range, on the order of about 120 decibels, while a typical output printer has a dynamic range of only about 20 to 30 decibels. The prior art systems have thus included different configurations of externally set time varying gain amplifiers to compensate for the large dynamic range of the acoustic input signals; however, these systems have not proven totally satisfactory because they require constant manual adjustment and intervention to provide an acceptable display. Also, the resulting display may be inaccurate because the amplifiers were not correctly adjusted.
Therefore, the principal objects of the invention are a side scan sonar apparatus and method which provide a linear plan view of the seabed topography having a selected aspect ratio, which provide an improved time varying gain control system for improving the repeatablity and reliability of the output record, and which provide an improved printer control. Another object of the invention is a reliable, flexible, and easy to operate side scan sonar system and method which do not require the intervention of a skilled operator.
Yet further objects of the invention are an apparatus and method which provide side scan sonar records having a controlled density and aspect ratio to enable the records to be combined with each other and provide an accurate composite picture or mosaic of the seabed topography.
Another object of the invention is a method and apparatus for enhancing the detail on the seabed.