TOPO-SS is the name given to a prototype sides can sonar for simultaneous measurements of the backscattered acoustic power from the sea bed and wide swath measurements of the water depths.
Depth profiles along the side-scanned area are obtained from phase measurements of a returning wave front in two parallel transducers. The present transducer configuration provides unambiguous measurements of the declination angle in a 30° vertical sector.
An experimental, towed version has been built and tested and preliminary field operation has been undertaken. The tow fish instrumentation also includes a roll-sensor for horizontal reference of the angular measurements and a high precession depth sensor for sea-level reference of the depth measurements.
The conventional side-scan sonar is a relatively simple and inexpensive technique for high resolution acoustic imaging of the sea bed with a high rate of area coverage. Although some topographical information is" obtainable from shadow effects, no direct measures of depth is normally provided. The inclusion of a depth measuring capability would greatly enhance the potential of the side-scanning sonar as an underwater mapping tool.
Several attempts have been made recently to develop a bathymetric system based on the side-scan sonar configuration. One approach is to use stereoscopic side-scan images and determine depths from parallel differences in overlapping scan lines. This is the acoustic equivalent to eristic photogram metric mapping. However, this method seems to give disappointing results due to inadequate positioning of the tow fish and poor correlation between the conjugate scan lines. Another, more successful attempt was accomplished with the Tele-sounder system2 which adds the output of two parallel side-scan receive arrays placed approx. 33 wavelengths apart. The effect of this is to divide the vertical beam pattern into a number of narrow beams, similar to the multi-beam sonar. However, the different beams are not readily identified and the deduction of declination angles has to be made indirectly. When presented on a graphical recorder, the ordinary monograph is broken up into a series of fringes which increase in width and spacing as the range increases. The fringes change range in accordance with the topography and the general appearance is to give a three dimensional impression of depths.
The system about to be described removes the ambiguities present in the Tele-sounder system by, placing the receiving arrays only a few wavelengths apart. The declination angles are determinate by measuring the phase difference between the signals from the two receivers. This method was first demonstrated by P.N. Denbigh with a 410 kHz sonar having a transducer separation of 3 wavelengths.
In order to arrive at the depth of a resolved, notified area on the sea bed to the side of a sonar, it is necessary to supplement the slant range measurements R with measurements of the declination angle G (fig.1). Modern sonar?s obtain bearing information either from scanning a narrow beam through a sector (sector-scanning sonar) or from preformed beams having a fixed angular displacement (multi-beam sonar). The measurement principle illustrated in fig. 1 offers a simple alternative to these rather complex systems.
