effects of such on the oblique vantage point is needed to correctly interpret 

 the images. A reasonable description of the side-scan sonar beam is arrived 

 at by comparison to the light areas and shadows formed by an obliquely held 

 flashlight in a darkened room. Figure 3 shows the geometry of side-scan sonar 

 during normal operation. 



^-r$ 



A - TOWFISH DEPTH BELOW SURFACE 

 B - TOWFISH ALTITUDE ABOVE BOTTOM 

 C - SLANT RANGE TO TARGET 

 D - ACOUSTIC SHADOW LENGTH 



Figure 3. Side-scan sonar geometry 

 (after Klein 1985) 



SHADOW ZONE 



11. A sonograph usually contains two channels of sonar information rep- 

 resenting the bottom to the right and left of the towfish. Two dark parallel 

 lines, representing the initial acoustical pulse, run just right and left of 

 the center of the sonograph (Figure 4). The track of the boat and towfish are 

 along these center lines (line A). The surface return (line B) is often the 

 next line closest to the center line (line A). Line C, the initial bottom 

 return, is recognizable as the start of the darker tone. Total water depth 

 can be calculated by adding the distances on the sonograph of the output pulse 

 to the surface (A to B) and the output pulse to the bottom (A to C). Scale 

 lines (D) are at slant range increments of 15 m (50 ft). As the recorder 

 range settings change (25 to 200 m per channel on most units), so does the 

 spacing on paper of the range lines since they are always 15 m (50 ft) apart. 



12. The dark line perpendicular to the line of travel (line E) is an 

 event mark created by the operator for later reference. Event marks are 



10 



