source towed below the water surface, are directed at a low angle to either 

 side, unlike downward- directed Fathometer and seismic reflection signals. 

 Side -scan sonar thus searches a broad swath on one or both sides of a track- 

 line, and detects such features as rock outcrops, boulders, man-made objects, 

 and other features that project above the sea floor. It is capable of resolv- 

 ing very small features such as ripple marks , and can discriminate between 

 broad differences in bottom sediment character. It is a useful adjunct to 

 seismic reflection and Fathometer equipment, especially where rock outcrops or 

 boulders that would limit or preclude dredging occur. 



103. Side-scan sonar systems consist of a control and recorder unit and 

 a towfish unit that is towed behind the survey vessel at a desired depth below 

 the water surface. Transducers in the towfish generate acoustic pulses both 

 to port and starboard, and receive reflected signals that are converted to 

 proportional electric energy, which is transmitted to the recorder unit. The 

 acoustic pulses emitted by the transducer are reflected from any object rising 

 above the general level of the sea floor, and recorded on the strip chart as a 

 dark tone. The area immediately beyond in the shadow of the object is 

 recorded as a light tone. Depressions are likewise recorded, but the light 

 tone is nearest the trackline and the dark tone outlines the far wall. The 

 strength of the reflection is indicated by the intensity of the dark tone, 

 which is a function of the acoustic impedance between the water and the 

 reflecting object, and the angle between the object and the incident acoustic 

 signal. Signal strength increases with increasing acoustic impedance and with 

 approach to a 90-deg relationship between the object face and the acoustic 

 beam. 



104. Available side-scan sonar equipment is capable of high resolution 

 imaging of the sea floor up to over 200 m to either side of the vessel track- 

 line; thus, a total swath of 400 m or more can be covered at each pass. In 

 common with seismic reflection profilers, the range increases and resolution 

 decreases progressively as the operating frequency is lowered. To alleviate 

 this problem, some systems are capable of dual operation by using both high- 

 and low- frequency signals that are simultaneously recorded on separate chan- 

 nels of a recorder having four or more available channels. Units are also 

 available that can correct slant ranges to objects to true horizontal position 

 and to correct vessel speed so that the records show true position, which 

 makes the data analogous to maps or aerial photographs. 



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