example at Crescent City, California, the 500-kHz system may not provide any 

 resolution advantage, and clarity may be lost due to record smearing. 



Vessel Requirements 



71. The vessel to be used in a side-scan sonar survey needs to have 

 power, and stability. Covered deck space capable of protecting the recorder 

 is recommended for most operations, though an open deck can be used when con- 

 ditions are calm. Ship motion is transmitted to the towfish through the 

 cable. If the motion is of sufficient magnitude or speed, the resulting image 

 on the recorder will be degraded. The size of the vessel in relation to the 

 wave climate is therefore important; i.e., a smaller boat can be used in mild 

 waves, while a larger boat is needed in larger waves. The effects of large 

 waves may force the use of the lower resolution 100-kHz system, as was done at 

 Crescent City, California. 



72. Optimum speeds for normal operations is felt by the authors to be 

 between 2 and 4 knots. Faster speeds can be used for general reconnaissance 

 or may be required under certain operating conditions. It is difficult to go 

 much slower than 1.0 knot and still maintain a stable, straight course. 



Site Limitations 



73. As stated earlier, the most severe site limitations are wave 

 climate and current conditions. While a larger boat can offset the wave 

 conditions to some degree, once the waves get over several feet high it is 

 nearly impossible to get a quality image in shallow water (< 100 ft) due to 

 the short cable length which effectively transmits vessel motion to the 

 towfish. Quality surveys of coastal structures are usually performed in waves 

 of 2 ft or less, while nearly calm conditions are preferred. Consequently, 

 the period for successful surveys is limited to seasons of low wave energy, 

 i.e. the summer months. It is prudent, then, to allow a wide weather window, 

 if possible, when planning a side-scan sonar survey. 



74. Strong currents can also present side-scan sonar survey problems. 

 The major problem occurs when the current is perpendicular to the path of the 

 survey vessel. At low survey speeds, the towfish will tend to orient itself 

 with the direction of a strong current rather than the survey vessel path. To 



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