avoid this problem, it is best to run the survey into the current which may 

 require adjusting the time of the survey to take advantage of the optimum 

 current direction. If the survey must be run perpendicular to a strong cur- 

 rent, it may be necessary to use a rigid mount (e.g. a pipe clamped to the 

 rail) to negate the current effects. 



75. Lack of contrast or too much contrast between the target of 

 interest and the surrounding bottom can be potential problems in side-scan 

 sonar surveys. It may be difficult to clearly define the toe of a stone 

 breakwater if it is situated on a rough bedrock base. In such cases, it may 

 be necessary to make runs with different towfish elevations and positions to 

 highlight the change in slope between the bottom and the structure. The 

 opposite problem can occur with a structure that is a very strong reflector of 

 acoustic energy, such as a sheet-pile wall. If the gain on the recorder is 

 turned down so that a clear image of the wall is obtained, bottom features 

 near the structure may not be visible. In this case, it may be necessary to 

 make runs with different gain settings to view both the wall and the bottom. 



76. Other site limitations can also make it difficult to maintain the 

 constant speed and towfish elevation required for good results. Presence of 

 other vessels, both commercial and recreational, can make it difficult to keep 

 the vessel on track. Proper planning in the time of the survey can reduce 

 these problems. Irregular bottom topography and floating debris can also 

 create potentially hazardous situations for the towfish. 



Operator Experience 



77. An experienced operator is needed to obtain quality records. 

 Recent advances in digital electronics technology have made some side-scan 

 sonar units produced since 1985 easier to tune for a clear record. However, 

 an understanding of the principles involved, instrument capabilities, and 

 experience greatly increases the probability of producing quality images. 



78. Experience is also a critical factor in interpreting the image. 

 Recent construction drawings along with a knowledge of the history of the 

 structure are a great help in interpreting the significance of anomalies in 

 the image. Persons familiar with the area and the structure should be 

 consulted. Local Corps personnel, marina operators, local fishermen, and 

 divers, to name a few, may also provide information that can aid interpreting 



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