PART VI: SURVEY PURPOSE 



80. CERC experience has shown that varying the operating variables 

 (e.g. vessel speed, trackline, etc.) and control settings of the side-scan 

 sonar assists in exploring the different details of the structure. This 

 technique allows a more informed interpretation of the surveyed structure. 

 Consequently, the purpose of the survey and the structural details of interest 

 will dictate the procedures used. The authors view the potential purposes of 

 a side-scan sonar survey to include reconnaissance, inspection, and qualita- 

 tive monitoring. The majority of the following discussion is oriented toward 

 sloping, rough-faced coastal structures and provides some general guidelines. 

 Specific cases may involve changes to those recommendations. Some recently 

 developed acoustic imaging alternative tools which have the potential to 

 overcome some of the limitations of side-scan sonar are presented at the end 

 of the monitoring section. A short discussion on inspecting vertical wall 

 structures concludes this portion of the report. 



Reconnaissance 



81. Side-scan sonar is best known as a reconnaissance tool. Possible 

 applications to coastal structures are numerous and include locating channel 

 debris, mapping bottom materials, locating lost objects or old structures, 

 and identifying major inconsistencies in the underwater portion of a coastal 

 structure. Larger range scales (50 m or greater) and higher speeds (4 to 



6 knots) are possible since fine detail is not required. Runs will generally 

 be made parallel to the structure and some distance seaward of the toe. The 

 approximate position of the boat should be noted on the record. Location of 

 the towfish relative to the boat should also be noted. Positioning could be 

 done without electronic survey control if survey stations on the structure can 

 be observed from the water. In this case the approximate station and esti- 

 mated distance offshore from the structure can be noted on the record. 



Inspection 



82. Use of side-scan sonar as an inspection tool for coastal structures 

 will often require some experimentation with the operating parameters. 



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