then composited to create a map showing cover over the specified stone blanket 

 area (Figure 17). Divers verified the results shown on the side-scan sonar 

 images by traversing along the three tracklines shown in the figure. Some of 

 the areas identified as sketchy cover were determined to be areas with ade- 

 quate stone cover but where drifting sand had covered the stone. 



36. The Ocean City experience showed how side-scan sonar can be used as 

 a quality control inspection tool. Another important lesson learned was the 

 need to schedule surveys around the tidal cycle to limit current velocities at 

 the time of the survey and to take advantage of the water depths. 



Burns Harbor, Indiana, and Calumet Harbor, Illinois 



37. During September 1985, the breakwaters at Calumet Harbor, Illinois, 

 and Burns Harbor, Indiana (Figure 18), were inspected with side-scan sonar to 

 assess their conditions. Specifically, the surveys were conducted as a part 

 of US Army Engineer District, Chicago's (NCC's), studies in preparation for 

 rehabilitation of the breakwaters. Morang's (1987) report provides a good 

 example of the detail possible when time and conditions allow a complete 

 structure inspection using side-scan sonar. The following paragraphs review 

 pertinent sections of that report. 



38. Both surveys were conducted using an EG & G Model 260 image cor- 

 recting side-scan sonar. The signal processing capabilities of the unit 

 allowed it to produce images corrected for slant range, ship speed, and alti- 

 tude. Consequently, the resulting records show an accurate plan view of the 

 lake bottom and structural features. The surveys were run using 100-kHz 

 transducers, and images were recorded at 25- and 50-m ranges. Surveys were 

 conducted from a US Army Engineer District, Detroit (NCE), survey vessel, 

 approximately 50 ft long and at speeds of 2 to 3 knots. Positioning was 

 accomplished by observing passage of 100-ft station markers painted on the 

 breakwater and by manually triggering the event marker at every fifth station. 

 The triggering was time delayed to allow the towfish to be even with the sta- 

 tion, allowing accuracies estimated to be ± 15 ft. Survey conditions were 

 excellent with waves of less than 1.5 ft. 



39. Sections A, B, and C of the Calumet Harbor breakwater were sur- 

 veyed. Sections A and B consist of wooden cribs (Figure 19), completed in 

 1904 and capped with concrete structures during the 1920's. Section C, the 



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