Varying boat speed, location relative to the structure, approach angle, 

 towfish elevation, etc., along with paper speed, range, and gain can all be 

 used to enhance the image or add a new perspective to the structure. The 

 total picture is often arrived at by piecing together observations from 

 several different runs. 



83. Inspection requires finer control of the operating parameters than 

 does reconnaissance and is usually done at slow speeds (2 to 4 knots) in 

 ranges of 25 to 50 m. Electronic positioning is strongly recommended, 

 although stations at 100-ft spacing along a structure can be used to determine 

 location with less accuracy. Runs will often be made parallel to the struc- 

 ture, and at least two passes should be made. Runs seaward of the toe with 

 the fish close to the bottom should provide good definition of the toe and 

 berms (Figure 39). By paralleling the structure near the water's edge with 

 the towfish near the surface, it should be possible to look down the structure 

 slope (Figure 39) to observe slope irregularities. 



Monitoring 



84. As a monitoring tool, side-scan sonar is far more limited because 

 it cannot be practically used to measure change. Accurate control and dupli- 

 cation of the moving towfish position through space and time are very diffi- 

 cult to accomplish. Slight variations in the towfish location and speed can 

 translate into obvious changes in object perspective and shadow length and 

 shape between different runs. Recently developed acoustic imaging tools aided 

 by microprocessors have the potential to remove some of the side-scan sonar 

 limitations allowing "mapping" of the targets (briefly discussed at the end of 

 this section) . 



85. Qualitative monitoring is possible and practical, particularly for 

 documenting changes to the structure toe and if the armor cover units are 

 large and distinctive. The changes to structure toe line and objects on the 

 bottom can be identified between surveys. Major changes in the slope relief 

 may also be identified between surveys. However, direct overlaying of the 

 images is not practical. The monitoring value of side-scan sonar is only 

 realized if all operation parameters (i.e. boat speed, towfish depth, posi- 

 tion, paper speed, range, etc.) are duplicated as closely as possible between 

 surveys. Short-range microwave positioning should be used and readings taken 



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