PART VIII: CONCLUSION 



93. Side-scan sonar has been shown to have value as a tool for evaluat- 

 ing coastal structures. However, it is not an answer to all inspection prob- 

 lems because of several limitations due to environmental conditions and the 

 physics of sound transmission in water. The main limitation is wave effects. 

 Wave motion is transmitted from the survey vessel through the cable to the 

 towfish. Large towfish motions severely reduce sonograph quality. This 

 problem can be mitigated somewhat by increasing vessel size, but for many high 

 energy locations the periods of calmer seas needed to produce good quality 

 images are limited. Another limitation is the resultant geometry of the side- 

 scan sonar transmission pattern which sometimes makes it difficult to get the 

 detail needed to interpret the image in areas where there is air entrained in 

 the water column, strong currents, or poor acoustical contrast characteris- 

 tics. Several methods to improve sonograph quality, such as slow towspeeds, 

 planning to avoid tidal currents and wave entrained water, etc., are presented 

 in the text. 



94. The main advantage of side-scan sonar is the speed at which a 

 lengthy structure can be surveyed at a relatively low cost. One or more 

 passes along a structure during a reconnaissance survey may be used to iden- 

 tify areas with questionable images that need additional site-specific inspec- 

 tion. The records from a side-scan sonar inspection may show a change in 

 slope that requires a conventional cross-section survey or a scour hole devel- 

 oping that requires a hydrographic survey. However, it is very expensive to 

 inspect an entire structure using divers or conventional survey techniques. 

 Side-scan sonar can identify the small percentage of a total structure that 

 needs to be investigated by these expensive methods at a low cost. 



95. Additional side-scan sonar surveying of the specific site of 

 interest, varying vessel speed, towfish attitude, approach angles, and 

 recorder settings can help define the structure condition beyond the recon- 

 naissance survey. Detailed quantitative monitoring of coastal structures is 

 presently beyond the capability of side-scan sonar. The difficulties of mea- 

 suring objects with accuracy from a moving platform now prevents determining 

 slope or percent armor units displaced from a structure. As field technology 

 and electronics improve, determining these types of quantities with some 

 degree of accuracy may be possible in the future, particularly if supplemented 



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