frequently. The side-scan sonar signal should be recorded on magnetic tape 

 for later playback and possible enlargement of key images. 



86. Until a few years ago, the only alternative to side-scan sonar 

 inspection of coastal structures was the use of divers. The rapid expansion 

 of ROV technology can, in low turbidity water, allow the engineer on the 

 surface to directly observe the structures by using a small, tethered submer- 

 sible with a low light TV camera. In clear water an ROV can operate more 

 efficiently and safely (especially in deep water) than a diver. 



87. Other types of acoustic imaging instruments are now also bridging 

 the gap between side-scan sonar and visual observations. Scanning sonars, 

 which use high frequency (675 kHz to 2 MHZ) sound waves and a rapidly sweeping 

 head, provide acoustic TV pictures on a cathode ray tube (CRT) with poten- 

 tially greater detail than side-scan sonar, though with less range (up to 



25 m) . The images are recorded on video cassette recorders (VCR's). They can 

 be used while the vessel is still and placed on stationary bottom resting 

 frames to reduce motion to a minimum. Depending on the orientation of the 

 acoustic head, images can be displayed in several modes, including a profile 

 slope cross section. Further investigations and field tests are needed to 

 define the usefulness of scanning sonars, but they have definite potential. 



88. Profiling sonar works like conventional fathometers, with the 

 exception of having a rotating head which allows a cross section of the bottom 

 to be surveyed instead of a single point. However, the instrument appears to 

 be sensitive to vessel motion effects, particularly roll, which would limit 

 its effectiveness in the open ocean (Kucharski and Clausner in preparation). 



Techniques for Surveying Vertical Walls 



89. Vertical walls can be viewed using the same basic techniques as 

 those used for inspecting sloping structures. However, vertical walls present 

 problems not found in sloping structures. Vertical walls are often made of 

 steel or concrete which are strong reflectors of acoustic energy often 

 creating "cross talk". Additionally, they often have features that are not 

 easily discernible by horizontally moving towfish. 



90. However, the low wave environment where vertical wall structures 

 often are found allows the use of alternative inspection methods. Often 

 vertical walls are constructed in areas of low wave energy, allowing the 



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