4 Acoustical Systems 



Side-Scan Sonar 



Side-scan sonar was used during the pilot study for several purposes: to provide 

 a general "picture" of the site including bed forms, to note large obstructions which 

 may need to be avoided, and to test the capability to detect small (ordnance-type) 

 objects on the bottom. The latter goal would require identifying a pattern df returns 

 in a specific area that was more likely to be a cluster of hard, cylindrical objects than 

 normal returns from bottom roughness elements. Throughout the survey area there 

 were hard, dark targets that appeared on the sonographs as 0.25-m-long, relatively 

 strong backscatter signals. Because these areas were observed throughout the 

 survey area and appeared with no particular pattern, their source may be accredited 

 to a natural effect of bottom roughness. Without additional ground-truthing, it is 

 not appropriate to identify these returns as pieces of ordnance. Larger objects with 

 patterns that were likely of man-made origin were observed in the study area. These 

 included what appeared to be a small sunken boat partially buried and a subsurface 

 buoy. In the case of the subsurface buoy, the magnetometer detected the presence of 

 metal in approximately the same area. Sand waves were prevalent over several 

 sections of the study area (Figure 8), which tended to dominate the acoustical signal 

 in these areas, obliterating any smaller returns. To the north of borrow area 1 A, the 

 bottom had a mottled appearance which suggests the presence of circular zones 

 containing a different (finer-grained) material than the surrounding sandy bottom 

 (verified by video camera crossings of the same area). The side-scan sonar did a 

 satisfactory job in locating larger objects and illustrating changes in bottom texture, 

 but it is not appropriate as an instrument for independently detecting the classes of 

 ordnance present at this site. As with all applications of side-scan sonar, a full- 

 survey use of this instrument would need to include a "ground-truthing" phase 

 where divers or other forms of bottom imaging would be collected and used to verify 

 record interpretation. 



X-star 



The purpose of testing the X-star was to determine the ability of this instrument 

 to detect hard return objects buried within the upper (say, 2-m) portion of a sandy 

 bottom. The potential value of X-star in characterizing the ordnance contamination 

 at Sea Bright would be realized if it was able to document whether or not suspected 

 ordnance was buried beneath the sand surface which would complicate any 



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Chapter 4 Acoustical Systems 



