is directed downward toward the sea floor along a vertical axis; whereas, for 

 side-scan sonar the very narrow acoustical pulses are directed in opposing 

 directions from the towfish transducers at an angle of about 10° below the 

 horizontal axis (Fig. 5) . Side-scan sonar equipment was developed during 

 World War II to locate and identify enemy submarines. However, researchers 

 realized during field trials that the equipment was also capable of distin- 

 guishing between major sediment types, and capable of locating underwater 

 objects with negative and positive relief from the sea floor, such as rock out- 

 crops, sand shoals, channels, and wrecks (D'Olier, 1979). The first side-scan 

 sonar built strictly for scientific use became commercially available in 1962 

 from the British firm, Kelvin Hughes, Ltd. (Flemming, 1976). During the past 

 20 years, a number of companies have continued to refine and develop their own 

 side-scan sonar equipment. Today, there are a half-dozen systems available that 

 are capable of giving good results. The major differences are: frequencies 

 used that determine record resolution, the selection of range scanning widths, 

 and more recently the availability of mapping unit recorders, which can yield 

 true scale mosaics of the sea floor. 



Most of the recent side-scan sonar units use piezoelectric transducers that 

 vibrate at precise frequencies when electrical power is supplied. The vibra- 

 tions from the transducer create pressure waves that are transmitted through 

 the water column, reflected and backscattered from the seabed, and finally picked 

 up by the hydrophone and recorded on continuous chart paper (Fig. 6). 



The transducers for side-scan surveys can be either attached to the hull 

 of the survey vessel where they can scan only in one direction and records 

 may be distorted by ship motion, or the transducers can be mounted on a towfish 

 that is pulled at varying depths astern (Fig. 5) . This alternative allows for 

 dual-channel scanning on both sides and also enables the operator to vary the 

 tow depth above the sea floor to achieve optimum data return. Transducers 

 usually vibrate in the frequency range of 50 to 500 kiloh^rtz; however, the most 

 commonly used frequency of about 100 kilohertz offers a compromise between maxi- 

 mum range coverage and good resolution of features on the seabed. The rela- 

 tively new high frequency transducers (500 kilohertz) are most useful for short 

 range, 150 to 450 feet (46 to 138 meters) , scan coverage where the maximum reso- 

 lution and clear definition of detail is desired. The lower frequency systems 

 are better utilized in regional surveys where wider scanning paths on the order 

 of 1,000 to 1,300 feet (305 to 396 meters) total width are desired (Fig. 7) 

 (National Oceanic and Atmospheric Administration, 1976) . 



2. Operation of the Equipment and Sonograph Interpretation . 



As stated, it is important prior to surveying to clearly define the desire 

 objectives. For side-scan data collection, there is always a trade-off between 

 the width of the coverage for each transect and the degree of resolution of fea- 

 tures on the seabed. For detailed site surveys it is desirable to have a track- 

 line spacing such that the sonograph records can fit into a mosaic to produce 

 a plan view of the survey area with no gaps. Such an integrated view of the 

 study area can greatly aid in identifying complex topographic features. 



If a towfish-type transducer is used, the best results are achieved when it 

 is towed above- the sea floor at 10 to 20 percent of the range scale used. Thus, 

 in surveying areas of high seabed relief the position of the towfish must be 

 adjusted by altering the length of the cable used for towing. In all surveys. 



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