PART II: BACKGROUND AND PRINCIPLES OF OPERATION 



5. Side-scan sonar evolved from the echo sounding depth finders devel- 

 oped during World War II. In the early 1960's, the first commercial side-scan 

 sonars were used to map the sea bottom and search for submerged objects 

 (Flemming 1976). Although these two functions remain the primary operating 

 areas for side-scan sonar, recent electronic advances and an increased aware- 

 ness of the capabilities of side-scan sonar have led to its use as a valuable 

 inspection tool for coastal structures. 



Theory 



6. In side-scan sonar systems, acoustical energy is projected laterally 

 from a pair of transducers mounted in a towed cylindrical body or "towfish." 

 Each horizontal beam of energy is from 0.2 to 1.5 deg wide (Figure 1). Verti- 

 cally, the main lobe of the beams covers an angle of approximately 40 deg and 

 is usually aimed 10 deg below the horizontal. Energy in the vertical side 



Figure 1 . Side-scan sonar in operation 



