borrow and placement areas, determination of optimum orientation and spacing 

 for bathymetric sounding lines), lost instrument search, and bottom evolution 

 monitoring. 



Background 



4. SSS was originally developed as a tool to assist in the search and 

 recovery of objects resting on the bottom, such as shipwrecks. Like echo- 

 sounding fathometers to which SSS is related, the transducer emits a series of 

 acoustic -frequency pulses (typically 50 to 500 kHz) traveling outward and 

 encountering surfaces that transmit, absorb, or reflect the energy. The ter- 

 minology "side scan" refers to the fact that the pulses are emitted laterally 

 to the forward motion of the transducer; as such, the area of seabed scanned 

 has a limited width and a length corresponding to the path length traveled by 

 the transducer. Using the strength of the reflected energy and the time 

 required for the return trip to the transducer, an image, typically a plan 

 view of the bottom and objects resting on the bottom, is produced by the pro- 

 cessor unit. If automatic correction for transducer position and forward 

 speed is included in the processing software and if input from position- fixing 

 equipment is fed to the unit, the resulting overlapping strip records can be 

 pieced together to form a mosaic image which is an approximately accurate 

 (scale) map of the bottom and its features. Additionally, the vertical dimen- 

 sions of some features can be estimated from the length of shadows they cast, 

 and other qualitative information can be inferred from the images, such as 

 shape and relative position, of large and isolated objects. 



