subsequent return recorded. Although this normal operation is 

 adequate for bathymetric mapping, substantial insight into the 

 physical properties and microtopography of the bottom can be 

 derived from the interpretation of the character of the return 

 pulse. 



SYSTEM DESCRIPTION 



Sea Beam is a multibeam sounding system developed and 

 manufactured by General Instruments Corporation. The geometry of 

 the system is described by Glenn (1970) and is illustrated 

 (Fig. 1). The hull mounted array uses 20 acoustic projectors 

 mounted along ship and 40 receiving hydrophones mounted athwart- 

 ship to preform 16 beams of 2-2/3' solid angle dimension and 

 spacing. The 54' range of the beam pattern allows ensonif ication 

 of the seafloor for a width equal to approximately 75 percent of 

 the water depth. The pings for the current experiment were 

 transmitted at constant time intervals of 6 or 12 seconds. 

 Throughout the ping cycle, sensors are interrogated to record the 

 ship's attitude (roll, pitch and yaw). All of the information is 

 transferred to the shipboard processing computer for analysis. 



The backscatter recording system, developed and described by 

 deMoustier (1986), accesses this information for recording to 

 magnetic tape. The backscatter envelope from each of the 16 

 beams is digitized and recorded with 512 samples at 2 millisecond 

 intervals centered on a window about the bottom return. During 

 post-processing, numerous corrections must be performed on this 

 recorded data. A time varying gain function is applied to 

 account for spherical spreading of the acoustic wave front. The 

 Sea Beam hardware assures that the outgoing pulse is near 

 vertical over the relatively short duration (7 milliseconds) of 

 projection. However, the ship's roll angle can vary 

 substantially during the receiving cycle, and must be modelled 

 and compensated. The corrected backscatter data from each beam, 

 when combined with ship's navigation, become the fundamental 

 information for sea-floor classification. 



GENERAL PRINCIPLES 



The physics of acoustic bottom interaction is a complex and 

 much studied subject and is described in some detail by Urick 

 (1983), Clay and Medwin (1977), and others. Research is 

 currently underway to study the physics of interaction of Sea 

 Beam with the seafloor (Robert C. Tyce, personal communication, 

 1987). The principles discussed here are greatly simplified, 

 however as will be shown later, the perspective is adequate for 

 the approach taken for this study. 



For the purposes of discussion, the outgoing acoustic signal 

 will be assumed to be an instantaneous planar wavefront. In 

 theory, the more complex form of this wavefront could be modelled 



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