SEABEAM BACKSCATTER ANALYSIS APPLIED TO THE 

 CLASSIFICATION OF DEEP-SEA VOLCANIC TERRAINS 



Christopher G. Fox and Marijke van Heeswijk 



National Oceanic and Atmospheric Administration 



Pacific Marine Environmental Laboratory 



Hatfield Marine Science Center 



Newport, OR 97365 



ABSTRACT 



Hull-mounted sonar systems, such as Sea Beam, are typically 

 used for mapping the bathymetry of the deep-sea floor. The same 

 digital information that is used for the measurement of depth can 

 be evaluated for the backscattering properties of the seafloor. 

 The unique morphologies and petrologies of volcanic and 

 hydrothermal terrains may produce identifiable backscatter 

 signatures. The Alvin support vessel Atlantis II is equipped for 

 digitally acquiring Sea Beam backscatter energy traces, allowing 

 sonar remote sensing and submersible exploration to be performed 

 in tandem. An experiment funded by the National Undersea 

 Research Program, at Axial Seamount, Juan de Fuca Ridge, has 

 produced the most extensive data set to date from a known 

 hydrothermal area, and these data are being calibrated using the 

 large groundtruth data base collected by NOAA ' s VENTS research 

 program. 



INTRODUCTION 



Hull-mounted sonar sounding systems have been used since the 

 1920 's for mapping the bathymetry of the deep-sea floor. The 

 technique involves projecting monochromatic sound (usually 12 kHz 

 in modern systems) directly downward from the ship and recording 

 the two-way travel time of the sound as it echoes from the 

 seafloor. By assuming a mean sound velocity for the water 

 column, the depth can be inferred. Since the 1960 's, digital 

 systems have been available which allow both a narrow beam 

 pattern to be generated, and the automated acquisition of the 

 returned acoustic energy to be processed via computer. In the 

 1970 's, a multibeam sounding system (Sea Beam) was made 

 commercially available. This system allows a wide swath of 

 individual soundings to be collected simultaneously. Tyce 

 (1986) describes many of the various sonar instruments currently 

 available with a historical perspective of their development. 



In conducting a typical Sea Beam bathymetric survey, the 

 returning acoustic pulse (backscatter echo) is digitized and 

 automatically analyzed to determine the arrival time of the 

 maximum (bottom) return. Following this determination for each 

 beam, the backscatter information is normally discarded and the 



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