and deconvolved with the received echo, however this level of 

 signal processing will not be attempted for the planned study. 

 Volume scattering of sound in the water column will also be 

 ignored- The primary remaining effects relate to the nature of 

 the seafloor ensonified; the relative orientation of the incident 

 wavefront with the bottom, the reflection coefficient of the 

 bottom, the roughness of the seafloor within the footprint, and 

 the interference of subsurface reflectors. Since the current 

 study area is dominated by fresh lavas, the subsurface 

 interference can be ignored. 



The relative orientation of incident waves and the reflecting 

 surface has a primary effect on the nature of the return pulse. 

 For a vertical incident wave (whether the center beam on a flat 

 bottom or a sidebeam on a properly sloping area of the bottom) 

 the reflected energies are directed toward the receiving array. 

 At any other angle, the primary energies are directed away from 

 the ship, with the majority of the energies being forward 

 scattered and the remainder back scattered. Because the Sea Beam 

 system provides a complete two dimensional model of the seafloor, 

 the orientation for the spatial wavelengths sampled can be 

 directly calculated. As described by Fox and Hayes (1985), 

 seafloor topography is composed of a continuum of wavelengths, 

 and therefore the boundary between the "orientation" and 

 "roughness" wavelengths of the seafloor are purely artificial. 



For vertically incident acoustic energy reflecting from a 

 perfectly smooth surface, the amount of energy returned depends 

 upon the physical properties of the reflecting surface and the 

 transmission medium. More specifically, the reflection 

 coefficient is a function of the relative densities and 

 compressional wave velocities of the two media. The properties 

 of the bottom water are essentially constant for most study areas 

 and the varying properties of the bottom generally control the 

 reflection coefficient. DeMoustier (1985) was able to map the 

 distribution of manganese nodules in a sedimentary environment 

 using these properties by mapping the peak amplitude of the 

 center beam return. The variations in rock properties for the 

 current study are probably minimal, being a neo-volcanic terrain. 



The microtopographic roughness of the seafloor within the 

 sonar footprint is the final primary factor affecting the nature 

 of the acoustic return. For a vertically incident plane wave, 

 the return pulse represents the convolution of an impulse 

 function with the distributions of depths within the footprint. 

 For a smooth bottom, the variance of depths is small and 

 therefore the returned energy is coherently received as a narrow 

 energy envelope. For a rough bottom, the variance of depths is 

 larger and the returned energy is spread out in time. For the 

 rough bottom case, additional energy is lost to scattering, which 

 with the spreading of the signal, results in a substantially 



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