r 





In the artificial circumstances of zero overburden pressure and a very 

 well - sorted sediment, the viscous mechanism may dominate; for "real" sedi- 

 ments at a depth of burial of 2 m, the solid-friction mechanism is dominant 

 (McCann and McCann, 1969). 



Thus, in general, fine-grain sediments attenuate acoustic energy to 

 a lesser degree than coarser matter. Therefore, sound may penetrate clays 

 and silts more deeply than sand or gravel. This result, of course, is 

 what is commonly observed. 



Other Acoustic Methods for Determining Seafloor p ronerties 



There are other methods of acoustically deriving information about the 

 physical and load-bearing properties of the seafloor, but these methods 

 have not been researched to the degree that the acoustic reflectivity, 

 velocities, and absorption have been. 



The Mine Defense Laboratory has developed a Sea Bottom Classifier (SBC) 

 which determines the softness of a sea bottom bv measuring the elongation 

 of reflected acoustic pulses using a standard depth sounder (Stanley, 1968). 

 If a short narrow-beam acoustic Dulse is transmitted into a hard bottom, 

 the received echo is about the same length as the transmitted pulse. On 

 the other hand, in a soft bottom the acoustic pulse penetrates the bottom, 

 and volume reverberation within the bottom causes elongation of the echo 

 pulse proportional to pulse penetration. 



Another approach employs a frequency analysis of the bottom-reflected 

 pulse. The frequency content of the pulse, as determined through a Fourier 

 analysis, may contain information which correlates with the physical or 

 engineering properties of the sediment. 



SUMMARY AND CONCLUSIONS 



The continuous reflection profiling system provides detailed geologi- 

 cal information about a potential seafloor construction area, as well as 

 quantitative data about its potential load-bearing capabilities. 



The marine construction engineer is supplied with a more accurate 

 and a continuous profile of the subbottom geologv than may be obtained 

 using only borings and depth sounding techniques. Core and borehole data 

 may be correlated with the seismic record. (Note here the immense poten- 

 tial value of using the NCEL Seafloor Deep Corer with a seismic survey.) 



In foundation studies, competent strata, sediment, and bedrock are 

 mapped. Slumping, faulting, and other geologic features are located. Dif- 

 ferential compaction of overburden, sediment erosion and deposition (scour 

 and fill) , and filled channels are among the other geological features 

 which may be delineated. Continuous reflection profiling, in short, pro- 

 vides information from which a detailed and accurate geologic map of a 

 potential construction area can be constructed. 



22 



