"good" or a "poor" reflector of sound. Although experience in interpreting 

 many seismic records is, admittedly, the best teacher, it is believed that 

 quantitative information about the attenuation properties of sediments is 

 useful, if only to serve as a primer to those individuals inexperienced in 

 such analysis. 



A number of researchers have performed attenuation measurements on 

 marine sediments. Shumway (1958, 1960) measured attenuation for a number 

 of sediment samples in the frequency range 20 to 40 kHz. Hampton (1967) 

 made a series of attenuation measurements on artificial sediments over 

 frequency range of 4 to 200 kHz. Wood and Weston (1964) obtained attenua- 

 tion data in muds over a frequency range of 4 to 72 kHz. Cole (1965) cal- 

 culated attenuation coefficients for various seafloor sediments from 

 reflectivity for frequencies below 4 kHz. 



McCann and McCann (1969) published a verv comprehensive article in 

 which the attenuation mechanism in marine sediments was described. 



There are three ways in which energy may be dissipated from a plane 

 wave propagating through a water-saturated sediment. 



1. Rayleigh scattering 



2. solid friction (particle-particle) losses 



3. viscous (particle-fluid losses 



Rayleigh scattering losses, characterized by a dependence on the fourth 

 power of frequency, are negligible for sediments of mean diameter less 

 than 0.1 cm at frequencies less than 1 mHz (Nolle et al., 1963). 



Solid-friction losses, which occur at the points of contact between 

 the particles, are characterized by a linear variation of attenuation 

 coefficient with frequency over a frequency range from 1 to 10° Hz 

 (Attewell and Ramana, 1966; McCann and McCann, 1969). The physical 

 mechanism of this loss does not appear to be well understood at this 

 time. 



Viscous losses arise because of the acoustic velocity differential 

 between the solid particles and the fluid in the sediment. McCann and 

 McCann (1969), using their own and Shumway 's (1960) experimental data, 

 have theoretically defined the ranges of sediment particle size over which 

 each of the latter two mechanisms is dominant. Their results will be 

 summarized. 



Figure 9 is a plot of the computed (McCann and McCann, 1969) and 

 experimental values of attenuation coefficient against sediment mean 

 diameter at a frequency of 30 kHz. It can be seen that attenuation coef- 

 ficient increases with increasing mean diameter up to a value of about 

 0.006 cm. 



20 



