relate the bulk density, P sec J, and the acoustic velocity, V, to the sedi- 

 ment porosity, n. From equations (9) and (10), the relationship between 

 reflection coefficient and porosity can be developed. Relationships 

 between porosity and impedances or bottom loss can also be developed. 

 Figure 2 is a plot of the theoretical curves relating porosity with these 

 acoustic properties of marine sediments. 



Figure 2 illustrates the linear (approximately) relationship between 

 porosity and sediment reflection coefficient. This relationship has been 

 substantiated by the empirical work of numerous researchers as 



R = 0.6727 - 0.696ln Hamilton et al. (1956) 



R = 0.6636 - 0.6478n Sutton et al. (1957) 



R = 0.6196 - 0.6277n Shumway (1960) 



R = 0.6634 - 0.6749n Morgan (1964) 



R = 0.6468 - 0.6456n Faas (1969) 



The recent work of Faas (1969) indicates that a 0.97 correlation coeffi- 

 cient exists between reflection coefficient and porosity (Figure 3). 



Breslau (1967), in a very comprehensive bottom reflectivity study, 

 obtained a relatively good (0.706) correlation between bottom loss and 

 porosity. Since porosity is related to grain size or textural properties 

 of marine sediments, Breslau (1967) has also established a general 

 relationship between bottom loss and the geological properties of 

 sediments . 



Generally, an increase in clay content will increase the sediment 

 porosity because of structural, size and shape effects, and a phenomenon 

 known as bridging (Terzaghi and Peck, 1948). Thus, the net result is 

 that porosity increases as the grain size decreases or as the percentage 

 of silt plus clay increases. 



The conclusion drawn by Breslau (1967) and others is that a signi- 

 ficant correlation exists between reflectivity of marine sediments and 

 their physical properties, particularly sediment porosity. As in any 

 generalization, caution must be exercised in applying such statements. 

 For example, the reflectivity data noted above have been collected at 

 widely different frequencies, 20 kHz-1 mHz, with no correction for 

 frequency. Since acoustic impedance increases slightly with frequency 

 (Smith and Li, 1966), some correction should be made. Additionally, 

 sediments containing gas derived from the decay of organic matter do not 

 adhere to the above generalizations (Jones, 1962; Smith and Li, 1966). 

 Sediments of this type may be fine-grained and highly porous, yet 



