ANDERSON: BOTTOM PROPERTIES FOR LONG-RANGE PROPAGATION PREDICTION 



Measurement information on the deeper sediments comes from sub- 

 bottom profiling of one kind or another; e.g., reflection and refrac- 

 tion profiling. Most of the information concerns the speed of 

 propagation of compressional waves which is converted into information 

 about depths to sub-bottom reflectors. 



Physical information comes from drilled samples. Considerable 

 information is being compiled by the Deep Sea Drilling Project. An 

 interesting idea might be velocity-logging these holes. 



Most available data on the bottom physical and acoustical param- 

 eter values are for high frequencies and compressional -wave speed. 



In surface sediments, Hamilton (1974) has added to our knowledge 

 of in situ values of surficial sediment speed and of techniques for 

 extrapolating laboratory measurements to in situ values. 



Figure 5 represents something like 3,000 measurements of speed 

 of propagation of compressional waves, and they are plotted as a ratio 

 of speed of propagation in the sediment to that in the water. They 

 show the well known, somewhat well defined relationship between speed 

 of compressional-wave propagation in sediments and porosity. These 

 are high-frequency values. 



The values go from something like 0.95 or about 5 percent lower 

 than the value in bottom water to almost 30 percent higher than the 

 value in bottom water. Sound-speed values outside this range of 

 variation are anomalous for unconsolidated sediments. Such values are 

 usually associated with gas in the sediment in some form. In shallow 

 water sediments, gas will exist as a phase, a gaseous phase, and it 

 will decrease the value of speed of propagation. In deeper water, it 

 is more likely to exist as a gas hydrate or clathrate, and it will 



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