ANDERSON: BOTTOM PROPERTIES FOR LONG-RANGE PROPAGATION PREDICTION 



Figure 4 presents a comparison (Bucker, 1974) with data and a 

 computation with a linear-gradient model of Morris. The data are for 

 one-third octave at 50 Hz and the theory (solid line) is for con- 

 tinuous wave. 



BOTTOM PHYSICAL PARAMETERS 



There are essentially three depth intervals into which sediment 

 can be broken for measurement techniques: surficial sediments, corer- 

 depth sediments, and deeper sediments. 



In the surficial sediments, acoustical properties have been 

 measured fairly extensively, especially the compressional-wave speed 

 at higher frequencies. The surficial sediment includes about the 

 first meter. These have been probed by everything from divers to the 

 diving saucer, with various types of probes, and with self -operating 

 units. Considerable information is available in the literature about 

 some of the parameters. Physical properties have also been studied 

 extensively using grab samples and cores. 



Corer-depth sediments extend from the one-meter depth to perhaps 

 30 or 40 meters for the very long cores. Acoustical information is 

 available to this depth from high-resolution sub-bottom profiling in 

 some regions. Core sediment samples are measured in the laboratory 

 and these values are then extrapolated to in situ values. Compres- 

 sional wave speed and attenuation are studied, especially again at 

 high frequencies. 



Recently, a different technique was instituted for corer-depth 

 sediments by ARL/Austin. It is a device called a profilometer which 

 projects a pulse across the diameter of the corer as the core is be- 

 ing taken and measures compressional-wave speed and attenuation in 

 the sediment at a carrier frequency of 200 kHz. 



304 



