high enough, the energy remaining after partial reflection at the 

 sea floor penetrates sediments and suffers partial reflections at 

 each successive boundary. The bedrock under the sediments also 

 reflects energy, marking the bottom of sedimentary material. Sound 

 travels at a determinable speed in water and in sediments; the time 

 interval between the outgoing pulse and return of the reflected 

 energy then determines distance between reflecting horizons. Subbottom 

 profiling yields information on the sediments, topography, and 

 structure of the oceanic crust. For determining crustal thickness 

 with depths of many kilometers, sound in the frequency range of 3 to 

 30 Hz is used; whereas for measurements of sediment thickness of the 

 order of tenths to a few kilometers of sediments, seismic reflection 

 profilers in the 30- to 500-Hz range are used. For studying the 

 detailed stratification of the upper sediments, in the top 100 m, 

 echo-sounders in the range 3.5 kHz to 12 kHz are used. As can be 

 inferred from the previous information, ocean bottom acoustic charac- 

 teristics are strongly frequency or wavelength dependent. 



From seismic studies it has been found that the average thick- 

 ness of the crust including the ocean sedimentary layer, down to the 

 mantle is 6.0 km, compared with an average crustal thickness of 33 km 

 on land. The average thickness of the present oceanic consolidated 

 or compacted sediments is 3 km. This is equivalent to 9 km of 

 unconsolidated sediment, which still has interstitial water in its 

 pore spaces. The thickness of sediments and rates of sedimentation 

 are used to estimate the age of an oceanic basin, with 200 to 600 

 years considered the average time for settling to a depth of 2,900 m. 

 This corresponds to a particle size of about 4 x 10~J cm, which is in 

 the silt-size range. Of course, this is a grossly oversimplified model. 



d. Bottom Acoustic Loss 



The loss of acoustic energy impinging on the bottom is quite 

 important for long-range sound propagation in the ocean, as well as 

 for understanding subbottom structure. Acoustic energy may be lost by 

 partial transmission into the bottom and subsequent absorption or by 

 scattering into nonspecular directions due to bottom roughness. Bottom 

 reflection loss can vary considerably from one location to another. 



Important parameters are the sound speed and density contrasts 

 between water and sediment layers in the bottom as well as porosity of 

 the sediments. Thicknesses of the bottom layers must be considered 

 also, especially in terms of acoustic wavelength or frequency. For 

 sediments with porosities greater than about 65 percent, sound speed 

 in the bottom is slower than in the deep bottom water. For such 

 sediments there is an angle of intromission, at about 10° grazing 

 angle, with very large losses of about 26 dB. The losses then decrease 

 to about 14 dB from 40° to 90° grazing angles. On the other hand "fast" 

 bottoms, with sound speed greater than water, will show a critical 

 grazing angle at about 30°, with practically dB loss from 0°-30° 

 grazing angles, and a sharp rise with losses of about 15 dB from 60° to 

 90° (fig. 2-9). 



