laboratory values of density and porosity versus depth in the sea floor. To construct in situ 

 profiles, the results of consolidation tests are used to estimate the amount of elastic rebound 

 (increase in volume) which has occurred after removal of the samples from overburden 

 pressure in the boreholes. In situ profiles of porosity and density versus depth are con- 

 structed for some important sediment types: calcareous ooze, siliceous oozes (diatoma- 

 ceous and radiolarian oozes), pelagic clay, and terrigenous sediments. There is less reduction 

 of porosity vv'ith depth in the first 100 meters in these deep-water sediments than previously 

 supposed: 8 to 9 percent in pelagic clay, calcareous and terrigenous sediments and only 4 to 

 5 percent in the sihceous sediments. From depths of 300 meters the most rebound is in 

 pelagic clay (about 7 percent) and the least in diatomaceous ooze (about 2 percent); 

 calcareous ooze and terrigenous sediment should rebound from 300 meters about 4 to 5 

 percent. Terrigenous sediment, from the surface to 1000 meters depth, probably rebounds 

 a maximum of about 9 percent. Methods are described and illustrated to predict density 

 and porosity gradients in the sea floor and to compute the amounts of original sediments 

 necessary to have been compressed to present thicknesses. Slightly over 2000 meters of 

 original sediments would have been required for compression to a present-day thickness of 

 1000 meters of terrigenous sediments. 



Two figures are reproduced here from Hamilton (1976c). Figure 10 illustrates 

 laboratory measurements which have been corrected to in situ values and compared with 

 data in shales (below 600 meters) from oil-industry explorations. Figure 1 1 illustrates 

 density versus depth for the five most common sediment types. These data and tables and 

 regression equations in the report should allow reasonable predictions of density at given 

 depths in the sea floor. 



SHEAR WAVE VELOCITY PROFILES IN MARINE SEDIMENTS 



Introduction 



The velocity of a compressional wave is dependent on the sediment bulk modulus, 

 rigidity and density. Given shear wave velocity and density, rigidity can be easily computed. 

 Given shear and compressional wave velocities and density, all of the other elastic properties 

 can be computed. When a sound wave is reflected within a sediment or rock layer, part of 

 the energy is converted to a shear wave. 



Studies at the Applied Research Laboratories of the University of Texas (Hawker 

 and Foreman, 1976; Hawker et al, 1976, 1977) found important effects when shear waves 

 were introduced into bottom loss models. At low grazing angles in the case of clay and 

 possibly silt (but not sand) overlying hard rock, it was found that a very large bottom loss 

 can occur over a narrow angular range through the production of a Stoneley wave (closely 

 related to the shear wave) along the sediment-rock interface. These dominant effects oc- 

 curred between the shear velocity critical grazing angle of about 50 degrees and the com- 

 pressional velocity grazing angle of about 70 degrees. 



17 



