Within homogeneous sea-floor sediments there are 

 at least four important factors resulting in sound-velocity 

 gradients. The effects of temperature and hydrostatic 

 pressure in the water have been noted above. Two others 

 are the decrease in sediment porosity resulting from the 

 intergranular pressures of the sediment mineral grains 

 (overburden pressure) and those components of sediment 

 strength resulting in rigidity of the sediment structure 

 (increasing cohesion, cementation, etc. ). The increase of 

 sound speed with decrease of porosity or pore space in 

 marine sediments is mainly due to the large effect of water 

 compressibility; it has been the subject of numerous 

 papers 33 ' 34 ' 39-43 and will not be reviewed here. 



When a sediment sample is removed from a bore hole 

 to the laboratory, it undergoes a small increase in porosity 

 and decrease in cohesion, as a result of expansion caused 

 by reduction in intergranular pressure. The sample never 

 expands enough to return to the porosity or cohesion it had 

 when it was at the sediment surface prior to burial and 

 imposition of intergranular pressures. Therefore, when 

 correcting laboratory measurements of sound speed to in 

 situ conditions at depth in the sediment, there is an in- 

 crement for decrease in porosity (because of overburden 

 pressure) which can be estimated with probable accuracy, 

 and an increment owing to increased cohesion, which at the 

 present time cannot be estimated with accuracy. These 

 porosity corrections are best estimated from soil-mechanics 

 consolidation tests; such tests were made on the Mohole 

 sediments. 9 



The laboratory measurements of the velocity of sound 

 in the Mohole samples have been corrected to in situ 

 conditions in the sediment body using the sum of three 

 corrections discussed above: temperature, hydrostatic 

 pressure in the pore water, and porosity-pressure effects. 

 These corrections do not include any factor of increased 

 sound speed owing to increased cohesion (in this case 

 probably small), and are thus minimum corrections. The 

 resulting in situ sound speeds are listed in table 8 and 

 plotted against depth in the sediment in figure 4. 



31 



