262 



Sediments 



10"'" 10" 



PERMEABILITY IN DARCYJ 

 10"'' 10"* 10"^ 



"1 \ \ 



I0"2 



COMPUTED 

 FROM ANNUAL 

 LOSS OF WATER 



LABORATORY 

 MEASUREMENTS 



Figure 212. Measured perme- 

 ability of Core 1941 from 

 Santa Cruz Basin (right) as 

 compared with "permeability" 

 computed from decrease of 

 water content with depth in 

 typical core (Fig. 213). From 

 Emery and Rittenberg (1952, 

 Fig. 23). 



fied for the basins containing sediments that 

 are very uniform with depth, except for oc- 

 casional abnormally coarse layers deposited 

 by turbidity currents. Santa Barbara Basin 

 is one of the best examples. The typical de- 

 crease in water content with depth of burial 

 in basin sediments (Fig. 213B and C) must 

 mean that each annual layer becomes more 

 deeply buried it is progressively thinned; 

 the same volume of sediment grains is pres- 

 ent in a given area of each layer, but the 

 volume of water in each layer decreases with 

 depth. According to Table 20, normal sedi- 

 ment in Santa Barbara Basin is deposited at 

 a rate of 85 mg/sq cm/yr. At a water con- 

 tent of 67 per cent by wet weight (200 per 

 cent by dry weight, or 82.5 per cent porosity), 

 a year's deposit has an original thickness of 

 0.200 cm. At depth the layer becomes 

 thinned in accordance with the decrease of 

 porosity at depth, so that at a depth of 500 

 cm the thickness is 0.200 x (100 - 82.5)/ 

 (100 - 73.0) = 0.130 cm thick (Fig. 213D). 

 Knowing the present thickness of annual 

 layers and their porosity, we can easily com- 

 pute the volume of water in layers at any 

 depth; it decreases markedly — from 0.168 ml 

 at the surface to 0.095 ml at a depth of 500 

 cm (Fig. 213E). Obviously, this decrease 

 means that water has been lost upward, as 



this is the shortest escape route, but the dif- 

 ference between 0.168 and 0.195 ml repre- 

 sents only a net decrease of water content 

 because, while the annual layer now at 

 500-cm depth was losing its original water 

 upward by compaction, other still deeper 

 layers were also losing some of their water 

 upward to overlying layers. The annual loss 

 for a layer at any depth is essentially the 

 slope of Figure 213E at that depth, and it is 

 expressed in Figure 213F. Clearly, the 

 greatest annual loss should occur in that part 

 of the core having the greatest decrease in 

 water content with depth. 



As shown by Emery and Rittenberg 

 (1952), if permeabiHty hmits compaction we 

 may compute the permeability for annual 

 layers at any depth using the foregoing data, 

 where the quantity of water annually dis- 

 charged Q is given by Figure 213F, length 

 of column L is given by Figure 213D, vis- 

 cosity ju for the temperature and salinity is 

 1.61 centipoise, time T is the number of sec- 

 onds in a year, area ^4 is 1 sq cm, and the 

 pressure difference H is the weight of grains 

 in a layer corrected for their buoyancy in 

 water and expressed in atmospheres — 

 5.8 X 10"'. The permeability computed in 

 this way varies between 5 X 10"^° and 

 500 X 10-^° (Figs. 212, 213G). Since this 



