by the outside clearance ratio, Cq, 



D - D, 

 C =-^ '- (3) 



where Dy^, is the maximum outside diameter of the core nose and D^- is the outside 

 diameter of the core barrel . The ratio of the volume of displaced sediment to the 

 volume of the sample is the area or Kerf ratio, Cq . 



D ^ - D ^ 



De 



Core shortening — Three different interpretations are shown graphically in 

 Figure 5 to relate the location of an increment of cored sample to its in-place loca- 

 tion: (I) The length of core recovered is equal to the same distance below the water- 

 sediment interface irrespective of how far the corer penetrated the bottom, an incre- 

 ment of core represents the same increment in place, and the gross recovery ratio is 

 100 percent. This interpretation has been assumed for piston submarine cores, as 

 previously mentioned. (II) A linear relationship exists between the increment of core 

 recovered and the distance penetrated, each Increment of core represents a less-thick 

 Increment in place, and the gross recovery ratio is always less than 100 percent. This 

 Interpretation has been demonstrated by Emery and Dietz (1941, p. 1706-1711) and 

 usually is favored for gravity corers (see, for example: Ericson and Wollln, 1956, 

 p. 107), (III) Little or no core shortening occurs down to a certain distance, about 

 40 to 75 cm (15 to 30 in) according to data presented by Hvorslev (1949, p. 105-119) 

 over which distance the gross recovery ratio Is about 100 percent; below this distance 

 the cumulative shortening is proportional to the additional distance penetrated, and 

 the gross recovery ratio is always less than 100 percent. This interpretation is favored 

 by Hamilton (1960, p. 377) for all gravity cores. A fourth interpretation Is a modifi- 

 cation of the third: Increments of core become successively smaller from top to bottom 

 compared to their thickness In place (Pratje, 1934; Piggot, 1941); a plot would fall 

 between 11 and 111 in Figure 5. 



Kallstenlus (1958, p. 9-10) reports that soft strata between rigid strata may be 

 squeezed out in piston samples without It being possible to detect the phenomenon; 

 his tests showed that 20 percent deformation (core shortening) in a compression test 

 produced distortion detectable with difficulty. The assumption that piston submarine 

 core recovery ratios are 100 percent. In consequence, is questionable. 



The amount of core shortening In gravity cores appears to be a function of corer 

 design, the better the engineering design the longer the core with 100 percent gross 

 recovery ratio. Richards and Keller (in press) have shown that In a large diameter 



13 



