*:Table T1 summarizes the permeability data for each sample, including the best estimate of permeability for each sample and the corresponding depths and porosities. Porosities were obtained from the shipboard data (Shipboard Scientific Party, 1999) and are not corrected for sediment rebound. The highest permeability at Site 1108, 1.0 × 10-16 m2, was noted in Sample 180-1108B-30R-6, 32-52 cm, a silty sand from a depth of 280 mbsf. Site 1108 samples from Bolton et al. (2000) at depths of 121 and 218 mbsf yielded permeabilities of ~5 × 10-18 m2. At Site 1115, permeability values from four samples ranged from 1.5 × 10-18 to 9.2 × 10-17 m2. Results from four samples from Site 1118 yielded permeability values ranging from 8.6 × 10-18 to 5.9 × 10-17 m2. No clear relationship between porosity and permeability is observed.
Permeabilities for the numerical model were based on results of the constant flow-rate tests (Fig. F2). For the section that was removed by erosion, a permeability of 1 × 10-16 m2 was used. Because the sample from 280 mbsf was from a permeable layer of silty sand, it was assumed not to be representative of the overall vertical permeability of that sediment packet. Numerical modeling was used to constrain the permeabilities of the undrilled section (485-900 mbsf) necessary to create overpressures. We could then assess whether these permeabilities were probable and, thus, whether overpressures resulting from sedimentation were likely.
Overpressures are
presented in terms of the excess pore pressure ratio, *:
*
= (Pf - Ph) / (Pl
- Ph), (11)
where
At Site 1108, slight
overpressures (*
> 0.1) initiate at k
= 3.4 × 10-19 m2 for the missing section. For significant
overpressures (*
> 0.7) to occur at the base of the sediment column, the permeability of the
missing sediment would have to be 2.0 × 10-19 m2 or less.
Results are not very sensitive to uncertainties in the sedimentation rate of the
undrilled section. With the sedimentation rate of the section from 485 to 900
mbsf doubled, slight overpressures (*
> 0.1) initiate at k
= 4.2 × 10-19 m2 for the missing section.
