The amounts of hydrate concentrated in the sediment pore spaces were estimated using Equation 1 with parameters shown in Table 2 and the results are summarized in Figure 1, Figure 2, and Figure 3 and Table 1. The hydrate concentration in pore space generally increases as depth increases at all three sites and is about 10%-15% near the BSR depths at the base of the hydrate zone. The average of gas hydrate concentration in the pore space varies between 12.1% and 3.8% at Site 997 depending upon the porosity data used. Table 1 indicates that the estimation based on the linear approximation of porosity trend is slightly higher than the estimation calculated from the core porosity, but they are very similar. The estimation based on the core porosities agrees with the estimation based on the chloride anomalies and resistivity logs (Shipboard Scientific Party, 1996a, 1996b, and 1996c). The average hydrate concentration in the pore space at all three sites, based on the core porosity, is 4.4%, and the average porosity is 57.5%. Therefore, about 2.5% of the sedimentary section (2.5 vol%) is occupied by hydrates.
The choice of weight W = 1.1 is based on the velocity and porosity samples acquired in this region (Lee et al., 1996) and the samples of Leg 164 above the hydrate-bearing interval. If a smaller weight such as W = 1 is used in the computation, then the amounts of hydrate are less than those shown in Table 1. For example, W = 1 yields a mean hydrate concentration of 7.19%-5.44% at Site 955 when using porosities computed from the bulk density, which is a 26% decrease in the hydrate concentration.
As indicated in Equation 1, if a higher exponent n is used, the hydrate concentrations become less than those shown in Table 1. As far as this exponent factor is concerned, there is no information to confirm the validity of n = 1 used in this estimation. If shear velocities are available, the exponent may be better constrained (Lee et al., 1996).