Edited and depth-corrected acoustic, core porosity, and bulk density data were obtained from T. Collett (pers. comm., 1998). The quality of the log measurements was significantly degraded at Sites 994 and 997, and moderately degraded at Site 995 by the size and rugosity of the borehole (Shipboard Scientific Party, 1996a, 1996b, 1996c).
Porosity of sediments primarily controls the acoustic transit time of the sediment. Thus, accurate porosity data are very important in estimating the amounts of hydrates when using acoustic logs. Unfortunately, the downhole logging measurements for porosity were significantly degraded because of the hole conditions (Shipboard Scientific Party, 1996a, 1996b, and 1996c). The porosities computed from density logs are displayed in Figure 1, Figure 2, and Figure 3 and their statistics are given in Table 1. Figure 1A, Figure 2A, and Figure 3A show the three porosity distributions, Figure 1B, Figure 2B, and Figure 3B show the acoustic velocity, and Figure 1C, Figure 2C, and Figure 3C show the estimated hydrate concentration using core porosities. As can be seen from these figures, the porosities derived from downhole measurements show significant scattering. Average porosities for the hydrated intervals derived from density logs are 61.0% ± 5.1%, 62.2% ± 6.2%, and 68.7% ± 9.1% for Sites 994, 995, and 997, respectively (Table 1).
On Leg 164, water content, wet bulk density, dry bulk density, and grain density were measured from recovered sediment cores. From these physical properties data, core porosities were derived and are shown as open ovals in Figures 1-3. Core porosities are much lower than the porosities computed from bulk density. Also in Figure 1, Figure 2, and Figure 3, linear porosity-depth trend estimated from the core porosities are shown. These linear porosity trends were also used in estimating the hydrate content.
Interval velocities computed from acoustic logs are shown in Figure 1, Figure 2B, and Figure 3B, and their statistics are shown in Table 2. At all three sites, the interval velocity is greater than 1.5 km/s and increases as the sub-bottom depth increases (toward BSR) and reaches about 2 km/s at Site 997 near the BSR. Average acoustic velocities are 1.7 ± 0.07 km/s, 1.72 ± 0.07 km/s, and 1.68 ± 0.09 km/s for Sites 994, 995, and 997, respectively. The average compressional velocity from the acoustic log at Site 994 is within ±0.01 km/s of the velocity derived from VSP data. The sampling interval and sample location of the core porosity are different from those of the downhole log measurement; therefore, the core porosities were linearly interpolated between measured porosities to match the sampling of the downhole log.