Analyses of sediment physical properties during Leg 204 yield useful insight to the nature of the near-surface sediment interval just beneath the seafloor. Using the ODP standard MST core logger with P-wave velocity, GRA density, and magnetic susceptibility together with the newly developed Geotek NCR sensor allowed unique opportunities to compare and correlate in detail different physical properties. It was also the first test of the NCR sensor during an ODP leg.
At Sites 1244, 1245, and 1246, a complete set of physical property data were acquired including MAD samples, MST data, and velocity and shear strength from split-core analyses. At several other sites (1247, 1248, and 1252) only subsets of physical property data could be acquired, limited by gas expansion cracks.
Resistivity data were used to estimate porosity following Archie's relation using the MAD samples as calibration points. The results from this analysis were in good agreement with Leg 146 Site 892 studies. The NCR-derived porosity values agree well with other independent porosity estimates, such as GRA density porosity. NCR data further suggest that the resistivity-porosity relation described by Archie's law is different in the near-seafloor sediment section (a = 1.35, m = 1.76) than within the deeper sediment column (a = 1.575, m = 1.73), as has also been observed on the northern Cascadia margin. Merging the two different depth ranges and estimating the best-fit Archie parameters for all data (a = 1.37, m = 1.93) artificially increases the porosity estimates for the near-seafloor sediments. The difference in the estimated Archie parameters has no implications on gas hydrate concentration estimates because the upper 10–20 mbsf did not contain any gas hydrate at the sites considered in this study.
NCR porosity appears well correlated with P-wave velocity, but shows a different relation than previously published (e.g., Hyndman et al., 1993; Jarrad et al., 1995). We observed no strong correlation between porosity and shear strength. Overall, P-wave velocity appears to uniformly increase with depth at all three sites, ranging from ~1480 m/s at the seafloor to ~1550 m/s at 8–10 mbsf. Shear strength also increases with depth at all sites, from ~20 kPa near the seafloor to 60–80 kPa at 8–10 mbsf, and thus a clear linear correlation is observed between shear strength and P-wave velocity.
Downcore seismic impedance and reflection coefficients were calculated at Sites 1244, 1245, and 1246 and can be used to calibrate seismic reflection surveys. However, the frequency content of the seismic data has to be considered in the calibration to account for the depth variation of seismic impedance over the upper 10 mbsf.
From all the analyses it can be concluded that the sediments at the slope basin (Sites 1251 and 1252) have much higher porosity (lower bulk densities) in the upper 10 mbsf than those close to or at the summit of southern Hydrate Ridge associated with low AOC. This may reflect a lower compaction state combined with higher sedimentation rates for the slope basin sites. The other sites located at the flank or on top of Hydrate Ridge may not receive as much sediment (i.e., they are detached from recent Holocene turbidite sedimentation). The observed high state of AOC may also be interpreted as a result of erosion, which may also be indicated by the relatively older age of the near-seafloor sediments at Sites 1244, 1245, and 1246 compared to basin Sites 1251 and 1252.