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doi:10.2204/iodp.proc.314315316.136.2009

Physical properties

At Site C0008, physical property measurements were made to provide basic information characterizing lithologic units, states of consolidation, deformation, and strain and to correlate coring results with downhole logging data. After capturing X-ray CT images and letting the core reach thermal equilibrium with ambient temperature at ~20°C, gamma ray attenuation (GRA) density, magnetic susceptibility, natural gamma radiation, P-wave velocity, and noncontact electrical resistivity were measured using a multisensor core logger (MSCL) system on whole-round core sections (MSCL-W). Thermal conductivity was measured using either a full-space needle probe method or half-space line source method on split working halves. The half-space method was used on lithified sediments deeper in the hole that were impenetrable with the needle probe. Cores were split in two longitudinally, one half for archiving and one half for sampling and analysis. A photo image capture logger (MSCL-I) and a color spectrophotometer (MSCL-C) were used to collect images of the split surfaces of the archive halves. Moisture and density (MAD) were measured on discrete subsamples collected from the working halves as well as from "clusters" adjacent to whole-round samples removed before splitting. Vane shear and penetration experiments were performed on the working halves. Additional discrete subsamples from working halves were used to perform electrical conductivity measurements, P-wave velocity measurements, and anisotropy calculations.

Density and porosity

Values of bulk density at Site C0008 were determined from both MSCL GRA measurements on whole cores and MAD measurements on discrete samples from the working halves of split cores (see “Physical properties” in the “Expedition 316 methods” chapter). A total of 186 discrete samples were analyzed for MAD (172 from Hole C0008A and 14 from Hole C0008C).

MAD wet bulk density increases continuously with depth from ~1.60 g/cm³ just below the seafloor to 1.95 g/cm³ at ~272 m CSF (Fig. F34A). Maximum MSCL bulk density values also show similar trends. GRA bulk density is generally lower than MAD bulk density and reveals a larger degree of scatter compared to MAD bulk densities. The scatter in MAD bulk density is likely due to lithologic variations among the silty clay sediments at this site. Grain density values were also determined via MAD measurements on discrete samples and are consistent throughout the hole (Fig. F34B).

Porosity values were estimated from whole-round core MSCL scan data and calculated from MAD measurements on discrete samples. MAD porosity (see “Physical properties” in the “Expedition 316 methods” chapter) generally decreases with depth (Fig. F34C) and varies inversely with MAD bulk density. Minimum MSCL porosity corresponds well with MAD porosity (Fig. F34C).

As shown in Figure F34C, porosity decreases monotonically with depth from ~63% just below the seafloor to ~47% at 270 m. No significant deviations from the porosity versus depth trend are observed.

P-wave velocity and electrical conductivity in discrete samples

At Site C0008, P-wave velocity and electrical conductivity data in discrete samples were acquired from 225 to 265 m CSF (Subunit IB, cf. lithology) in Hole C0008A. The small amount of data acquired does not allow any description in terms of variation with depth. P-wave velocity values (Fig. F35A) average 1750 m/s, which is significantly lower than average values obtained at Sites C0006 and C0007. Electrical conductivity data (Fig. F35C) show high values, which could reflect relatively high porosity compared to that at other sites. Transverse electrical conductivity anisotropy (Fig. F35D) shows high values at ~250 m CSF with a very high conductivity parallel to the bedding surface.

Thermal conductivity

Thermal conductivity measurements were conducted on whole-round HPCS and ESCS cores. Thermal conductivity values associated with Hole C0008A vary between 0.7 and 1.2 W/(m·K) (Table T19; Fig. F36A) and modestly increase with depth but also show a great deal of scatter. Thermal conductivity values associated with Hole C0008C vary between 0.7 and 1.2 W/(m·K) (Table T20; Fig. F37A), decrease with depth, and also show significant scatter. Increased gas hydrate concentration was found in Hole C0008C, which may explain both the decrease in thermal conductivity with depth and the increased scatter because gas expansion cracks were prevalent throughout most of the core. Significant negative spikes at 140 m CSF in Hole C0008A and 90 m CSF in Hole C0008C may be associated with the occurrence of hydrates.

In situ temperature

In situ temperature was measured using the advanced piston coring temperature tool (APCT3) in Holes C0008A and C0008C. In Hole C0008A, APCT3 measurements were supplemented with the sediment temperature (SET) tool. All measurements were made in calm to moderate seas and were successful (Tables T21, T22). Temperature-time series for each temperature measurement in Holes C0008A and C0008C are shown in Figures F38 and F39. The temperature tool was stopped at the mudline for up to 10 min prior to each penetration. The average apparent bottom water temperature is 2.0° and 1.8°C in Holes C0008A and C0008C, respectively (Tables T21, T22). Determinations of bottom water temperature were much more consistent in Hole C0008C than in Hole C0008A. The variability in measured bottom water temperature is likely due to uncertainty in the core winch depth meter. Significant frictional heating occurred on all penetrations with the temperature versus time records exhibiting the characteristic probe penetration heating pulse and subsequent decay (Figs. F38, F39). Equilibrium temperature estimates are based on a 1/time approximation from the temperature-time series while the tool is in the bottom. The effective origin time of the thermal pulse associated with tool penetration was estimated by varying the assumed origin time until the thermal pulse decay followed the theoretical curve. A delay of 55–94 s from the initial penetration heating time gives a best fitting linear 1/time plot for both holes. In Hole C0008A, the temperature measurement at 119.0 m CSF shows multiple temperature spikes near the beginning of the penetration, and the tool may have double penetrated. This would put excess heat into the formation relative to theory and may explain the slightly high temperature relative to the best-fit gradient (Fig. F36B). Additionally, this equilibrium temperature is based on a shorter time series than the other measurements, further degrading the measurement. In Hole C0008A, the one SET tool measurement at 243.8 m CSF is consistent with APCT3 measurements. The best-fit thermal gradients are 51° and 57°C/km in Holes C0008A and C0008C, respectively (Figs. F36C, F37C).

Heat flow

If heat transfer is by conduction and heat flow is constant, the thermal gradient will be inversely proportional to thermal conductivity according to Fourier’s law. This relationship can be linearized by plotting temperature as a function of summed thermal resistance (Bullard, 1939) (see the “Expedition 316 methods” chapter). For Holes C0008A and C0008C, the least-squares fit to the temperature and thermal resistance data indicates a heat flow of 52 and 46 mW/m², respectively (Figs. F36C, F37C). In Hole C0008A, the estimated bottom water temperature is the same as the average of the mudline temperatures, whereas in Hole C0008C the estimated bottom water temperature is 0.2°C greater than the average of the mudline temperatures (Tables T21, T22). A constant conductive heat flow appears to describe the overall thermal structure for both holes quite well.

Shear strength

Shear strength measurements were conducted using a semiautomated miniature vane shear device and a pocket penetrometer. Measurements were made at discrete locations on the working halves of split cores at a frequency of approximately three measurements per core (generally in sections 2, 5, and 7). Tests were conducted on relatively intact and homogeneous regions of these sections, generally somewhere between 50 and 100 cm below the top of the section. To minimize error induced by disturbance of the core by the measurement technique, vane shear tests were conducted first, followed by penetrometer tests 1–2 cm from the site of the vane shear penetration. No vane shear measurements were conducted on whole cores. Both instruments were inserted orthogonally to the surface of the working halves. The rotation rate of the semiautomated vane shear apparatus on board the Chikyu is 71°/min. At Site C0008, shear strength was measured to 220 m CSF in Hole C0008A and 137 m CSF in Hole C0008C (Fig. F40).

Shear strength increases linearly overall in both holes along a trend of 0.45 kPa/m. In Hole C0008A, measured shear strength decreases past 180 m CSF, but this is inferred to be an effect of tensile cracking in the split cores rather than a real strength decrease at depth. In Hole C0008C, a gap in the core record below 60 m CSF precedes a zone of decreasing shear strength from 70 to 90 m CSF. Beyond this zone, shear strength increases at ~0.45 kPa/m to the bottom of the hole at 137 m CSF.

Color spectrometry

Results from the measurement of color reflectance are presented in Figures F41 and F42. Trends between Holes C0008A and C0008C are similar with the L* and b* values decreasing slightly with depth. The L* values range from ~20 to 50. The a* values range from –2 to 2, and the b* values range from 0 to 4. There are no significant anomalies.

Magnetic susceptibility

Volumetric magnetic susceptibilities were measured using the whole-round core MSCL in all recovered cores from Site C0008 (Fig. F43). Uncorrected magnetic susceptibility values are presented in this chapter. Magnetic susceptibility varies between ~0 and 600 × 10^–3 SI with a mean of ~137 × 10^–3 SI. Magnetic susceptibility values are low from the seafloor to ~150 m CSF where there is a positive excursion centered at ~175 m CSF. Values between 200 and 260 m CSF appear cyclic and may reflect interbedded conglomerates and silty clay.

Natural gamma ray

Natural gamma ray (NGR) results are reported in counts per second (cps). The background scatter, produced by Compton scattering, photoelectric absorption, and pair production, was measured at the beginning and subtracted from measured gamma ray values. In general, NGR decreases from the seafloor to ~235 m CSF at the Subunit IA/IB boundary (Fig. F44). At this boundary, NGR values are offset toward lower values and then rise relatively quickly to ~272 m CSF.

Integration with seismic data

There are no logging-while-drilling (LWD) data at this site and core-seismic integration between these holes is mainly based on prestack depth-migrated seismic profiles. The distance between Holes C0008B, C0008C, and C0008A is 215 m (Fig. F1). Seismic profile Inline 2675 from the CDEX 3-D seismic survey (Fig. F2) passes through Holes C0004B, C0008B, and C0008C and lies 10–30 m from Holes C0004C, C0004D, and C0008A, which are projected onto the seismic profile.

Generally, core recovery is good at Site C0008 and provides continuous NGR logs for correlations between holes. NGR data from core logs are superimposed on the seismic data (Fig. F45) The seismic data highlight some key reflections that match well with core data and image key sedimentary facies and faults.

In Hole C0008A between ~50 and 150 m CSF, interbedded mud and turbidite sand create high amplitude reflections that are continuous to Hole C0008C. The NGR logs show cyclic variations through this interval. Between 150 and 234 m CSF, continuous seismic reflections caused by thick sands and volcanic ashes provide references for correlation between these two holes. NGR logs show characteristics that can also be well correlated between these holes. However, depth shifts between the seismic reflections and NGR peaks are up to 15 m.

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