PHYSICAL PROPERTIES

At Site 1246, we used Hole 1246B for physical property analyses. This site is close to the previously cored Site 1244 and allows a lateral comparison of their lithologic characteristics, especially regarding Horizon B.

Track-mounted IR imaging of the core liner on the catwalk was used to determine the thermal state of the cored sections, whereas the handheld IR camera was used for rapid detection of cold anomalies. Four hydrate samples were selected based on IR images and subsequently sampled at this site.

Standard measurements for physical properties were carried out (see "Physical Properties" in the "Explanatory Notes" chapter). In addition, the automated shear vane and the three different velocity probes mounted on the Hamilton Frame were used in the upper two cores to determine shear strength and velocity anisotropy (PWS1, PWS2, and PWS3), respectively.

IR imaging of the cores drilled in Hole 1246B provided identification of hydrate zones in each core on the catwalk, as described in "Infrared Scanner" in "Physical Properties" in the "Site 1244" chapter and in "Infrared Thermal Imaging" in "Physical Properties" in the "Explanatory Notes" chapter. This information was used to facilitate hydrate sampling and preservation for all cores. The IR thermal anomalies are cataloged in Table T9, including an interpretation of the overall hydrate texture for each anomaly. The majority of the hydrate detectable by IR imaging at this site (76%) is present as disseminated layers. Veins, parallel to or crosscutting bedding, account for 16% of IR-detected hydrate. In contrast to Hole 1248B, nodular textures account for only 8% of hydrate detected by IR.

Successive thermal images were used to produce a downcore thermal log for each core recovered in Hole 1246B (Fig. F17). The logs show the overall thermal structure of each core. Strong cold anomalies are present from 66 to 109 mbsf, which correspond to the locations of hydrate samples. The temperature anomalies created by the hydrate were extracted by examination of the downcore temperature data and by direct examination of IR images. Figure F18 shows the magnitude of the temperature anomalies as function of depth. Subtle cold anomalies are first present at 16 mbsf, and distinct anomalies start at ~33 mbsf. The magnitude of T increases toward the BSR, and minor cold anomalies extend below the BSR by ~3 m. Below 117 mbsf, temperature anomalies are probably a result of causes other than hydrates (e.g., contact with cold drilling fluid during extended core barrel coring or gas expansion or exsolution). The IR thermal anomalies that are attributed to hydrates are consistent with pore water saturation (S_w) from LWD (see Fig. F18), except from 15 to 45 and 106 to 118 mbsf, where the current interpretation of LWD logging results shows no hydrate, whereas IR results suggest the presence of relatively small amounts of hydrate.

The extent of cold thermal anomalies to a depth of 117 mbsf, which is 3 m below the BSR depth of 114 mbsf is probably within the combined uncertainty of the estimated BSR depth and the curated core depth.

The preponderance of both disseminated and stratigraphically conformable veins of hydrate in Hole 1246B suggests that differences in permeability and porosity related to bedding may control the presence of hydrate at Site 1246. Stratigraphic control of the presence of hydrate in the overall context of Site 1246 is discussed below.

The presence of concentrated hydrate is often associated with intensive gas expansion cracks and voids, as illustrated in Figure F19 for Section 204-1246B-8H-4. These voids and cracks have a much higher IR temperature than the surrounding hydrate-bearing sediment. Thus, the overall cold-spot anomaly is often broken up into small intervals interrupted by the relatively higher temperature voids. By removing the voids and artificially compressing the hydrate-bearing intervals, gas hydrate in Section 204-1246B-8H-4 would yield a cold-spot thermal anomaly with a total length of 55 cm.

Sediment density values were measured with the multisensor track (MST) gamma ray attenuation (GRA) sensor (GRA density) and were also derived from discrete moisture and density (MAD) samples (bulk density) (Table T10). Both curves are in very good agreement and correlate to the LWD density values (Fig. F20), except that the correlated LWD data are slightly deeper by ~2-3 m. Sediment density values generally increase with depth from around 1.6 g/cm³ at the seafloor to ~1.8 g/cm³ at 140 mbsf. Two distinct zones of increased densities are identified at 55 and 65 mbsf. These zones correspond to intervals of high MS (see below). MAD indicate that these zones are also characterized by very low porosity values at ~45%-50% compared to the intervals above and below, which show porosities in the range of 55%-65%. The grain density does not show much variation with depth, with most of the values falling in the range between 2.6 and 2.8 g/cm³.

The most prominent features in the MS record are the two strong events at 55 and 65 mbsf (Fig. F20). These two MS anomalies can be correlated to the seismic Horizon B (Fig. F21). Close inspection of the sedimentological record indicates that the MS anomalies correlate to large turbidite events. Those turbidite sequences are dominated by quartz-rich layers (see "Lithostratigraphic Unit II" in "Lithostratigraphic Units" in "Lithostratigraphy"). The MS signals of both events are sharply truncated at the top and bottom, and they extend over a depth range of ~2.5 m each (Fig. F22). MAD data in this interval indicate an increase in density of ~0.2 g/cm³ and a decrease in porosity of ~15%. Grain densities remain near constant across this interval, however. The turbidite records are also closely related to the presence of hydrate.

Close inspection of the IR images showed that the base of each of the MS anomalies associated with Horizon B correspond to a cold-spot anomaly, which is an indication of the presence of hydrate (Fig. F22). The base of the MS anomaly is an interval where the more coarse-grained material is deposited. This is in general agreement with the hypothesis that gas hydrates form preferably in coarser-grained material (e.g., Lee and Dillon, 2001; Dallimore et al., 1999).

There is a slight mismatch in aligning the sedimentological record with the measurements on the MST and the IR images. It should be noted that the IR images were taken on the catwalk before the sediment cores were curated. A typical procedure after IR imaging is to close gas-expansion voids by compressing the sediments in the liner with a handheld piston. This results in fewer but larger voids, which fall at section boundaries. When core sections are split, the relative position of voids can shift, causing offsets between MST and the visual core descriptions. This explains the mismatch between MST measurements made on the whole-round core and the sedimentological record determined on the archive half of the split cores.

The Non Contact Resistivity (NCR) system is very sensitive to cracks and voids induced by gas expansion of the cores. The voids act like insulators and result in high resistivity values dominating the downcore trend (Fig. F23). The lower limit of the resistivity values can be used to estimate sediment resistivity; however, data values scatter significantly as a result of the effects of gas expansion. Within the upper 10 mbsf, where the gas expansion is less dominant, good correlation between the noncontact resistivity (NCR), compressional (P-) wave velocity (V_P), and gamma density is observed (Fig. F24).

A detailed investigation of V_P was carried out in Cores 204-1246B-1H and 2H, which did not suffer from strong gas-expansion cracks (Table T11; Fig. F24). Velocities were measured using the MST on the whole-round cores and also using the three different velocity sensors of the Hamilton Frame on the split cores. Measurements were only reliable to a depth of ~4.5 mbsf for the MST, but we were able to measure velocities to a depth of 10 mbsf from the split-core sections. Note that the reported velocity values are uncorrected for in situ temperature. Cores were measured after a temperature equilibration time of >4 hr.

In general, V_P measured from the MST are slightly lower than those measured on the split cores. On average, the difference is ~30-40 m/s. This could be due to either the effect of a systematic shift of the Hamilton Frame sensors (inaccurate calibration) or drying of the split core over time. It was noted that the cores lost considerable amounts of moisture during storage on the core racks before splitting. The loss of water may explain the slightly higher velocity values measured on the split cores. The MST velocity record shows some high-resolution variability with intervals of increased values at ~1 and 3.5 mbsf. These intervals correspond to zones of higher density as seen in the GRA record (Fig. F24) and slightly higher resistivity (Fig. F24).

The three V_P sensors on the Hamilton Frame measure the velocity of split cores in the x-, y-, and z-directions, respectively, and these measurements can be used to analyze the presence of any velocity anisotropy. It was observed that the PWS1 velocity (z-direction) (i.e., sound waves travel perpendicular to the bedding plane) was consistently slower than the other two velocities. Velocities in the x-direction were always higher by ~20 m/s. This can be an effect of velocity anisotropy caused by bedding. Within the upper 10 m, bedding is almost parallel to the seafloor. Therefore, the velocity measured across the sedimentation direction (z-component) is lower than that along the bedding (x-direction). The velocity along the y-direction was observed to be close to that in the x-direction, which is an expected trend; the two values begin to differ at depths below 6 mbsf. This could be the effect of developing gas-expansion cracks, which can also be seen in the NCR record, as the resistivity values begin to show significant scattering (Fig. F24B).

Thermal conductivity was routinely measured on the temperature-equilibrated whole-round core sections. Measured values vary between 0.81 and 1.07 W/(m·K) with a mean of 0.965 W/(m·K) (Fig. F20; Table T12). Thermal conductivity decreases from the seafloor to ~35 mbsf and is slightly higher in the interval from 40 to 80 mbsf. Below 80 mbsf, thermal conductivity increases almost linearly with depth.

Shear strength was measured in cores from within the upper 40 mbsf of Hole 1246B (Table T13; Fig. F20). We used the handheld Torvane as well as the automated shear vane. The two different systems are generally in very good agreement. Shear strength increases with depth from values of ~30 kPa at shallow depth to ~80 kPa at 40 mbsf. Extensive gas expansion of the cores resulted in inaccurate values (3.0-5.8 kPa) at greater depth.

Physical properties measured at Site 1246 match well with the major lithostratigraphic units, especially Horizon B. Horizon B can be traced over a length of several kilometers on 3-D seismic profiles and was cored at Sites 1246 and 1244 (see Fig. F5 in the "Leg 204 Summary" chapter). It is mainly characterized by the presence of large turbidite events that are associated with high MS values. Sediment density increases within these sequences with values ~ 0.2 g/cm³ larger than the over- and underlying sediments. The horizon consists of a minimum of two large turbidite events, each ~2.5 m thick, with a clear fining-upward sequence. The bottom of each sequence is associated with the presence of gas hydrate.