PHYSICAL PROPERTIES

Two holes were drilled at Site 1248 and sediments from them were analyzed for physical properties. Hole 1248B consists of three cores, but only Cores 204-1248B-1H and 2H were analyzed (Core 3H was too short for physical property analysis). Hole 1248C was cored to a TD of 148.5 mbsf, but in the upper 50 mbsf, recovery was <25% and only five dispersed sections out of this interval were recovered and were available for physical property analyses. Standard IR imaging was performed on all sections recovered, and these images were used for identification of cold-spot anomalies associated with the presence of hydrate. Physical property measurements were performed on whole-round core sections, and discrete samples were taken for moisture and density (MAD) analyses from the working half of the split core sections (see "Physical Properties" in the "Explanatory Notes" chapter).

IR imaging of cores recovered at Site 1248 provided on-catwalk identification of hydrate zones in each core as described in "Physical Properties" in the "Site 1244" chapter and "Physical Properties" in the "Explanatory Notes" chapter). This information was used to facilitate hydrate sampling and preservation as applied to all cores from this site.

Most of the hydrate detectable by IR imaging at this site (50%) is present as disseminated layers. However, an unusually high percentage of hydrate appears as nodular textures in the IR images (26%). Veins or layers account for 25% of IR-detected hydrate. No thick zones of concentrated hydrate were detected. Poor recovery in the upper 48 mbsf of Holes 1248B and 1248C prevented direct observation of high concentrations of hydrate as estimated from LWD data (see "Downhole Logging"). However, the core that was recovered in the upper 48 mbsf exhibited extensive cold anomalies even in partially filled core liners, a result typical of disrupted or dissociated hydrate and consistent with poor recovery in a zone of abundant hydrate.

Typical hydrate thermal anomalies are shown in Figure F22, including examples of the nodular texture that is relatively abundant in Hole 1248B. The IR thermal anomalies from this site are cataloged in Table T10, and a qualitative interpretation of the hydrate texture is provided for each anomaly. An example of the size and geometry of hydrate nodules detectable by IR is illustrated in Figure F23. Two IR scans of anomaly IR117 (Table T10) taken ~19 min apart show a T of 5.5°C in the first scan and ~1°C in the second scan. The second scan has also developed a warm spot at the center of the anomaly that is interpreted to be a void developing where hydrate previously existed. The archive half of the split core clearly shows a small area of mousselike texture, ~2.5 cm in diameter, associated with the hydrate. The hydrate did not fully penetrate the core, and the disrupted zone (or IR anomaly) is consistent with a variety of shapes, including spherical, cylindrical, or blade shaped.

Circular-shaped anomalies in Hole 1248C suggest a spherical shape for these nodules; if they were commonly blade shaped, a larger number of oblate shapes would have been observed. Figure F23 also illustrates that a spherical nodule of hydrate 2 cm in diameter in the center of a core would probably exhibit the IR signature of disseminated hydrate because low-temperature hydrate would not be in direct contact with the core liner. Thus, thermal anomalies classified as "disseminated" may represent a range of hydrate textures, from a single nodule in the center of a core to distributed particles of millimeter-size hydrate grains to dispersed pore-filling hydrates present on the submillimeter to micrometer scale. Such anomalies are not veins or layers large enough to transect the core or nodules larger than a few centimeters because these features would show up as well-defined large T anomalies.

Successive thermal images were used to produce a downcore thermal log for each core recovered at Site 1248 (Fig. F24). The logs show the overall thermal structure of each core. The dominant features identified in the thermal logs are cold anomalies for the limited amount of core recovered in the upper 16 and 48 m in Holes 1248B and 1248C, respectively, and relatively abundant discrete cold anomalies from 48 to 124 mbsf in Hole 1248C. Cold anomalies are observed below the BSR for ~5 m. Minor cold anomalies below 124 mbsf are only present at the top of individual cores and are explained by the impact of drilling fluid in the deeper parts of the hole or perhaps by gas expansion or exsolution. This phenomenon is well documented by the IR temperature data and is most common for XCB cores (see "Physical Properties" in the "Site 1251" chapter). Results from Hole 1248C indicate that cold core tops can also be present with APC coring. The IR temperature anomalies are plotted as a function of depth in Figure F25. These results demonstrate that the distribution of hydrate is consistent with LWD pore water saturation (S_w) data (Fig. F25) (also see "Downhole Logging").

The presence of gas hydrate 5 m below the BSR may be within the combined uncertainty of the BSR depth and core depth. However, if we take the available depths at face value, the lower two IR anomalies fall below the GHSZ, as inferred from seismic (BSR depth estimated at 119 mbsf) and LWD data (estimate based on Archie saturation at 118 mbsf). They fall within the stability zone of hydrates of higher molecular weight hydrocarbons (see "Organic Geochemistry") and within the methane/seawater/hydrate stability zone predicted by in situ temperature measurement (129 mbsf) (see "Downhole Tools and Pressure Coring"). Considering that there may be a 4-m mismatch between the cored hole and the LWD hole (Fig. F26), it is premature to conclude that these two IR anomalies were present below the methane hydrate stability zone, but it is possible.

Poor core recovery resulted in only sparse physical property data from the upper 50 mbsf in Holes 1248B and 1248C. Bulk density values increase with depth from ~1.6 g/cm³ at the seafloor to ~1.75 g/cm³ at 150 mbsf (Table T11; Fig. F27). A notable feature at this site is Horizon A, which is identified from the seismic data at ~128 mbsf. Logging data indicate a large decrease in sediment density at this depth (Figs. F26, F27), which is inferred to be associated with sand/ash layers described in the core descriptions (see "Lithostratigraphic Subunit IIIA" in "Lithostratigraphic Units" in "Lithostratigraphy"). However, this interval was not fully recovered in the cores. The GRA density data do indicate a decrease in density around this interval, but there is a depth discrepancy of ~4 m for the top of Horizon A between the LWD and GRA density measurements. This discrepancy may be explained by uncertainties associated with the coring depth and the poor core recovery. The 4-m discrepancy may also be caused by structural offset between Holes 1248A and 1248C. The poor core recovery in the interval around Horizon A did not allow high-resolution sampling for MAD analyses, and the few data points available do not indicate a prominent change in the bulk density. There are two data points in this interval that show reduced grain densities, possibly associated with the ash layers. The interpretation of the low-density ash layer as the major cause of Horizon A is consistent with the reversed polarity nature of this seismic reflector, which is reinforced by the presence of free gas trapped in this layer.

The boundary between the lithostratigraphic Units I-II and III is defined at 39 mbsf (see "Lithostratigraphy"). Because of poor core recovery, few density data from the multisensor track (MST) or MAD analyses are available for correlation to the lithostratigraphic units.

The MS data below 50 mbsf do not show large variability. Data values range between 20 and 60 x 10^-7 SI, which are relatively low compared to other sites. Within lithostratigraphic Unit III (39-150 mbsf), the interval between 105 and 125 mbsf shows slightly higher variability in the MS record. These variations were correlated to the presence of sand layers (see "Lithostratigraphy").

The Non Contact Resistivity (NCR) system was fully implemented at this site as part of the MST measurements and worked well. In particular, it proved to be very stable (i.e., very little drift between sections). Despite the fact that the sensor system was working well, it only shows that the data are dominated by the effect of abundant cracks caused by gas expansion. This cracking results in apparently high and very variable resistivity values (Fig. F28), which although representative of the sediment in the core does not accurately represent the in situ lithologies. A close inspection of the two cores recovered in Hole 1248B at a depth above 10 mbsf (where gas expansion is less strong) does show detailed downcore variation of resistivity values ranging between 0.5 and 1.5 m, which reflect subtle but real lithologic variations. This demonstrates the potential use of the NCR system for fluid-saturated core sections.

Because of the extensive gas-expansion effect, compressional (P)- wave velocities (V_P) could not be measured with the MST; no measurements were possible with the Hamilton Frame velocimeter either.

Only a limited number of measurements were conducted to determine thermal conductivity values at this site because of the extensive gas expansion cracks (Table T12; Fig. F27). In general, the measured values are slightly higher than those reported from the other sites, with an average value of 0.99 W(m·K). As a result of the limited number of measurements, no correlation to other physical properties was performed.

No measurements with the hand-held Torvane were conducted at this site because of extensive gas expansion cracks.

Section 204-1248C-4X-1, which contained disseminated hydrate, was used to test whether the dissociation of hydrate is detectable by repeated measurements of physical properties using the MST sensors over a time period of several hours. Section 204-1248C-4X-1 was measured five times with the MST and was scanned in the X-ray line scanner (see "Operations" in the "Site 1249" chapter) four times during a time period of 160 min. GRA density and MS profiles remained stable with time. However, an analysis of the NCR data shows a response in conductivity over time (see Fig. F29). Part of this change could be associated with the increase in core temperature. However, a change of ~10°C, as observed during this experiment, has an effect on the resistivity value of only ~0.1 m. The resistivity values changed by ~0.5 m, on average, in this section over time. Assuming this effect is from the dissociation of small amounts of hydrate, then the best structural model is one where the hydrate is present as veins or veinlets. Disseminated small nodules would have a negligible effect on resistivity upon dissociation.

Poor core recovery in both Holes 1248B and 1248C yielded only a limited amount of useful physical property data from Site 1248. No correlation to the lithostratigraphic units was therefore possible. At this site, Horizon A was drilled at a depth of ~128 mbsf. The top of this horizon was recovered, and the MST data show a decrease in the sediment density, which agrees with what was seen in the LWD data. No MS anomaly could be correlated to this horizon. Thermal IR imaging provided a robust and rapid tool for hydrate identification on the catwalk. Post-catwalk processing of thermal anomaly data from the IR images shows a good correlation to the S_w derived from LWD resistivity logs.