PHYSICAL PROPERTIES

In this section, we describe the downhole distribution of physical property data collected from Site 1244. Three holes were drilled at this site. Hole 1244A contained only one core and missed the mudline. Hole 1244B was drilled to a depth of 62 mbsf. Hole 1244C, which was drilled to a depth of 330 mbsf, contains a total of 39 cores. Several pressure cores were taken with the PCS (Cores 204-1244C-14P, 16P, and 18P) in Hole 1244C. No continuous measurements with the multisensor track (MST) and thermal conductivity were possible on these PCS cores. However, subsampling for moisture and density (MAD) was conducted. This site was revisited on 19 August 2002, and Holes 1244E and 1244F were cored. Physical property measurements were acquired from Hole 1244E to a total depth of 140 mbsf, and complete IR scans of all cores recovered were acquired.

Routine physical property measurements were collected on whole-round core sections at this site (see "Physical Properties" in the "Explanatory Notes" chapter). A scan of IR emission along the entire core-liner surface was recorded for most cores at this site prior to sectioning and sampling on the catwalk.

Samples were taken from split cores to measure sediment MAD properties. We also measured compressional (P)-wave velocities (V_P) on the split cores using the standard ODP Hamilton Frame device, and occasionally, we used the handheld Torvane to measure shear strength on selected core sections.

The sampling strategy at this site was designed to address specific leg-related research objectives as well as to provide a first test for all systems (Holes 1244A-1244C). Sampling rates for MAD measurements were much higher at this site (minimum of one sample per section) compared to the other sites. The objective of this strategy was to obtain a high-resolution data set of bulk density and related properties (porosity, grain density, and moisture content) to compare to the LWD measurements. The MAD data provide good ground truth and a reliable comparison to the in situ measurements of the logging tools.

Measurement spacing, count times, and data acquisition schemes (DAQ) used for the MST were the following: MS: spacing = 2.5 cm, count time = 3 x 1 s; gamma ray attenuation (GRA): spacing = 2.5 cm, count time = 5 s; and PWL: spacing = 2.5 cm, DAQ = 50.

In Hole 1244A and partially in Hole 1244B (up to a depth of ~35 mbsf), we also began testing the new Geotek Non Contact Resistivity (NCR) system. The new ODP software and electronics interface was not functional at this stage, and hence, there is no usable data at this site. There was a problem with acquiring MS data while the NCR was running, but this was resolved by turning off the NCR system below 35 mbsf.

Most of the physical property data show breaks or variations in downhole trends that match well with the data acquired by LWD and partially agree with the lithostratigraphic boundaries defined by the sedimentologists (see "Lithostratigraphy").

IR imaging of cores drilled at Site 1244 provided on-catwalk identification of hydrate zones in each core (see "Physical Properties" in the "Explanatory Notes" chapter). Dissociation of hydrate is an endothermic reaction that produces decreased temperature in intervals of the core containing hydrate. The butyrate core liners are opaque in the IR range detected by the camera used (8-12 µm). However, the cooled zones associated with hydrates are transmitted through the core liner by thermal conduction, creating an image of the core temperature on the surface of the core liner, which is then detected by the IR-imaging camera.

Each thermal image covered ~20 cm of core. The spatial resolution of the thermal images is lower than a direct image of the core itself but, nonetheless, provides previously unavailable information on the overall shape and character of hydrate occurrences. This information was used to facilitate hydrate sampling and preservation starting at Core 204-1244C-8H.

The shapes of thermal anomalies in the images were compared to the actual hydrate samples observed and photographed after they were taken from the core liners (Fig. F26). The thermal images provide a distinction between nodular, vein-filling, and layered hydrate, if hydrate abundance is relatively low, as is the case at this site. In addition, disseminated hydrate is detectable on the IR images as thermal anomalies with a T of ~1°C or less. The IR thermal anomalies are cataloged in Table T2, including an interpretation of the overall hydrate texture.

Successive thermal images were also used to produce a downcore thermal log for each core recovered at Site 1244 (Fig. F27). The logs show the overall thermal structure of each core and include both positive thermal anomalies associated with voids and the tendency of cores to be warmer at the bottom than at the top. This tendency becomes stronger with depth and is especially true for XCB cores. The warmer base of each core may result from the shorter time since exposure to the frictional heating of the bit as well as local frictional heating associated with removal of the cutting shoe and core catcher from the bottom of the core barrel. In addition, the warmer temperatures at the base of the core could reflect thermal transfer associated with gas expansion. The extreme positive thermal anomalies are artifacts associated with the partial absence of core in the last image of the core sequence. Also, note that the XCB-cored interval is cooler than the APC-cored interval at this site, which is presumably caused by the poorer quality of the XCB core and greater contact with cool drilling fluid during core recovery.

A more detailed plot of the zone of negative thermal anomalies in Holes 1244C and 1244E (Fig. F28A) shows the distribution of IR temperature anomalies as a function of depth. The larger anomalies are present between 47 and 85 mbsf, indicating that this is the zone containing the greatest abundance of hydrate at this site. Figure F28B also shows the LWD resistivity log for Hole 1244D, which has been interpreted using Archie's Relation to predict the abundance of hydrate (see "Downhole Logging"). Recognizing that Hole 1244D is 15 m away from Hole 1244C, the plot shows that observed hydrate corresponds reasonably well to the resistivity estimate. The poorest matches are from ~15 to 40 mbsf, from ~100 to 115 mbsf, and below ~125 mbsf. Lack of agreement between the two methods is likely to reflect a combination of disseminated hydrate not detected by IR imaging, uncertainty of hydrate resisitivity logs, heterogeneity in hydrate occurrence between Holes 1244A and 1244D, and the response of the resisitivity log to the presence of gas below the BSR.

The absence of negative thermal anomalies below 126 mbsf is consistent with a BSR depth of 125 mbsf derived from seismic data. PCS and XCB drilling were conducted between 141 mbsf and the bottom of the hole (BOH). At present, it is not known if the use of the XCB reduces the sensitivity of the IR image, but it seems likely given the greater temperature reduction in XCB cores in Figure F27. PCS cores do not permit collection of IR data.

GRA densities were measured by the GRA instrument on the MST, and bulk density and grain density were calculated from the MAD data (Table T15; Fig. F29). These density values and inferred porosities are compared to the LWD data (Fig. F30).

In general, all three data sets are in good agreement. However, there are distinct discrepancies between the GRA measurements and the MAD/LWD data. The GRA measurements are most affected by gas expansion effects and voids in the core liner ("Physical Properties" in the "Explanatory Notes" chapter). This data set shows the largest scatter (accurately reflecting the core material in the liner) and consistently produces much lower density values than the two other techniques, especially at greater depth. However, the measurements for Holes 1244A and 1244B show that for depths shallower than 50 mbsf and especially for the upper 10 mbsf before gas expansion occurs, the MAD and GRA measurements are a perfect match (Fig. F31). At those shallow depths, the LWD did not produce reliable results because of poor borehole conditions. The LWD and MAD data do not always show a perfect match. For example, the two data sets show similar density to 175 mbsf, below which the MAD data are consistently lower than LWD data, to 245 mbsf. The two data sets are, again, in good agreement below 245 mbsf. We have no explanation for these differences. They do not coincide with a change in coring technique from APC to XCB at 145 mbsf or any obvious lithologic boundaries.

Overall, the density increases slightly with depth, starting with values at ~1.6 g/cm³ near the seafloor and reaching maximum values of ~1.85 g/cm³ at a depth of ~155 mbsf. This general increase is an expected effect of compaction of soft marine clayey-silty sediments. Just above the boundary separating lithostratigraphic Units I and II, a low-density layer is present, which is best resolved with the high-resolution LWD data.

Between 155 and 180 mbsf, a unit characterized by high density values (~1.9 g/cm³) and low porosity values of ~50% is found. This unit has a relatively sharp top and smooth base. Below 175 mbsf, density values start to decrease downhole to a value of ~1.85 g/cm³. Density then remains almost constant to a depth of 235 mbsf. A second high-density, low-porosity unit, with similar density and porosity values as the shallower unit, is present from 235 to 245 mbsf. This sequence is sharply truncated at the bottom, where density values drop to ~1.72 g/cm³. This drop in density is associated with the boundary between slope-basin sediments and the deeper accretionary wedge complex (see "Lithostratigraphy"). The density increases within lithostratigraphic Unit III from 1.72 (245 mbsf) to ~1.8 g/cm³ (~285 mbsf). Below 285 mbsf, there are large fluctuations in density, with values varying from ~1.4 to ~2.1 g/cm³. There is partial agreement between the discrete MAD, LWD, and GRA density measurements within this unit, especially at 300 mbsf, where all three independent techniques gave very high density values of ~1.9 g/cm³. This suggests that the fluctuations may not be an artifact of poor borehole condition (in case of the LWD), gas expansion, or drilling slurry (in case of the XCB coring). However, the origin of these fluctuations remains enigmatic.

The discrete measurements of MAD were used to calculate grain densities and porosities. The inferred grain density values are rather uniform throughout the hole (values of 2.7 ± 0.2 g/cm³), with the largest deviations occurring at depths >210 mbsf. Since the grain density lacks any significant variation, porosity is essentially proportional to bulk density.

MS was not measured in Hole 1244A or in the uppermost 35 m of Hole 1244B. Multiple measurements between 35 and 53 mbsf in Holes 1244B and 1244C show the same data trend (Fig. F29). However, minor deviations occur as a result of difference in hole locations as well as difference in gas-expansion cracks and voids.

Since the sediment sequence of Hole 1244C provides the most complete MS profile, we describe the MS characteristics based on results of this hole. Overall, three MS units can be identified (Fig. F29). MS Unit I has relatively uniform MS values of ~40 x 10^-7 (SI). This unit extends from the seafloor down to 160 mbsf. MS Unit II is characterized by large variations of up to 380 x 10^-7 (SI). This unit is truncated sharply at 233 mbsf and is followed by MS Unit III, which again has relatively uniform susceptibility of ~40 x 10^-7 (SI).

V_P was measured with the Hamilton Frame (PWS3) on split cores only in the upper 12 m of Holes 1244A-1244C. In deeper parts of the holes, velocity measurements were strongly affected by gas expansion in the cores and by the XCB coring technique, which disturbed the entire core.

The V_P measured with the PWS3 device vary between 1530 and 1600 m/s (Table T16). These values are, however, ~50 m/s higher than the V_P measured with the MST (Fig. F32). It was subsequently discovered that the PWS3 measurement error of 50 m/s was caused by a worn displacement transducer.

In Hole 1244E, a new set of velocity measurements were carried out to investigate the structure in the upper 10 mbsf (Fig. F33). The measurements in Hole 1244E show an almost linear increase with depth. This increase is consistent with the trend observed in the GRA density, shear strength, and electrical resistivity, which is related to the early compaction of the sediments.

Thermal conductivity measurements were made routinely after the cores were equilibrated to ambient room temperatures. The general procedure was to measure thermal conductivity once per core in Section 3. On cores adjacent to downhole temperature measurements, thermal conductivity was measured at Sections 1, 3, and 5 of the core.

Measured values vary between 0.64 and 1.12 W/(m·K), with an average value of 0.94 W/(m·K) (Table T17; Fig. F29). A small increase in thermal conductivity was observed downhole to a depth of 240 mbsf. However, data are scattered, probably as a result of gas expansion. No correlation between bulk density and thermal conductivity was therefore observed. Thermal conductivity apparently decreased at depths below 240 mbsf. However, we believe this to be mainly the effect of gas expansion and the XCB coring technique, which introduces cracks and lower-density drilling slurry.

Vane shear strength measurements were not routinely conducted because of the disturbed nature of the sediments. Occasionally, a handheld Torvane was used to measure shear strength (see "Physical Properties" in the "Explanatory Notes" chapter). Measurements were made on the working half of the split-core sections at locations where the sediment appeared undisturbed. Intervals within the core sections with abundant cracks and voids resulting from gas expansion were avoided. Measured values vary between 23 and 160 kPa (Table T18; Fig. F29). Within the upper 10 mbsf, there is good correlation between shear strength and V_P , bulk density, and electrical resistivity. All parameters indicate compaction of the sediment. However, no further correlation to other physical properties or lithostratigraphic units at greater depth was attempted because of the sparse and scattered data set.

At this site, we implemented the use of an IR camera as a tool to identify the presence of gas hydrate in the cores. These data provided a new means of estimating the spatial distribution of hydrate in cores. Further analyses should result in quantification of hydrate concentrations.

The physical properties measured at Site 1244 are generally in good agreement with the other data acquired at this site. The high-resolution sampling procedure for discrete MAD analyses provides ground truth for the LWD experiment. Density measurements from the MST, the MAD analyses, and the LWD data agree very well and show similar trends; however, there are discrepancies between MST and MAD/LWD densities that were caused by gas expansion and voids in the cores.

There is good correlation of density and MS with major lithostratigraphic units. The boundary between lithostratigraphic Units I and II at 62 mbsf was not clearly detected in the MS. The MS data suggest a boundary at a depth of 165 mbsf where a sudden change in the MS pattern was observed. Unit II shows higher MS values and is characterized by large variations, which may correlate to individual turbidite sequences. Although turbidites were also observed in Unit I, the MS data do not show as strong variations here. This may be an indication of different source material.

The boundary between lithostratigraphic Units II and III is identified as the change from slope-basin sediments to the AC. This boundary is marked by a sudden drop in average density and MS values.