In this section, we describe the downhole distribution of physical property data collected from Site 1251. Site 1251 is located in the basin to the east of the southern summit of Hydrate Ridge. Samples from three holes (Holes 1251B-1251D) drilled at this site were used to make physical property measurements. Holes 1251E-1251G were devoted to special microbiology and high-resolution turbidite studies, and no physical property measurements were performed on the cores from these holes. LWD data were available from Hole 1251A.
Routine physical property measurements were collected on whole-round core sections (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. Special comparisons of IR scans between Holes 1251B and 1251D were performed to study the effects of different coring techniques (APC vs. XCB) on core temperatures.
Samples were taken from split cores to measure moisture and density (MAD). Compressional (P)-wave velocities (VP) were not measurable at this site because of gas expansion, with the exception of the upper 50 cm of Core 204-1251B-1H. Shear strength was measured with the handheld Torvane on selected core sections to a depth of 180 mbsf.
The sampling for MAD at this site was mainly from Hole 1251B. For those depth intervals in Hole 1251B where core recovery was low or special tools were used, MAD samples were collected from Hole 1251D to fill the sampling gaps.
Measurement spacing, count times, and data acquisition schemes (DAQs) used for the multisensor track (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; P-wave logger (PWL): spacing = 2.5 cm, DAQ = 50. At this site, the Non Contact Resistivity (NCR) system was fully operational and was used in Holes 1251B and 1251D. A calibration was run for the NCR system to convert uncalibrated conductivity into resistivity of several standards of seawater with varying salinity (see "Physical Properties" in the "Explanatory Notes" chapter).
IR imaging of cores drilled at Site 1251 provided identification on the catwalk of hydrate zones in each core. This information was used to facilitate hydrate sampling and preservation for all cores from this site. Thermal images suggest that most of the hydrate observed at this site is present as disseminated layers or zones, except for a major hydrate zone with interlayed sediment in Hole 1251D from 190 to 202 mbsf (near the BSR). An example of a typical hydrate thermal anomaly is shown in Figure F24. The IR thermal anomalies from this site are cataloged in Table T12, including an interpretation of the overall hydrate shape.
Successive thermal images were used to produce a downcore thermal profile for Holes 1251B and 1251D (Fig. F25). The profiles show the overall thermal structure of each core. The dominant features of the profiles are similar to those described for Site 1244, except for the zone of high hydrate abundance noted above. The downcore temperature profiles also include artifacts such as large positive anomalies caused by sun illumination and overall temperature trends that are caused by daily ambient temperature changes on the catwalk. Calibration data, to eliminate the atmospheric and ambient temperature effects, have been collected and will be applied during later data analysis. The artifacts are spatially limited or they create systematic differences in background temperatures that did not impact the identification of hydrates on the catwalk. The detailed analysis of hydrate thermal anomalies for estimating the concentration of hydrate in the subsurface was not affected. The temperature anomalies created by hydrate have been extracted from the downcore temperature data and from direct examination of IR images. Results are displayed graphically in Figure F26 and demonstrate the overall low abundance of hydrate at this site, which is broadly consistent with pore water saturation (Sw) from the LWD data (see Fig. F46). Comparison of the thermal anomalies from Holes 1251B and 1251D (24.3 m apart) shows that hydrate zones match in general, but specific zones do not correlate between the holes. Comparison of Sw (Hole 1251A) with thermal anomalies shows a lack of exact depth correlation. Possible reasons for the lack of detailed correlation include (1) hydrate zones that are not stratigraphically controlled, (2) stratigraphically controlled zones that are laterally discontinuous, or (3) uncertainty in core depth due to poor core recovery. Note that the ~12-m-thick hydrate zone near the BSR in Hole 1251D was not detected in Hole 1251B, where there was nearly zero recovery at the equivalent depth. We suspect but cannot be certain that a thick zone was actually present in Hole 1251B and was not recovered. Sw from LWD resisitivity data (Hole 1251A) does show hydrate present over the depth intervals 187-197 and 202-205 mbsf but not with a response consistent with the thermal response in Hole 1251D. There is clearly significant hydrate near the BSR in Holes 1251A and 1251D and possibly in Hole 1251B, but it is likely that there is also significant lateral heterogeneity in hydrate concentration on a scale of ~25 m. This is important because the presence of a 10-m-thick zone of hydrate near the BSR is a significant contribution to the total volume of gas hydrate estimated for slope basins.
The absence of negative thermal anomalies below ~205 mbsf in Hole 1251D is consistent with the BSR depth and measured in situ thermal profile (see "Downhole Tools and Pressure Coring") at this site. Minor cold anomalies are present at 213.8 and 350.1 mbsf in Hole 1251B (Anomalies 204-1251B-IR42 and IR43) (Table T12). Both of these anomalies are present at the top of XCB cores, which are commonly slightly cooler than other parts of the core. Likely explanations include a larger than normal quantity of relatively cold drilling fluid entering the top of the core barrel during retrieval or gas expansion during retrieval. Recorded drilling parameters did not change significantly before, during, and after retrieval of these cores. Cooling from gas expansion, similar to that observed at Blake Ridge (Paull, Matsumoto, Wallace, et al., 1996), is perhaps less likely given the relatively low permeability of these sediments and the location of the anomalies at the core tops. In the case of the deep cold anomalies at Blake Ridge, some cores were actually retrieved frozen, apparently as a result of gas expansion in situ. Examples of gas-expansion cooling of the core liner have been observed in IR images (Core 204-1244A-7H), but neither of the anomalies noted here has features suggesting an obvious connection to gas expansion.
The IR data also show the thermal difference between XCB- and APC-cored intervals at this site, similar to that observed at Site 1244. This difference is shown in Figure F27, where XCB and APC cores were taken under similar ambient temperature conditions. There are at least four possible explanations for the temperature difference: (1) circulation of drilling fluid near the bit face during XCB coring; (2) greater tolerance and size variability between the core liner and the diameter of XCB core, resulting in greater movement of drilling fluid along the core during core recovery; (3) greater frictional heating during collection of APC cores; and (4) greater gas expansion or gas exsolution in XCB cores. At present, we cannot determine which of these four explanations is most important. For future hydrate drilling, it will be important to develop a better understanding of the relative thermal impacts of APC and XCB drilling. In some instances, the XCB is more effective for retrieving and preserving hydrate. For example, there was greater hydrate recovery just above the BSR in Hole 1251D (where the XCB was used) compared to Hole 1251B (where the ACP was used on the same interval). However, the circulation of drilling fluid in the case of XCB coring almost certainly produces a significant physical disruption of hydrate that needs to be considered in selecting the coring method for these types of sediments.
The thick thermal anomaly (hydrate zone) near the BSR in Hole 1251D provides an opportunity for relating chlorinity of IW to the presence of hydrate. Figure F28 shows this relationship, clearly linking a zone of thermal anomaly with a negative chlorinity anomaly. These results demonstrate the importance of selecting some IW samples within a meter or less of gas hydrate to increase the probability of detecting chlorinity anomalies (see also Fig. F25 in the "Site 1245" chapter).
A density profile was available prior to coring from the LWD program. In addition to the LWD data, routine sediment density measurements were carried out with the MST on whole-round cores (GRA density measurements) and on discrete samples (MAD measurements) (Table T13). The bulk density values show a perfect match with the LWD data (Fig. F29). The GRA density data show identical trends compared to those of MAD and LWD; however, the values were larger by ~0.3 g/cm3.
Sediment density at this site generally increases with depth from values of ~1.4 g/cm3 near the seafloor to ~1.9 g/cm3 at 300 mbsf. This is the expected trend resulting from normal compaction of the sediments. The boundary between lithostratigraphic Units I and II (defined at 130 mbsf) is not evident in the density records. However, the boundary between lithostratigraphic Units II and III is marked by a decrease in density. The density drop at 300 mbsf is well resolved in all three types of density measurements (MAD, LWD, and GRA). Values drop from ~1.9 g/cm3 at 320 mbsf to almost 1.5 g/cm3 at 370 mbsf. This boundary is also associated with a strong seismic reflector dipping east. (Fig. F30). The change in physical properties is caused by higher amounts of biogenic opal. This sediment component has a low grain density of ~2.0 g/cm3, resulting from the amorphous character of opal-A and a certain amount of structural water (up to 10%) within the silica phase. Within lithostratigraphic Unit III, a change in density occurs at ~360 mbsf. Density values increase at this depth to ~1.75 g/cm3 and are constant for the remainder of Hole 1251B. No seismic reflection can be associated with this increase in density. The deeper seismic record is characterized by a chaotic reflection pattern typically observed in accreted sediments.
Porosity decreases almost linearly from values of ~65% at the seafloor to ~50% at 300 mbsf. The porosity values in the low-density layer located between 320 and 370 mbsf are slightly higher at ~65%. Below 365 mbsf, porosity is nearly constant at values of ~55%. Calculated grain densities vary between 2.6 and 2.9 g/cm3 and are uniform for the entire site with an average value of 2.7 g/cm3.
The MS profile at Site 1251 is characterized by relatively uniform values of <40 x10-7 (SI) but with occasional susceptibility peaks of up to 200 x 10-7 (SI) (Fig. F29). Most of the peaks observed on the MS profile can be correlated to sand layers (e.g., at 20 and 170 mbsf) or to the zones of abundant sulfides (e.g., at 130 mbsf). However, close comparison with the sedimentological record shows that there are no sand layers or sulfide zones below 400 mbsf that could explain the spikes in the MS.
The boundary between lithostratigraphic Units I and II (unconformity) is marked by a spike in MS. However, an expanded view of the MS record at this depth shows that this spike is formed by two separate peaks (Fig. F31). A comparison with the core description shows that the high MS values correlate to the abundance of sulfides. This correlation was further investigated using the MS point sensor and X-ray images to help identify intervals with high sulfide concentrations (Fig. F31). The X-ray images are sensitive to variations in density. In general, sulfides form along small cracks or fill in former bioturbation features. On the X-ray images, darker spots (equivalent to higher-density material) are present at locations that also have larger MS values. Those spots are most likely sulfides, which have a much higher density than the surrounding sediments. It should be noted that pyrite (a common sulfide) does not have high MS and, thus, cannot be the cause of the anomaly. However, XRD analyses of a sulfide concretion found in Section 204-1244C-5H-4 showed a high abundance of pyrrhotite, a highly magnetic mineral. Sections 204-1251B-15H-2 through 15H-6 do not contain enough sulfide to be detectable by the onboard XRD. We were therefore not able to identify the magnetic mineralogy within this interval.
The NCR sensor was implemented for the first time as part of the suite of MST sensors at Site 1251. Early interfacing difficulties were overcome while drilling Hole 1251B, and it was possible to ensure consistent readings by "zeroing" before each core section was measured. This sensor system was still in a testing phase; therefore, we did not attempt any detailed interpretation of the data or correlate it to other physical properties at this site. We measured the resistivity for all cores recovered from Hole 1251B (Fig. F32). The resistivity was calculated from the raw values using the calibration function explained in "Physical Properties" in the "Explanatory Notes" chapter. The measurements are very sensitive to the distance between the sensor and the surface of the core liner. The position of the sensor changed during the measurements of Hole 1251B between 90 and 173 mbsf, resulting in a shift of the raw values (Fig. F32). Because the relative position of the sensor to the core liner was not measured, no calibration is available for this range of depth.
The NCR sensor produces data dominated by the effects of gas expansion in these sediments. It is, in fact, yet another MST "crack detector" similar to the GRA sensor and PWL. Gas voids in the core are electrical insulators and, hence, will provide very high resistivity values. It is clear from Figure F32B, which shows just the upper 10 m of data, that the cracks caused by gas expansion occur below 3 mbsf in Hole 1251B. Above 3 mbsf, the resistivity shows a discernible structure that is related directly to the sediment lithology rather than to the core disturbance caused by gas expansion. The conclusion from these first tests is that the NCR system is stable and can reliably measure the resistivity of the core, even if most of the measurements respond to the presence of gas expansion cracks or are dominated by other coring artifacts.
It is almost impossible to measure VP at this site because of gas expansion. The VP was measurable at only four points within the first meter below seafloor at Hole 1251B. The average value of these four data points is 1533 m/s. Only one velocity measurement was performed using the Hamilton Frame velocimeter, which gave a result of 1557 m/s (Table T14).
Thermal conductivity measurements were made routinely after the cores were equilibrated to ambient room temperatures. The general procedure was to measure thermal conductivity in one section per core (normally in Section 3). However, in cores where special downhole temperature measurements were conducted, three thermal conductivity measurements were made in Sections 1, 3, and 5, respectively.
At Site 1251, the measured thermal conductivity values vary between 0.721 and 1.036 W/(m·K), with an average value of 0.933 W/(m·K) (Table T15; Fig. F29). Data are scattered in the upper 160 mbsf, possibly because of gas expansion in the sediment. A small downhole increase in thermal conductivity was observed to 200 mbsf. The thermal conductivity values are reduced slightly between 300 and 450 mbsf, with an average value of 0.945 W/(m·K).
A plot of average normalized thermal conductivity vs. bulk density shows a good correlation between the two, indicating that the thermal conductivity is a direct function of water content in the sediments (Fig. F33).
Vane shear strength measurements were conducted on samples in Hole 1251B to a depth of 175 mbsf. Below 175 mbsf, the XCB coring technique disturbed the sediments too strongly to make reliable measurements. Measurements were made on the working half of the split core sections in locations where the sediment appeared generally undisturbed. Intervals within core sections exhibiting abundant cracks and voids resulting from gas expansion were avoided. Measured values vary between 4 kPa at shallow depths and 162 kPa at 175 mbsf (Table T16; Fig. F29). The shear strength increases linearly with depth (gradient = 0.5 kPa/m) from the seafloor to 80 mbsf, where a sharp decrease in shear strength is observed. This depth does not coincide with a change in shear vane size nor does it correspond to a change in stratigraphic unit or lithology. Below 80 mbsf, shear strength increases again linearly with depth but at a larger gradient (0.75 kPa/m) than that observed at shallow depth.
The variations in physical properties at Site 1251 are consistent with the three main lithostratigraphic units discussed in "Lithostratigraphy". The upper two units are characterized by uniformly increasing density values, probably a function of normal consolidation. The boundary between Units I and II is an unconformity, which is well defined in the seismic record. However, there is no apparent change in density. The unconformity correlates to a spike in MS, which can be explained by diagenetic mineral formation (differences in sulfide abundance) within the sediments. The X-ray images show a high abundance of sulfides in those intervals that correlate to high MS values. One possible mineral is pyrrhotite, but it was not confirmed by XRD analysis at this horizon. A sulfide concretion found in Section 204-1244C-5H-4 was determined by XRD analysis to have a high concentration of pyrrhotite, but this was not large enough to be detected with the MS sensor of the MST. Small individual concretions can only be detected with the MS point sensor, which has the required spatial resolution. Another option to explain higher MS values is a change in sedimentation rate, which can result in changes of the relative concentration of magnetic minerals. However, with onboard techniques no definitive interpretation for the high MS values is possible.
Thermal imaging using the track-mounted IR camera on the catwalk provided the best method of detecting zones of gas hydrate in the cores at this site, especially when the hydrate occurred in disseminated form. Relatively low concentrations of hydrate were observed at Site 1251, except near the BSR, where IR anomalies and chlorinity indicate a zone of high hydrate concentration. The lateral extent of this zone is uncertain and important, as its presence or absence results in a relatively large difference in the total amount of hydrate estimated in the slope basin. Discrete samples of hydrate were not found in Hole 1251B, although several zones with cold anomalies were identified. The temperature anomalies observed in Hole 1251B were relatively small compared to the main anomaly encountered at Hole 1251D just above the BSR depth. These small thermal anomalies are interpreted to be indicative of disseminated hydrate.