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Physical properties

Physical properties measurements of core samples from Site C0022 provide insight into the evolution of a splay fault and associated deformation by combining results from the adjacent sites (Sites C0004 and C0008), where coring and logging were conducted during NanTroSEIZE Stage 1 Expeditions 314 and 316. Physical properties on core samples also help calibration as well as correlation with LWD data (see “Logging while drilling” and “Core-log-seismic integration”).

Whole-round multisensor core logger and split core multisensor core logger (whole-round cores and working halves)

Whole-round cores were analyzed by the whole-round multisensor core logger (MSCL-W). The results of gamma ray attenuation (GRA) density, magnetic susceptibility, natural gamma radiation (NGR), and electrical resistivity measurements (see the “Methods” chapter [Strasser et al., 2014a]) are summarized in Figure F33. GRA density increases with depth similar to bulk density of MAD measurements discussed in “Moisture and density measurements (discrete cores).” From the seafloor to 210 mbsf, magnetic susceptibility slightly increases; below 210 mbsf, magnetic susceptibility increases rapidly. NGR is mostly constant above 210 mbsf and starts decreasing below 210 mbsf. This boundary suggested by magnetic susceptibility and NGR is consistent with a subunit boundary defined by LWD data (see “Logging while drilling”). Electrical resistivity slightly decreases with depth, although resistivity measurements on working halves and discrete samples increase with depth as discussed later.

VP measurements on whole-round cores and working halves were conducted using the MSCL-W and the split core multisensor core logger (MSCL-S), respectively. VP measurements using the MSCL-W are shown in Figure F34A. VP data are filtered by amplitude because the MSCL-W picked up some arrivals even for locations with lots of voids or poor contacts between liner and sediment core. VP measurements using the MSCL-S were attempted for the first time in the NanTroSEIZE project on the D/V Chikyu to have better contact with cores, liners, and transducers. No other physical properties measurements using the MSCL-S were conducted. Although we attempted VP measurements using the MSCL-S on a selected section of each core from Cores 338-C0022B-1H to 6H, good quality MSCL-S data were obtained only on Section 1H-4 (Fig. F34B).

Moisture and density measurements (discrete cores)

A total of 400 discrete samples from Hole C0022B were measured for MAD. MAD data from Hole C0022B are summarized in Table T16 and Figure F35. Between 4.25 and 415.32 mbsf, bulk density ranges from 1.51 to 2.05 g/cm3, grain density ranges from 2.46 to 2.89 g/cm3, and porosity ranges from 40% to 69%. Bulk density and porosity show less scatter with increasing depth. Grain density values are generally constant with depth, scattered about an average value of 2.70 g/cm3. Both bulk density and porosity generally change linearly with depth; bulk density increases and porosity decreases. Taking a closer look, porosity decreases quickly from 69% at the seafloor to 45%–55% at ~100 mbsf, and then increases to 60% at 150 mbsf. Interestingly, the minimum porosity of this trend at ~100 mbsf is close to the megasplay fault inferred in LWD data at 100.0–100.8 mbsf (see “Logging while drilling”). One potential cause for this trend is that sediment near the splay fault has experienced shear-enhanced compaction. Future postexpedition research, including laboratory experiments and detailed microstructural analyses, will shed light on the evolution of physical properties associated with shear deformation.

Thermal conductivity (whole-round cores and working halves)

Thermal conductivity was measured on whole-round cores from 4.4 mbsf (Core 338-C0022B-1H) to 369 mbsf (Core 36X) using a needle probe sensor and on working-half cores from 338 mbsf (Core 33X) to 414 mbsf (Core 41X) using a half-space line source probe. Thermal conductivity generally increases from ~1.01 to 1.53 W/(m·K) with depth (Fig. F36). Thermal conductivity measured on working-half cores is higher than that measured on whole-round cores.

Electrical resistivity and ultrasonic P-wave velocity (working halves and discrete cores)

A total of 353 electrical resistivity measurements were conducted on working-half cores between 0 and 370 mbsf, using the Wenner four-pin array probe. Each measurement was recorded in the dominant lithology types per section. In addition, nine cubic samples obtained between 335 and 415 mbsf were used to measure electrical resistivity and VP.

During the measurements on working-half cores from Site C0022, the pins of the Wenner probe were incorrectly connected to current and potential ports. This could affect resistivity absolute values toward lower values. Several measurements with both correct and incorrect configurations of the four pins were conducted on several points on Sections 338-C0022B-10T-5 and 11T-CC. The obtained data show no systematic difference between the two configurations (Table T17). Thus, we conclude that the measured data of resistivity with incorrect configuration of four pins are valid and report them. More importantly, we found later that the correct values of standard seawater resistivity were not obtained because of the use of a small container when the measurements on cores from Sites C0002 and C0022 were conducted on the Chikyu. Unexpectedly low resistivity values obtained for cores at Sites C0002 and C0022 are probably due to overestimation of seawater impedance. This problem was resolved when resistivity was measured on cores from Site C0021 using a larger container of seawater (see also “Physical properties” in the “Site C0021” chapter [Strasser et al., 2014d]).

Electrical resistivity ranges from 0.103 to 1.738 Ωm, with an average of 0.498 Ωm, and increases with depth as expected from the progressive densification and porosity loss (Fig. F37; Table T18). Above 280 mbsf, resistivity increases from 0.1 to 0.7 Ωm, and below 280 mbsf, more scatter is present, with an average resistivity of ~0.5 Ωm.

Resistivity measured with the Wenner probe is lower than resistivity measured on cubic samples and resistivity measured with the MSCL-W at the same depth (Fig. F38). However, despite low resistivity values, resistivity variations with depth are similar between the MSCL-W and Wenner probe results. The MSCL-W resistivity shows a progressive increase from 0 to 280 mbsf with values between 0.5 and 1 Ωm and stays constant below 280 mbsf at ~0.8 Ωm (Figs. F33, F38). The results of resistivity on cubic samples from Expedition 316 Sites C0004 and C0008 are similar to the resistivity measured on cubic samples at Site C0022 but have higher resistivity than the Wenner probe in Hole C0022B (Fig. F38).

Resistivity results obtained using the Wenner probe from Sites C0002 and C0022 during Expedition 338 were compared with those obtained from IODP Expedition 333 Sites C0011 and C0012 that were also measured using the same Wenner probe (Fig. F39). Resistivity results from Expedition 338 Sites C0002 and C0022 are lower than those from Sites C0011 and C0012 at a given porosity value. As mentioned above, lower resistivity values for Sites C0002 and C0022 were possibly due to the incorrect values of standard seawater resistivity from the use of a small container. However, Archie’s cementation factor (m) determined for Site C0022 (m = 2.17) from the exponent of the power law fit is similar to the values determined at Sites C0002 (m = 2.4) and C0010 (m = 2.3) during previous IODP NanTroSEIZE expeditions (Expedition 314 Scientists, 2009a; Expedition 319 Scientists, 2010) and at Site C0012 (m = 2.95) during Expedition 338 (see “Logging while drilling” in the “Site C0012” chapter [Strasser et al., 2014c]).

A total of nine discrete cubes were extracted from Hole C0022B (335–415 mbsf) when the sediment was too consolidated to allow Wenner probe insertion for electrical resistivity measurements. These cubes were analyzed for electrical resistivity and VP along three orthogonal directions (x, y, and z). The results of electrical resistivity and VP are summarized in Tables T19 and T20 and Figure F40.

Electrical resistivity measured on cubes ranges from 0.44 to 1.703 Ωm. All except two samples (338-C0022B-38X-5, 32 cm, and 40X-1, 63 cm) record an anisotropy such that electrical resistivity in the vertical z-direction is higher than that in the horizontal x- or y-direction because of the bedding oriented within the x-y plane (Fig. F40). Resistivity is usually the lowest along the bedding plane in sedimentary rocks because of better pore connectivity. Vertical anisotropy is between 0.8% and 38.8% with negative values, except for the two cubes that probably have a bedding plane within x-z or y-z planes with anisotropy up to 113.7%. The measurements with bedding subparallel to the x-y plane present a transverse anisotropy (i.e., lineation) with a horizontal anisotropy from 2% up to 50.2%.

VP increases with depth and ranges from 1.349 to 2.031 km/s between 335 and 415 mbsf. Horizontal and vertical anisotropies range from 0.37% to 29.7% in Hole C0022B (Fig. F40). Electrical resistivity and VP increase with depth. This is probably related to densification and porosity reduction.

Shear strength (working halves)

Shear strength measurements using a vane shear device and a pocket penetrometer (see the “Methods” chapter [Strasser et al., 2014a]) were made on working halves from 4.24 to 306.91 mbsf. One measurement for each method was made per core. Penetrometer measurements range from 27.8 to 228.9 kPa, whereas vane shear measurements range from 3.6 to 118.7 kPa and are consistently lower than the penetrometer measurements (Fig. F41; Table T21). There is considerable scatter in the data, which increases with depth. One trend that may be observed is an increase in the maximum penetrometer values with depth. There are no definite trends in the vane shear measurements. Sediment below ~307 mbsf was too consolidated to make penetrometer and vane shear measurements; unconfined compressive strength (UCS) measurements were made on these deeper sediments (see below).

Unconfined compressive strength (discrete cores)

Unconfined compression tests (see the “Methods” chapter [Strasser et al., 2014a]) were conducted on five cubic samples (2 cm × 2 cm × 2 cm each) used for VP and electrical resistivity measurements and two cuboid samples (1.5 cm × 1.5 cm × 3 cm each) (Table T22). A vertical load is applied along the long axis of the sample, which is parallel to the core axis. UCS (maximum force per unit area) ranges between 2.0 and 3.9 MPa, with an average of 2.8 MPa (Fig. F41; Table T22). The variability in UCS data is probably due to sample heterogeneity, sample size, control of loading rate (manual versus computer control), and sensitivity of the load cell (±0.02 kN for the load cell on the Chikyu).

Color spectroscopy (archive halves)

The results of color reflectance measurements using the color spectroscopy logger (MSCL-C) are summarized in Figure F42. General trends are that L* and b* decrease slightly with depth and a* is approximately constant with depth. L* ranges from 14 to 64, a* ranges from –6.2 to 8.6, and b* ranges from –2.82 to 7.21. The results in Hole C0022B are similar to those in Holes C0008A and C0008C because of the same lithologies (Expedition 316 Scientists, 2009b).