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doi:10.2204/iodp.proc.314315316.204.2012

Methods and materials

Specimen preparation

Soon after recovery of a given core aboard the D/V Chikyu, the whole-round samples were cut, capped and taped in their plastic core liners, and sealed in aluminum vacuum bags with moist sponges to prevent moisture loss. The samples were stored at 4°C until immediately prior to trimming. To extract each specimen, the plastic core liner was cut lengthwise along two lines 180° apart using a hacksaw and then removed. Cylindrical specimens for permeability tests in the vertical core direction were trimmed using a wire saw and soil lathe from the top 7 cm of each whole-round sample. Sample dimensions were measured at several points using a caliper to a resolution of ±0.03 cm and averaged to obtain values used for subsequent calculations. Specimen length after trimming ranged from 4.4 to 5.9 cm and averaged 5.3 cm (Table T1). Specimen diameter ranged from 3.4 to 4.1 cm and averaged 3.8 cm. Separate specimens for permeability tests in the horizontal direction were trimmed from the remaining material located immediately below the top 7 cm taken for vertical permeability testing. These specimens were trimmed perpendicular to the core axis. Bedding orientation (dip angle) with respect to the core axis (Fig. F1) was not taken into account during trimming, but that factor could exert some influence on the apparent magnitude of permeability anisotropy.

Specimen trimmings were retained and used for measurements of liquid limit (LL), plastic limit (PL), and corresponding plasticity index (PI) following procedures outlined in ASTM Standard D4318-05 (ASTM International, 2005) (Table T1). Initial specimen porosity (n_i) was estimated from gravimetric water content (w_i) of the specimen trimmings by assuming 100% pore water saturation and a specific gravity of the mineral solids of 2.70. Gravimetric water content of the specimen trimmings was determined by oven-drying at 105°C. Additional information reported on Table T1 includes Skempton’s B-value determined to assess specimen saturation (see subsequent discussion) and final water content and porosity (w_f and n_f) measured from oven drying portions of the specimens after permeability testing. Values reported on Table T1 are those taken after isotropically consolidating the specimens to ~0.55 MPa effective stress. Computed porosities were not corrected to account for the presence of tightly adsorbed water associated with smectite content (e.g., Brown et al., 2001; Gamage et al., 2011), nor was porosity corrected for the presence of salt in the pore water.

Constant-flow apparatus

Constant-flow, flow-through permeability tests were used to determine hydraulic conductivity in the vertical and horizontal core directions. The constant-flow method employs an infuse/withdrawal syringe pump to simultaneously inject and extract pore fluid from the top and bottom of the specimen. The closed system in use at the University of Missouri consists of an acrylic confining cell to contain the specimen and provide isotropic effective stress, a constant-flow infuse/withdrawal syringe pump, one differential pressure transducer to measure hydraulic head loss between the specimen top cap and bottom cap, and an air/water interface panel for regulating the confining fluid pressure and pore fluid backpressure (Fig. F2). A digital interface displays and records effective stress (σ′), change of hydraulic head (Δh), and time duration measurements made during each test run. Signals from the pressure transducer were acquired to obtain head loss at a precision of ±1 cm H₂O over a range spanning ±1000 cm H₂O. The flow pump (KDS Scientific, Model 260) holds two syringes (Hamilton GasTight Series 1000) and has the capability to cycle continuously back and forth in a push-pull action. As one syringe infuses pore fluid into the specimen, the other withdraws an equal volume of fluid from the other end of the specimen at the same rate. At the end of the set volume, the direction is automatically reversed and the next cycle begins. With the use of three-way valves, the pump can empty and refill syringes for a continuous dispense. Volumetric flow rate (Q) for the series of tests described here ranged from a minimum of 0.00025 cm³/min to a maximum of 0.010 cm³/min, resulting in a corresponding discharge (Darcy) velocity (v) that ranged from approximately 3.50 × 10^–7 to 1.37 × 10^–5 cm/s.

Backpressure saturation

Prior to testing, all permeant lines and porous stones were saturated with simulated seawater (25 g NaCl to 1 L tap water). A specimen was placed on the pedestal, the top cap applied, and a latex membrane placed on the specimen using a vacuum membrane expander. The confining chamber was then sealed and the cell was filled with tap water. Saturation of the specimen was achieved by ramping pore fluid backpressure to 70 psi (0.48 MPa) while also ramping the confining pressure to maintain an effective isotropic confining stress of 5 psi (0.034 MPa). Elevated backpressure was maintained for at least 24 h. Saturation of the specimen was checked by increasing the confining pressure (σ) to 80 psi (0.55 MPa) and measuring the corresponding pore pressure (u) response and calculating Skempton’s B-value (B = Δu/Δσ). Specimens were considered saturated if B ≥ 0.95 (Table T1). Once saturation was achieved, the cell confining pressure was increased to consolidate the specimen at a desired effective stress. Pore water was allowed to drain during consolidation from both the top and bottom of the specimen.

Constant-flow permeation

Constant-flow tests were performed for each of the 10 specimens (5 trimmed vertically and 5 trimmed horizontally) at five increasing levels of effective stress: 5 psi (0.034 MPa), 20 psi (0.138 MPa), 40 psi (0.276 MPa), 60 psi (0.414 MPa), and 80 psi (0.551 MPa). Tests at each level of effective stress were conducted using four flow rates to obtain replicate permeability values, including two tests conducted with a top-to-bottom flow direction (denoted subsequently as a negative flow value) and two tests conducted with a bottom-to-top flow direction (denoted as a positive flow value) to obtain replicate permeability values. Transient response from the differential pressure transducer was monitored for steady-state head difference (Δh_s), typically requiring 75–100 min per test run (e.g., Fig. F3). Values of applied discharge velocity (v) and steady-state hydraulic gradient (i_s) were plotted to assess consistency among the four test runs and linearity in the relationship (Figs. AF1, AF2, AF3, AF4, AF5, AF6, AF7, AF8, AF9, AF10). Coefficient of determination values (R²) by least-squared linear regression of these v and i_s values were ≥0.9835, indicating good repeatability among the four flow tests conducted at each level of confining stress and the validity of Darcy’s Law (Equation 1) for calculating hydraulic conductivity.

Data analysis

Hydraulic conductivity (K) was calculated for each specimen using Darcy’s law:

where

• Q = applied volumetric flow rate (cm³/s),

• is = steady-state hydraulic gradient equal to the ratio of the steady-state head loss (Δh_s) to the length over which that head loss occurs (ΔL) (taken as the initial height of the specimen), and

• A = cross-sectional flow area (cm²) (taken as the initial specimen area).

The corresponding discharge velocity is v = Q/A. Hydraulic conductivity values under laboratory test conditions (units for K = cm/s) were converted to intrinsic permeability (k) (m²) values using

where

• µ = viscosity of permeant (0.001 Pa·s at room temperature conditions),

• ρ = density of permeant (1027 kg/m³), and

• g = gravitational acceleration (9.81 m/s²).

Imaging of grain fabric

Specimens for imaging of grain fabric were cut from the whole-round samples with a razor blade at vertical orientation and horizontal orientation relative to the axis of the cylindrical samples (Fig. F4). Grain fabrics of wet, uncoated, and unfixed specimens were imaged using an FEI Quanta 600 FEG scanning electron microscope (SEM). The instrument operates in environmental mode (ESEM) at 30 kV, with the specimen chamber pressure set at 700 Pa. Water vapor (~98% humidity) from a built-in reservoir keeps the specimen from losing moisture. The temperature of the cooling stage was set to 2°C. The specimens were imaged with a gaseous backscatter electron detector, spot = 3.0 at a working distance of ~10 mm. This combination generates an imaging resolution of ~4 nm, and the dimensions of the field of view are about 145 µm × 130 µm at 2000× magnification. Specimens were placed in the holder on the stage with the imaged surface facing upward. “Center stage” and “Tilt” commands of the ESEM controlling software were used to manually adjust the imaging face to an orientation as close to perpendicular as possible to the imaging beam. All the image files were saved with color gray mode in tiff format.

Digital images were processed using ImageJ software (available at rsbweb.nih.gov/ij/index.html). Our processing steps adhere to the following:

1. Contrast enhancement: linear stretching of the gray-level histogram in order to use 256 gray-level values.

2. Median filter: moving each pixel value to the median values of 9 closest pixels (to reduce noise).

3. Mean filter: moving each pixel value to the mean values of 9 closest pixels weighted by its coefficient (to preserve subtle details).

4. Median hybrid filter: moving each pixel to the median values of middle horizontal 3 pixels, center vertical 3 pixels, and center pixel of those 9 closest pixels (to reduce noise while preserving linear features).

5. Threshold: adjusting and picking up one point of gray-level histogram (to select objects).

6. Make binary, to transform the gray image to white and black image (e.g., Fig. F5A).

7. Overlap the image onto the original image and set its alpha value (transparency) to 60% in Photoshop software, then separate objects that touch, by manual adjustment with eraser tool (Fig. F5B, F5C).

8. Median filter with ImageJ, to remove objects <8 pixels in size (because measurements on small objects are mostly biased).

9. Fill the holes on the objects (Fig. F5D).

10. Measure automatically, to obtain the long-axis and short-axis dimensions and long-axis orientation of an object.

The software can automatically determine the long or short axis (apparent dimensions) of the objects in the 2-D image. Results are saved in a text file automatically after measurement.

Orientation of grain fabric was quantitatively characterized in the form of rose diagrams depicting orientations of the apparent long particle axes. In petrography, SEM, and transmission electron microscopy studies, most investigators measure between 100 and 500 grains per thin or ultrathin section (Krumbein, 1935; Friedman, 1958; van der Plas, 1962; Griffiths, 1967; Chiou et al., 1991). The orientation of each particle (apparent long axis) was assigned to an angle between 0° and 180°. For the vertical section, the core axis is oriented at 90°. We depict all the measured orientations as rose diagrams using Rozeta software (available at www.softpedia.com/get/Science-CAD/Rozeta.shtml). This software automatically counts the number of particles according to their orientation and combines data into bins of 10°. In addition to the rose diagram, the number of values in each bin was summed and normalized to a total of 100%. Cumulative frequency curves were constructed to show the distribution of grain orientations (Chiou et al., 1991).

Various statistical methods can be used to characterize the degree of orientation, such as the formulas of Folk and Ward (1957), Martínez-Nistal et al. (1999), and Zaniewski and Van Der Meer (2005). The Folk and Ward (1957) formula was proposed originally to graphically compute values of sorting (standard deviation) for grain size data. The equivalent equation for standard deviation of grain of orientation (d) equals

where φ₈₄, φ₁₆, φ₉₅, and φ₅ represent the angle of orientation at the 84th, 16th, 95th, and 5th percentiles, respectively, on the cumulative frequency curve. This graphical technique avoids the laborious calculations required by moment statistics (Chiou et al., 1991). If the fabric of sediment shows strong preferred orientation, then the sorting of orientation angles will be better and the slope of cumulative frequency curve will be steeper near the median (50th percentile). In theory, the maximum value of d is 72.3° (e.g., a case in which φ₁₆ and φ₅ = 0° and φ₈₄ and φ₉₅ = 180°). We normalized each standard deviation to this maximum value by calculating the “index of microfabric orientation” (i) as shown the following formula:

The closer i is to 1, the more particles are aligned in a preferred direction. For a highly random arrangement of particles, the cumulative curve generally has a slope of <0.75 near the median, the standard deviation of orientation is >35°, and the index of microfabric orientation is <0.51. For well-oriented clay particles, the slope of the cumulative curve is generally >1.00 near the median, the standard deviation of orientation is <25°, and the index of microfabric orientation is >0.65.

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