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doi:10.2204/iodp.proc.314315316.122.2009

Physical properties

Continuous physical property measurements provide basic information to assist characterization of lithologic units and states of consolidation and deformation, as well as correlation of cored materials with downhole logging data. First, X-ray CT images were captured for all core sections. Then, gamma ray attenuation (GRA) density, magnetic susceptibility, natural gamma radiation, P-wave velocity, and electrical resistivity were measured using an MSCL system (Geotek Ltd., London, UK) for whole-round core sections (MSCL-W) after thermal equilibration at room temperature, ~20°C. After MSCL-W measurements, thermal conductivity measurements were carried out on whole-round core sections for soft sediments and on split working-half cores for hard sediments and rocks. Digital photo image scanning and color spectrophotometry were carried out on the split surfaces of archive-half cores using the photo image capture logger (MSCL-I) and the color spectrophotometer (MSCL-C), respectively. MAD were measured on discrete subsamples collected from working-half core samples, as well as from clusters next to whole-round samples. Sediment shear strength was measured on working halves using vane shear and a penetrometer. P-wave velocity and electrical conductivity were measured in three orthogonal directions in 20 mm cubic samples. Details about each measurement are given below.

MSCL-W

Gamma ray attenuation density

A thin gamma ray beam was produced by a ¹³⁷Cs gamma ray source at a radiation level of 370 MBq within a lead shield with a 5 mm collimator. The gamma ray detector comprised a scintillator and an integral photomultiplier tube. Calculation of bulk density from gamma ray attenuation was by the following equation:

ρ = 1/(µ × d) × ln(I₀/I),

where

• ρ = sediment bulk density,
  
• µ = Compton attenuation coefficient,
  
• d = sample thickness,
  
• I₀ = gamma source intensity, and
  
• I = measured intensity through the sample.

Because µ and I₀ are treated as constants, ρ can be calculated from I. We used a set of aligned aluminum cylinders of various thicknesses, surrounded by distilled water in a sealed core liner used for drilling, for calibration. Gamma counts were taken through each cylinder for long count time (60 s), and ln(I) was plotted against ρ × d. Here ρ of each aluminum cylinder was 2.7 g/cm³, and d was 1, 2, 3, 4, 5, or 6 cm. The relationship between I and ρ × d can be expressed as follows:

ln(I) = A (ρ × d)² + B (ρ × d) + C,

where A, B, and C are coefficients determined during calibration. These coefficients varied slightly during the expedition. The MSCL provided the values of I and d, and ρ was calculated with the equation above.

This density measurement was conducted every 4 cm for 4 s. The spatial resolution was 5 mm, so each data point reflects the properties of the closest 5 mm interval.

Porosity (ϕ) is calculated from MSCL density assuming a solid grain density (ρ_s) of 2.7 g/cm³ and a pore fluid density (ρ_f) of 1.024 g/cm³:

ϕ = (ρ_s – ρ)/(ρ_s – ρ_f).

Magnetic susceptibility

Magnetic susceptibility is the degree to which a material can be magnetized by an external magnetic field. A loop sensor (MS2C; Bartington Instruments Ltd.) with an 8 cm loop diameter was used for magnetic susceptibility measurements. An oscillator circuit in the sensor produces a low-intensity (8.0 × 10^–4 mA/m RMS) nonsaturating, alternating magnetic field (0.565 kHz). Any material near the sensor that has a magnetic susceptibility causes a change in the oscillator frequency. This pulse frequency is then converted into magnetic susceptibility values. The spatial resolution of the loop sensor is ~4 cm and accuracy is 5%.

Like GRA density data, magnetic susceptibility data were obtained at 4 cm intervals with a 41 s acquisition time.

Natural gamma radiation

Natural gamma ray (NGR) emissions were recorded from all core sections to determine variations in the radioactive counts of the samples and for correlation with downhole NGR measurements. A lead-shielded counter, optically coupled to a photomultiplier tube and connected to a bias base that supplied high-voltage power and a signal preamplifier, was used. Two horizontal and two vertical sensors were mounted in a lead cube-shaped housing. Most X-ray emissions from rocks and sediment were produced by the decay of ⁴⁰K, ²³²Th, and ²³⁸U, three long-period isotopes. Spatial resolution was 120–170 mm, and NGR was measured every 15 cm for a 30 s period. Background radiation noise was 38 cps, measured by inserting a blank filled with distilled water.

P-wave velocity

The basic relationship for sonic velocity is

v = d/t,

where

• d = distance traveled through the core and
• t = traveltime though the core.

P-wave velocity transducers are mounted on the MSCL system and measure d and t horizontally throughout the whole core. Total traveltime measured between the transducers includes three types of “delay” as

• t_delay = delay related to transducer faces and electronic circuitry,
• t_pulse = delay related to the peak detection procedure, and
• t_liner = transit time through the core liner.

The effects of delays are calibrated using a core liner filled with pure water. For routine measurements on whole-round cores in core liners,

v_core = (d′_core – 2d_liner)/(t₀ – t_pulse – t_delay – 2t_liner) × 1000,

where

• v_core = corrected velocity through core (km/s),
• d′_core = measured diameter of core and liner (mm),
• d_liner = liner wall thickness (mm), and
• t₀ = measured total travel time (µs).

Electrical resistivity

The noncontact resistivity sensor on the MSCL system operates by inducing a high-frequency magnetic field in the core from a transmitter coil, which in turn induces electrical currents in the core which are inversely proportional to the resistivity. Very small magnetic fields regenerated by the electrical current are measured by a receiver coil. To measure these very small magnetic fields accurately, a different technique has been developed that compares readings generated from the measuring coils to readings from an identical set of coils operating in air. Electrical resistivity data were obtained at 4 cm intervals.

Thermal conductivity

Thermal conductivity measurements were conducted on whole-round core samples from relatively shallow depths (<230 m CSF) and on split halves of cores from depths >230 m CSF.

A full-space single-needle probe TeKa TK04 unit (Blum, 1997) was utilized to measure thermal conductivity of unconsolidated sediments at three per core interval under conditions of full recovery. A small hole was drilled in the core liner, usually 26 cm from the top of each section. A 2 mm diameter temperature probe was inserted into the working half of the core section. At the beginning of each measurement, temperature in the samples was monitored automatically without applying a heater current until the background thermal drift was determined to be <0.2 mK/h. The heater circuit was then closed, and the temperature increase in the probe was recorded. During each 24 h period, three standard blocks with thermal conductivities of 0.517, 1.237, and 1.623 W/(m·K), respectively, were probed. Measurement results were then plotted against true values, and the slope of the linear regression was obtained. This slope was used to calibrate core sample measurements. The reported thermal conductivity value for each sample is an average of three repeated measurements.

Because sediments become stiffer with increasing depth, thermal conductivity measurements were conducted on the split halves. A QTM-500 quick thermal conductivity was utilized to measure hard samples. At the beginning of each half-space thermal conductivity measurement, a 10 cm long split-core piece was taken from the working half of the core and placed in seawater at ambient temperature for 15 min. The sample was then wrapped in stretchable plastic wrap. Care was taken to remove any visible air bubbles between the plastic wrap and the sample surface. The half-space probe was placed on a flat surface of the sample, and heating and measurements were done automatically. Calibration procedures are same as those used for whole-round samples.

Moisture and density measurements

MAD of rocks and sediments were calculated by measuring wet mass, dry mass, and dry volume. Approximately 5 cm³ samples were taken from two intervals (at ~25 and 100 cm from the top of the section as a convention) for each working-half section. For Hole C0002D cores, samples were taken from one interval for each section. In addition, MAD samples were routinely taken from the “cluster” slices next to whole-round samples (except for whole-round samples for microbiology). If the whole-round sampling location overlapped the regular MAD sampling intervals, no additional sample was taken.

In general, care was taken to sample undisturbed parts of the core and to avoid drill mud. Immediately after the samples were collected, wet sediment mass (M_wet) was measured. Dry sediment mass (M_dry) and volume (V_dry) were measured after drying in a convection oven for 24 h at 105° ± 5°C. Wet and dry masses were weighed using paired electronic balances, which compensated for the ship’s heave. Dry volume was measured using a helium-displacement pycnometer (Quantachrome penta-pycnometer) with a nominal precision of ±0.04 cm³. Measurements were repeated four times and the average of the last three measurements was used. Bulk density, dry density, and density of the solids, as well as porosity and moisture content, were computed, taking into account the precipitation of dissolved salts during drying (Blum, 1997).

Shear strength measurements

Undrained shear strength measurements were determined using a semiautomated laboratory vane shear device (Wykeham Farrance, model WF23544) and a pocket penetrometer (Geotest Instrument Co., model E-284B). Measurements were made at discrete locations on the working halves at a frequency of approximately three per core (at 100 cm from the top of Sections 2, 5, and 7 as a convention). In general, measurements were made adjacent to MAD sampling horizons. Care was taken to conduct tests within undisturbed and homogeneous parts of the core. To minimize disturbance effects resulting from the measurement itself, vane shear tests were generally conducted first, followed by penetrometer tests. Measurements were made with the vane rotation axis and penetrometer penetration direction perpendicular to the split surface.

Vane shear strength S_u(v) (kPa) is calculated as

S_u(v) = T/K_v,

where

• T = torque required to fail the material (N·m) and
• K_v = constant depending on vane dimensions (m³) (Blum, 1997).

All measurements reported here were obtained using a vane with height and diameter of 12.7 mm. Failure torque was determined by measuring the degrees of rotation of one of four torsional springs and a linear calibration equation (manufacturer specified) relating the rotation angle to torque for the particular spring being used. Selection of the appropriate spring was based on the anticipated shear strength of the material. Vane shear results were generally considered reliable for shear strength values less than ~150 to 200 kPa, above which excessive cracking and separation of the core material occurred.

The pocket penetrometer provides a measure of unconfined compressive strength in units of kilograms per centimeter squared. Compressive strength is calculated from the penetration resistance generated by pushing a cylindrical probe into the split core surface (Blum, 1997). Penetrometer-based shear strength (S_u(p)) is calculated as measured compressive strength converted to units of kilopascals and divided by two. Shear strength values reported here were calculated from the average of three penetration trials conducted at adjacent points on the core. Typical spatial separation between trials was on the order of 1 cm. All measurements were obtained using a probe with a nominal diameter of 6.4 mm (0.25 inches). Tests were conducted for shear strength values up to a maximum of ~300 kPa.

MSCL-I: photo image logger

The MSCL-I scans the surface of archive-half cores and creates a digital image. The line-scan camera equips three charge-coupled devices; each charge-coupled device has 1024 arrays. Light reflection from the sample surface passes through the lens and is split into three paths (red, green, and blue) by a beam splitter inside the line-scan camera. Then, each reflection is detected by the corresponding charge-coupled device. Finally, the signals are combined and the digital image is reproduced. Optical distortion downcore is avoided by precise movement of the camera. Spatial resolution is 100 pixels/cm.

MSCL-C: color spectroscopy logger

A color spectrophotometer (Konica-Minolta, CM-2600d) is included on the MSCL-C system. The xyz-type aluminum frame allows operators to set a maximum of seven core sections on the tray, and the sensor unit (including the spectrophotometer and small distance measuring system using a laser sensor) moves over each section and down at each measurement point to measure the split archive core surface.

Light reflected from the sample surface is collected in the color spectrophotometer’s integration sphere. The instrument’s structure allows for the specular component to be included (SCI setting) or excluded (SCE setting). The SCE setting is the recommended mode of operation, especially for sediments, to exclude glare. The light is then divided into wavelengths at a 10 nm pitch (400–700 nm), and the spectral sensors in the sphere convert the light to electrical currents proportional to the intensity of the light. Next, the color spectrum from the sample is normalized by the source light of the reflectance. The obtained spectrum is calibrated with the measurement of a pure white standard, which has a high reflectance true value at visible wavelengths and is measured by the vendor, and a black box (zero calibration). Measurements can be calculated based on the 2° or 10° standard observer and any of 11 illuminants.

Color reflectance is categorized as an IODP standard measurement, and the measured color spectrum is normally converted to L*, a*, and b* parameters. L*, a*, and b* provide relative changes in the composition of the bulk material and are widely used to correlate sections from core to core or hole to hole and to analyze the characteristic and cyclicity of lithologic changes.

Anisotropies of P-wave velocity and electrical resistivity

Three directional measurements on discrete samples of P-wave velocity and electrical conductivity were performed on RCB cores. Core pieces were cut with a saw equipped with two parallel disks set at 20 mm spacing. This sample preparation enables measurement of both electrical conductivity and P-wave anisotropies. If the core has no (or subhorizontal) apparent stratification or foliation, cubes are cut with faces 1, 2, and 3 orthogonal to the x-, y- and z-axes of the core reference, respectively. Orientation of the axes is the same as for paleomagnetism, with z- pointing down along the core axis, x- pointing into the working half, and y- in the core face.

The sample is held between two stainless steel electrodes covered with filters soaked in seawater and the complex impedance (R + jX) is measured at 10 kHz between opposite cube faces with an Agilent 4263B component analyzer. Three such measurements may be performed along directions x, y, and z. The conductance tensor component (e.g., σ_x) along a given direction (e.g., x) is computed from the impedance measured along this direction and sample dimensions according to the formula

σ_x = (L_x/L_yL_z)[(R_x – R₀) – j(X_x – X₀)/(R_x – R₀)² + (X_x – X₀)²],

where L is the length and R₀ and X₀ refer to the measured impedance of the filter. Conductance tensor components σ_y and σ_z are obtained by substitution in this formula.

To measure P-wave velocity along a given direction, the sample is held with a force of 49 N (corresponding to a pressure of 120 kPa) between two transducers covered with rubber spacers. The emitter is connected to a pulse generator (Physical Acoustics C-101-HV); the receiver is connected to an oscilloscope synchronized with the pulse generator. The oscilloscope signal is transferred to a computer, and the arrival time is picked and logged automatically. This setup has a delay of 2.10 µs, which is substracted from the arrival time to obtain the travel time. The velocity along a given direction is simply given by the length divided by the traveltime.

When three measurements are performed, the orientation of the tensor cannot be known. However, some simplification may be expected if the sample is almost transversely anisotropic around the axis perpendicular to the main foliation or stratification. The two following definitions appear convenient and are given here with electrical conductivity as example. Apparent anisotropy in the horizontal plane is

α_I = 2[(σ_x – σ_y)(σ_x + σ_y)].

Apparent transverse anisotropy is

α_T = {[(σ_x + σ_y)/2] – σ_z}/{[(σ_x + σ_y)/2] + σ_z}

for a truly transversely anisotropic medium, and in the core reference frame (x, y, and z) the anisotropy ratio α = α_I/α_T is a function of the dip of the foliation in the sample.

In situ temperature measurement

In situ temperature measurement was carried out using the advanced piston coring temperature tool (APCT3) (Fig. F20), which is the third-generation tool of its kind used with the HPCS to measure downhole in situ temperatures. The APCT3 consists of three components: electronics, coring hardware, and software. During this expedition, in situ temperature measurement was basically done for every third core during HPCS coring in Holes C0001E and C0001F from 13.60 to 170.98 m CSF and in Hole C0002D from 15.38 to 158.97 m CSF. The sensor was calibrated for a working range of 0°–45°C.

The electronics fit into a special cutting shoe, which was lowered to the seafloor and shot into the formation. To equilibrate with seafloor temperature, the cutting shoe was held at the mudline for ~5–10 min before penetration. After shooting, it takes ~10 min for the sensor to equilibrate to the in situ temperature of the formation. Mud pumps need to be off during temperature equilibration. Shooting the barrel into the formation normally causes a rapid increase in temperature due to frictional heating. After that, temperature decreases with time along a decay curve. Temperature was measured as a time series with a sampling rate of 1 s. Temperature data were logged onto a microprocessor within the downhole tool; when the tool was retrieved, data were downloaded to the computer.

A typical penetration curve exists of three parts: a rise in temperature at the beginning due to frictional heating, a decay curve as the sensor equilibrates to the in situ temperature of the sediment, and another rise while pulling the tool out (Fig. F21). In situ temperature was calculated by using the decay curve after penetration. Therefore, the interval used for calculation was not disturbed, for example, by vibration that causes frictional heating. Data were processed using the program TP-Fit, which runs on Matlab. The theoretical background of the program was provided by M. Heeseman (pers. comm., 2007). In situ temperatures could be calculated by using the decay curve.

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