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doi:10.2204/iodp.proc.325.102.2011

Physical properties

The principal objective of the physical properties measurement program for Expedition 325 was to collect high-resolution data that will allow

• Characterization of lithostratigraphic units and formation properties,

• Facilitation of hole-to-hole correlation and the retrieval and construction of complete composite stratigraphic sections together with lithological and sedimentological descriptions for each hole, and

• Provision of data for construction of synthetic seismograms and investigation of the characteristics of major seismic reflectors.

Off shore, the physical properties program involved collecting high-resolution, nondestructive measurements on whole cores using a GEOTEK multisensor core logger (MSCL). The MSCL is equipped with four sensor types in order to measure gamma density, transverse compressional wave (P-wave) velocity, noncontact electrical resistivity (NCR), and magnetic susceptibility. Note that “gamma density” refers to the bulk density of the core as derived from the collimation of gamma rays across the core (see “Gamma density”).

On shore, physical properties measurements included digital line-scan images and color reflectance. Lower resolution thermal conductivity measurements on unsplit cores, using a full-probe needle point inserted into whole cores, were conducted prior to the Onshore Science Party (OSP). Discrete samples taken from split cores during the OSP were measured for P-wave velocity and moisture and density (MAD). P-wave velocities were mostly measured on the same sample (in coral formations) or on a sample from the same depth (in sediments) as the MAD measurements. The resolution of these measurements is approximately one per section from the working half of split cores. Results from discrete sampling can be used to calibrate the high-resolution, nondestructive measurements made offshore on whole cores with the MSCL.

Offshore petrophysical measurements

Multisensor core logger

The MSCL has four primary measurement sensors mounted on an automated track that sequentially measure gamma density, P-wave velocity, NCR, and magnetic susceptibility at a 1 cm sampling interval. Two secondary measurement sensors are also present, which allow the primary measurements to be corrected for core diameter and temperature. Whole-core round sections were measured with the MSCL set up in horizontal mode. Automated measurements were taken on all core sections after they equilibrated to ambient temperature. Core catcher pieces and core shorter than 15 cm were not included in the logging process.

The quality and validity of the MSCL data are a function of both core quality and sensor precision. Optimal measurements for all sensors require a completely filled core liner and fully fluid-saturated cores. In sections where the core liner is insufficiently saturated, the quality of the measurements is compromised (see “Quality assurance and quality control” for an expanded discussion of this). In terms of sensor precision, gamma density and magnetic susceptibility sensors are affected by measurement time, whereas P-wave velocity and resistivity measurements are affected by temperature (see “P-wave velocity” and “Noncontact electrical resistivity”). To provide high-resolution data during Expedition 325, a downcore measurement sampling interval of 1 cm for all sensors was chosen on the basis of the amount of core and time available, coupled with the goal of attaining optimal resolution of data. Cores were allowed to equilibrate to room temperature for a minimum of 6 h prior to measuring.

A full calibration of the MSCL sensors (Table T4) was conducted at the start of the expedition, every time the system was rebooted (for example, after power failure or after transit), and when calibration checks revealed unacceptable departures from full calibration values (see “Quality assurance and quality control”). Checks on the full calibration were then performed approximately once every 5–6 cores or every 6 h. These calibration checks involved logging three calibration reference pieces (magnetic susceptibility piece, standard core liner saturated with distilled water, and standard core liner saturated with 3.5 g/L salinity fluid) and comparison with the values derived during the full calibration. This allows the functionality of the four primary sensors to be assessed quickly.

Measurement principles

Gamma density

Gamma density is measured by determining the attenuation of gamma rays (mainly by Compton scattering) that pass through the cores and is used as a proxy for bulk density. The degree of attenuation is proportional to the electron density in the gamma path (GEOTEK MSCL Manual; geotek.co.uk/downloads/). Gamma attenuation coefficients vary as a function of atomic number, but as most rock-forming minerals have similar and low atomic numbers, the correlation between gamma density and bulk density is generally very good.

A small (370 MBq) ¹³⁷Cs source (half life = 30.2 y) was used to produce a gamma beam with primary photon energies of 0.662 MeV. The 5 mm diameter collimator was used throughout MSCL measurement operations during Expedition 325. The sampling interval was set at 1 cm with count time set at 10 s (the same as for the magnetic susceptibility sensor). The resolution with this setup is 0.5 cm.

Calibration of the system for Expedition 325 was completed using a stepped water/aluminum density standard (provided by GEOTEK). Initial calibration was performed using a standard core liner (~0.3 m length) containing the stepped aluminum calibration piece centered inside the liner and filled with distilled water. Gamma counts were taken for 30 s through each of the five aluminum steps of known thicknesses (6, 5, 4, 3, and 2 cm). In addition, the gamma counts of a liner filled entirely with distilled water were recorded. Regular calibration checks (every 5–6 cores) were conducted during logging, using the distilled water calibration piece. All data were handled using the processing parameters from these wet calibrations. Dry calibrations (air/aluminum) were also conducted, and the processing parameters arising from these are available should reprocessing be required.

P-wave velocity

Transverse P-wave velocity was measured using two P-wave transducers aligned perpendicular to the core axis (in whole-core setup). A compressional acoustic wave pulse centered on a frequency of 230 kHz was transmitted horizontally across the core at each sample point (1 cm sampling interval). The P-wave transducers also function like displacement transducers, monitoring the variations in the outside diameter of the liner. These variations are ultimately used in processing of the gamma density, P-wave velocity, and magnetic susceptibility datasets (NCR is not affected by core diameter). Initial calibration was performed using a distilled water saturated standard core liner piece (~0.3 m length) at room temperature. This was repeated as necessary, following checks conducted by logging the calibration piece at regular intervals during the core logging process (calibration check pieces run every 5–6 cores or every 6 h, as with the other sensors).

This measurement is critically affected by the quality of the core. The data quality is poor where undersized core causes a separation between the core material and core liner and also where the core is insufficiently water-saturated to allow for successful propagation of the acoustic pulse through the core. Prior to Expedition 325, it was decided that core liners would not be resaturated. As a result, core liners were often insufficiently saturated, and P-wave measurement was often unsuccessful because of poor acoustic coupling.

Noncontact electrical resistivity

Electrical resistivity of sediment cores was measured using the noncontact resistivity sensor. The measurement is based on a technique using two sets of coils, allowing for the comparison of readings from the core (one set of coils) with readings from taken in air (one set of coils). This makes measurement of the very small sensor responses possible. In terms of operation, the transmitter coil induces a high-frequency magnetic field in the core. This magnetic field induces electrical currents in the core that are proportional to the core’s conductivity (inversely proportional to the resistivity of the core). Very small magnetic fields are then regenerated by these electrical currents, and these fields are then measured by the receiver coil. The spatial resolution of the measurement is ~2 cm.

Initial calibration was performed using six standard core liner sections (~0.3 m in length) containing fluid of known salinity. The six standards were made up to concentrations of 35, 17.5, 8.75, 3.5, 1.75, 0.35, and 0 g/L. This calibration procedure was repeated as necessary following regular checks every 5–6 cores (or every 6 h) by logging a piece of core liner filled with fluid of salinity in the middle of the anticipated salinity range (3.5 g/L) of the logged core.

Magnetic susceptibility

Whole-core bulk magnetic susceptibility was measured on the MSCL using a Bartington MS2 system composed of a MS2 meter coupled to a MS2C sensor coil. The sensor coil has a diameter of 88 mm, corresponding to a loop diameter of 80 mm. The 80 mm loop used for Expedition 325 is a nonstandard loop operating at a frequency of 513 Hz (standard 80 mm loops usually operate at 565 Hz). A correction factor (×0.908) is therefore applied to the processed data. The MS2 system operates on two fixed sensitivity levels, ×0.1 and ×1. These sensitivity levels correspond to count times of 10 and 1 s, respectively. For Expedition 325, all cores were measured for magnetic susceptibility using the ×0.1/10 s setting because the cores were expected to have very low magnetic susceptibility, giving a poor signal response, and measurements can be optimized in this instance by increasing the count time. Measurements were made at a sampling interval of 1 cm. The sensor automatically zeroes and takes a free air reading at the start and end of each run in order to account for instrument drift by subtraction of a linear interpolation between readings. Magnetic susceptibility data were recorded as corrected volume specific units (× 10^–5 SI).

The accuracy of the magnetic susceptibility sensor was checked using a calibration standard (made of impregnated resin) with a bulk susceptibility of 213 × 10^–6 cgs. This calibration piece was centered within a short section of core liner and logged at regular intervals during the core logging process to check functionality of the sensor.

Because of an intermittent sensor problem, the 80 mm loop was changed to a 90 mm loop for Hole M0047A and for all further holes. The slightly larger loop diameter means that the loop/liner diameter ratio is larger, and the quality of the data may therefore be affected.

Quality assurance and quality control

A quality assurance/quality control (QA/QC) check was carried out in three stages: during the offshore phase of the expedition, before the OSP, and during the OSP. During the offshore phase, QA/QC involved core quality description, use of hard copy and electronic MSCL log sheets and calibration sheets, and repeat MSCL logging of selected cores as described below. Postoffshore systematic cross-checks of electronic calibration, data files, and processed data were made. The final dataset was made available during the onshore phase as raw, processed, and filtered data.

The quality of the MSCL data is greatly affected by the quality of the core being measured. Core quality issues including undersized cores, small segments, and rubble were all prevalent in the Expedition 325 cores. The impact on the measurements is such that data are often underestimated or erratic, and as such, the dataset should be treated with caution.

In addition to sensor calibrations and calibration checks, a QA/QC repeat measurement plan was in place during the offshore phase of the expedition. Twenty-six core sections were relogged while offshore, with sections selected from a variety of depths but otherwise at random to avoid any bias. Gamma density and noncontact resistivity data were the most repeatable of the measurements. Those which gave large differences between the original and repeat measurements can be attributed to core quality issues. The magnetic susceptibility repeated measurements were much closer in value to the original measurements after the loop change (see “Magnetic susceptibility”). Those repeated measurements taken on the 80 mm loop had good agreement in terms of trends but not in terms of absolute values of the original data. Where available (three core sections), the repeated P-wave velocity data was very close in value and trend to the original dataset.

A roll test was also conducted on a section that highlighted the effect of core orientation on different measurements. NCR gave the greatest variation in repeated results, owing to a combination of depth of investigation and core heterogeneity. Gamma density exhibited good agreement in opposite pairs of results (north–south and east–west), which can be explained by the way in which the sensor works. Magnetic susceptibility gave consistently good agreement between all four roll test measurements, which, as with gamma density, can be attributed to the way the sensor operates.

Onshore petrophysical measurements

Onshore petrophysical methods are described in the order in which they were undertaken. Thermal conductivity required measurement on whole cores, and the measurements were therefore taken in advance of the OSP. Line-scanning and color reflectance measurements were conducted on split core sections, and it was necessary to do these measurements as soon after splitting as possible to preserve the integrity of the images and data. The remaining measurements (discrete P-wave and MAD) were conducted on discrete samples taken from the spilt cores and were therefore done last.

Thermal conductivity

Thermal conductivity was measured with the TeKa TK04 system using the needle-probe method in full-space configuration for soft sediments (Von Herzen and Maxwell, 1959). The needle-probe contains a heater wire and calibrated thermistor. It is assumed to be a perfect conductor because it is significantly more conductive than the sediments that it is measuring. Cores were measured cold, at a temperature of 4°–6°C in the reefer at the Center for Marine Environmental Sciences (MARUM) at Bremen University (Germany).

Thermal conductivity of whole cores from Expedition 325 was measured by inserting the needle-probe into the sediment through a small hole drilled into the core liner. The needle was always inserted parallel to the splitting plane of the core to ensure minimum disturbance of cores. Generally, the coefficient of thermal conductivity (k [W/(m·K)]) is calculated from the following:

where

• T = temperature (K),
• q = rate of heat flow through the material (W/m), and
• t₁ and t₂ = time interval (normally 80 s duration) along the heating curve(s).

The correct choice of t₁ and t₂ is complex. Commonly, thermal conductivity is calculated from the maximum interval (t₁ and t₂) along the heating curve where k(t) is constant. In the early stages of heating, the source temperature is affected by the contact resistance between the source and the full space, and in later stages, it is affected by the finite length of the heating source (assumed infinite in theory). The special approximation method employed by the TK04 software is used to develop a best fit to the heating curve for all of the time intervals where

and

A good measurement results in a match of several hundred time intervals along the heating curve. The best solution (output thermal conductivity) is that which most closely corresponds to the theoretical curve. Numerous measuring cycles were automatically performed at each sampling location, and, when obtained, the best five were used to calculate an average thermal conductivity.

Thermal conductivity measurements were taken at one location within a core, and cores were sampled where appropriate. Most of the Expedition 325 cores were corals, which are not appropriate for thermal conductivity measurement owing to voids in the core. Measurements were taken primarily in soft sediments, into which the TK04 needles could be inserted without risk of damage. However, if sediments were partially or completely lithified, a small hole was drilled into the core prior to inserting the TK04 needle.

Digital imaging

Digital linescan images of the split cores were obtained during the OSP using the Avaatech Superslit X-ray fluorescence (XRF) core scanner in operation at MARUM and supported by the DFG-Leibniz Center for Surface Process and Climate Studies at the University of Potsdam (Germany). The XRF scanner has an option for linescan camera and linear light source, and it should be noted that XRF was not taken on the cores. The linescanner produces high-resolution color images and also outputs accurate color data in red-green-blue (RGB) and Commission Internationale d’Eclairage lightness, a (green to red chromacity), and b (blue to yellow chromacity) (CIE-Lab) units because of individual charge-coupled device pixel calibration. The Linescan Program uses the Stemmer Common Vision Blox platform to acquire and process color images.

The camera system contains a three charge-coupled device camera with beam-splitter and a manual controlled Pentax 50 mm lens. The image resolution is ~150 pixel/cm in the crosscore and downcore directions. With an exposure time of 5 ms, a scan speed of 125 mm/s was achieved. The image coverage in the crosscore direction is ~13.5 cm and in the downcore direction a maximum 153 cm.

Every split core was imaged with a color/gray chart beside it, and this scan is available as the original file. Three output files are generated for each core section: a high-resolution bitmap file (.BMP), a compressed image file (.JPEG), and a numeric text file (.TXT). Numeric data are in RGB units. The linescan system was calibrated every 24 h with black and white calibration. All split cores were measured using aperture setting f/5.6+ (a fixed value between 5.6 and 8) and white calibration level f/11+ (a fixed value between 11 and 16). Consistency of equipment settings was chosen over custom settings in order to ensure uniformity of the dataset. Software features necessitate the length of linescan images to be a couple of centimeters longer than the curated core length. For Expedition 325, bitmap picture files were modified to match the length of the cores after the image was taken. Where a whole-round sample had been removed from the core, a foam placer was inserted in its place. The numeric data files were corrected to the same length as the modified .BMP picture files. Both original and corrected files were uploaded to the database. All images were checked by the operator to ensure that the full core section had been captured by the scan before acceptance of data.

Diffuse color reflectance spectrophotometry

Archive halves were typically measured at 2 cm intervals using a handheld Minolta spectrophotometer (model CM-2600d). Interval spacing was adjusted appropriately depending on the nature of the core. White calibration of the spectrophotometer was carried out twice per day, and a calibration for zero was performed once per day on starting up the machine. Prior to measurement, the core surface was covered with clear plastic wrap (Hostaphan) to maintain a clean spectrometer window.

Spectrophotometric analyses produce three types of data:

1. L* (lightness), a*, and b* values, where L* is a total reflectance index, a* is the green (–) to red (+) chromaticity, and b* is the blue (–) to yellow (+) chromaticity (Minolta Spectrophotometer CM-2600d Manual; www.konicaminolta.eu/​fileadmin/​CONTENT/​Measurement_Instruments/​Download/​NEU_Owners_Manuals/​CM-2600d_2500d_IM_English.pdf);

2. Munsell color values; and

3. Intensity values for 31 contiguous 10 nm wide bands across the 360–740 nm interval of the light spectrum with a reflectance range from 0% to 175% and a resolution of 0.01% (Minolta Spectrophotometer CM-2600d Manual).

When utilizing the spectrophotometric measurements, it is recommended that detailed examination of core photos/images and disturbance descriptions/tables is undertaken in order to cull unnecessary or spurious data. However, this screening process was minimized by targeting appropriate locations along the core for measurements with the handheld sensor. Measurements were taken with the instrument horizontal against the split core surface. The location in depth of the measurement was recorded. Measurements were taken in the most homogeneous areas at each depth downcore in order to obtain as pure a color reflectance signal as possible.

P-wave velocity from discrete samples

P-wave velocity in material removed from a split core can be derived from the traveltime of acoustic waves passing through a sample of known thickness. P-wave velocity varies with the lithology, porosity, saturation, and bulk density of the material, as well as state of stress and degree of fracturing. In marine sediments and rocks, velocity values are also controlled by the degree of consolidation and lithification, fracturing, occurrence and abundance of free gas and gas hydrates, and so on. Together with density measurements, sonic velocity is used to calculate acoustic impedance, or reflection coefficients, which can be used to estimate the depth of reflectors observed in seismic profiles and ultimately to construct synthetic seismic profiles.

P-wave velocity was measured during the OSP using a GEOTEK P-wave logger for discrete samples (PWL-D). The equipment consists of a mechanical section containing the transducers (between which the sample to be measured is placed), a set of electronics, and a computer. The two transducers also contain receivers. Acoustic coupling is achieved through solid neoprene surfaces (pads on the transducers) and can be improved by applying downward pressure on the sample between transducers and/or by wetting the neoprene with distilled water. A laser distance transducer measures the thickness of the sample. The PWL-D system can measure velocities on cubic or cylindrical, consolidated or lithified core specimens. Poorly consolidated samples are not suitable for measurement with the PWL-D because they tend to crumble when placed between the transducers. The system allows measurement on split cores in the direction perpendicular to the split core surface.

The basic velocity equation is

where

• d = distance traveled through the material (m) and
• t = traveltime of the P-wave through the material.

The PWL-D was calibrated at the start of each set of samples measured. Calibration involved measurement of a sample of known length and P-wave velocity, and a measurement was also taken with the transducers touching (zero distance). In order to monitor drift of the measurements, the velocity of the calibration piece was also noted at the end of each sample run.

Time delays that are subtracted to correct the traveltime are the delay related to the transducers and electronics (t_delay) and the delay related to the peak detection procedure (t_pulse). Delays were determined during calibration with zero distance. For routine measurements on discrete samples with the PWL-D system, the equation for the velocity is

where

• v_core = velocity through sample (m/s),
• d_core = measured thickness of the sample (mm),
• TOT = measured total traveltime (µs), and
• PTO = delay correction (µs).

A pulse is sent to the transmitter sensor, which generates an ultrasonic compression wave at ~230 kHz, which then propagates through the sample and is received by the receiver sensor. The received signal is processed through an analog to digital converter before being displayed in the software display. The signal is digitized at a sampling frequency of 12.5 MHz.

In the software, a threshold detector determines the first positive or negative excursion on the received pulse and can be adjusted by the operator. The pulse timing is achieved by measuring the time to the first zero crossing after the threshold has been exceeded. In this way, the traveltime measured is approximately one half of the wavelength after the start of the pulse but is measured without any errors caused by signal amplitude. A delay can be used to define the point at which the software should start its threshold detection. The delay should be set before the start of the signal.

Sample quality strongly affects the ability to acquire P-wave velocity data. It was important during Expedition 325 to prepare the sample correctly in order to get good contact between the transducers. Where appropriate, preparation involved cutting the sample to ensure there were two flat, parallel surfaces to aid in good acoustic coupling with the transducers. P-wave velocity is also sensitive to temperature (Leroy, 1969), with P-wave velocity increasing with increasing temperature. Temperature was recorded during every measurement. The P-wave system was calibrated before every measuring session.

P-wave measurements were made on discrete samples taken, on average, once per section. In areas of coral material, one discrete sample was collected and analyzed for both MAD and P-wave. In areas of sediment, separate P-wave and MAD samples were taken, in most cases, at the same depth or near each other. It was not possible to measure all samples because some samples were not well consolidated and crumbled when placed between the transducers. Measurements were performed first on samples straight from the sampling table (“initial”), followed by samples dried in the oven for 24 h (“dry”), and then finally fluid-saturated samples (“resaturated”). Saturation of pore spaces was achieved by placing the sample in a saline solution of 35 g/L (“seawater”) for 24 h while in a vacuum. The P-wave measurement was performed three times at each stage (initial, dry, and resaturated) for each sample for more reliable results.

Data files are in tab-separated value (.TXT) format and contain a header with the core followed by the measured data and the calculated velocity and sample stage (initial, dry, and resaturated). The waveform is recorded in two columns containing the time base and voltage changes.

Moisture content and density

MAD measurements (bulk density, dry density, grain density, water content, porosity, and void ratio) were determined from measurements of the wet and dry masses of core plugs and dry volumes.

Discrete samples of <10 cm³ were taken from the working-half sections and transferred into previously weighed and calibrated 10 mL glass beakers. Samples were taken at an interval of one sample per section, in a zone of geological interest, where core quality allowed. In areas of coral material, only one discrete sample was taken per section for both MAD and P-wave analyses. However, in areas of sediment, a further 3 to 15 cm³ sample was collected as close as possible to the MAD sampling point for discrete transverse P-wave velocity measurements.

Wet samples were weighed to a precision of 0.001 g using an electronic balance (M_wet). Samples were dried in a convection oven at 105° ± 5°C for a period of 24 h followed by cooling to room temperature in a dessicator. Dry sediments were successively weighed (M_dry) and measured using a Quantachrome pentapycnometer (helium-displacement pycnometer) with a precision of 0.02 cm³. This equipment allowed the simultaneous analysis of four different samples and one standard (calibration spheres). Volume measurements were repeated a maximum of five times, or until three consecutive measurements exhibited <0.02% standard deviation with a purge time of 1 min. Volume measurements were averaged per sample (V_dry). Calibration spheres were cycled from cell to cell of the pycnometer during each run to check for accuracy, instrument drift, and systematic error.

The mass of the evaporated pore water (M_pw) is given by

The volume of pore water (V_pw) is given by

where pw = pore water density at standard laboratory conditions (1.024 g/cm³ and 3.5% salinity).

Salt precipitated in sample pores during the drying process is included in the M_dry and V_dry values, resulting in the following approximations:

• The mass of solids including salt (M_solid) is given by the dried mass of the sample (M_dry = M_solid).

• The volume of solids including salt (V_solid) is given by the measured dry volume from the pycnometer (V_dry = V_solid)

• The wet volume (V_wet) is given by

For all sediment samples, water content (w) is expressed as the ratio of the mass of pore water to the wet sediment (total) mass:

Wet bulk density (w), dry bulk density (d), sediment grain density (g), and porosity (φ) are calculated from the previous equations (density is given in g/cm³):

and

Expedition 325 samples were selected from the least disturbed intervals. However, it was not possible to ensure that all were completely uncontaminated by fluid inundation during the core splitting and sampling process.

Porosity values derived from MAD measurements may be underestimated in particular coral units. This is a consequence of the high permeability of these sediments, which allow fluid to drain from the cores during the weighing process. Finer grained sediment cores are less susceptible to such draining, and as such, the porosity estimates are more accurate.

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