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doi:10.2204/iodp.proc.310.103.2007 PetrophysicsOffshore, the petrophysics program involved wireline logging and collecting high-resolution, nondestructive measurements on whole cores using the Geotek MSCL. While offshore, the MSCL was outfitted with four sensor types in order to measure gamma density, magnetic susceptibility, transverse compressional wave (P-wave) velocity, and electrical resistivity. Onshore, the MSCL was used for the acquisition of digital line scan images. Other onshore measurements include lower resolution moisture and density, color reflectance, and full-probe needle-point thermal conductivity measurements that were routinely performed manually. A helium gas pycnometer was used to measure the volume (for density determinations) of discrete samples at an approximate resolution of one per section from the working half of split cores. This allowed independent determination of bulk density, dry density, grain density, water content, porosity, and void ratio, which were used to calibrate the high-resolution, nondestructive measurements made with the MSCL. P-wave velocities were measured on the same samples used for density determination at an approximate resolution of one per section from the working half of split cores. Multisensor core loggingThe MSCL has four physical property sensors mounted on an automated track that sequentially measure gamma density, P-wave velocity, noncontact electrical resistivity, and magnetic susceptibility. Whole-core round sections were measured with the MSCL in horizontal setup mode. Standard mode measurements on all cores were undertaken on temperature-equilibrated cores. All core sections and sections from the core catcher were logged on the MSCL. Data quality is a function of both core quality and sensor precision, which is partly a function of measurement time for gamma density and magnetic susceptibility (but not for P-wave velocity). Optimal measurements require a completely filled core liner; additional notes are provided regarding core quality. Fully water-saturated cores provide optimal conditions for P-wave velocity and gamma density measurements. In sections where the core liner was insufficiently saturated, the resultant P-wave measurements were either unreliable or unobtainable (see more detailed notes in “Compressional wave velocity”). The downcore measurement spacing interval selected was 1 cm for all sensors. Calibration procedures for all sensors were performed prior to the start of core logging. Repeat calibrations were conducted by repeatedly logging (one run per ~5–6 core sections) three selected calibration pieces (water-saturated gamma calibration section, resistivity calibration section, and magnetic susceptibility section), providing a check on the calibration. Only if the values departed from an acceptable range was a repeat calibration performed. DensityGamma ray attenuation (GRA) density is measured by determining the attenuation of gamma rays that pass through the cores and is used to estimate bulk density. The degree of attenuation is proportional to the electron density in the gamma path. Gamma attenuation coefficients vary as a function of atomic number, but, as most earth-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 662 keV. Two collimators of 2.5 and 5 mm were available. The maximum measurement resolution obtainable by the sensor is 0.5 cm (using the 2.5 mm collimator), and the minimum integration time for a statistically significant measurement is 1 s. Measurement intervals were set at 1 cm with the count time set at 10 s (the same as for the magnetic susceptibility sensor), and the 5 mm collimator position was selected in order to maximize gamma counts. Calibration of the system was completed using known water/aluminium density standards. Initial calibration was performed using a standard core liner (~0.3 m length) containing a stepped aluminium calibration piece centered within the liner and filled with water. Gamma counts were taken through each stepped piece of aluminium (of different, known thickness) for 30 s. This calibration procedure was repeated as necessary following checks by logging the calibration piece at regular intervals during the logging process (1 calibration run per ~5–6 core sections). In cases where the core was left unsaturated with water, “dry” calibrated processing parameters were used and a cautionary note was added to the file. P-wave velocityTransverse P-wave velocity was measured using two P-wave transducers aligned perpendicular to the core axis with P-waves passing through the core horizontally (in whole-core setup). A compressional wave pulse centered on a frequency of 320 kHz was transmitted through the core. A pair of displacement transducers monitored the separation between the P-wave transducers so that variations in the outside diameter of the liner did not degrade the accuracy of the measured velocities. Measurement spacing was set at 1 cm. This measurement is critically affected by the quality of the core, such that undersized core, where the core is not directly in contact with the liner, severely reduces the quality of this measurement. Water-saturated cores allow optimum quality of P-wave measurements; accordingly, Tahiti cores were seawater-saturated prior to measurement on the MSCL. This was carried out only after considering the effect on the geochemistry of the core; it was thought to be negligible because the highly porous cores were inundated with seawater during drilling. In cases where the core was insufficiently water-saturated and/or contact with the liner was poor, the quality of the P-wave data was considered to be unreliable and either the data were deleted from the processed data or a cautionary note was added to the file. Initial calibration was performed using a distilled water–saturated standard core liner piece (length = ~0.3 m) at known temperature. This calibration procedure was repeated as necessary following checks by logging the calibration piece (for time considerations, the end of the water-saturated gamma calibration piece was used if it was of sufficient length) at regular intervals during the logging process (1 calibration run per ~5–6 core sections). Electrical resistivityElectrical resistivity of sediment cores was measured using the noncontact resistivity (NCR) sensor. The sensor operates by inducing a high-frequency magnetic field in the core, which in turn induces electrical currents in the core. The small magnetic fields generated by these induced electrical currents are measured on a receiver coil and normalized with a third set of identical coils operating in air. The spatial resolution of measurement is ~2–4 cm. The measurement interval selected was 1 cm, as for the other sensors. Initial calibration was performed using five standard core liner sections (length = ~0.3 m) containing water of varying but known salinity. The five standards were made up to concentrations of 35 (35,000 ppm), 17.5, 3.5, 1.75, and 0.35 g/L from an initial concentration of 35 g/L that was successively diluted. This calibration procedure was repeated as necessary following checks by logging a piece of core liner filled with water of a salinity in the middle of the anticipated salinity range of the logged core at regular intervals during the logging process (1 calibration piece per ~5–6 core sections). Magnetic susceptibilityWhole-core magnetic susceptibility was measured on the MSCL using a Bartington MS2 meter coupled to a MS2C sensor coil. The loop sensor had an internal diameter of 80 mm, corresponding to a coil diameter of 88 mm. It normally operates at a frequency of 565 Hz. As a result of the requirements for Expedition 302, two nonstandard loops at slightly offset frequencies were used, and, although this two-loop sensor system was not required for Expedition 310, one of these same loops was used. The loop therefore operates at a slightly offset frequency (513 Hz). This necessitates a correction factor (×0.908) to be applied to the processed data. The MS2 system operates on two fixed sensitivity levels (×0.1 and ×1), corresponding to 10 and 1 s sampling integration periods, respectively. The higher sensitivity (0.1 range) setting results in measurements to the first decimal place, and the resolution of the loop is 2 × 10–6 SI on this range with an effective sensor length of 4 cm. Measurements were made at a spacing of 1 cm on the 0.1 range. 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 these two 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 with a bulk susceptibility of 213 × 10–6 cgs. This calibration piece was centered within a core liner and logged at regular intervals during the core logging process (1 calibration run per ~5–6 core sections). Digital color imaging systemWhile onshore, systematic high-resolution line-scan digital core images of the archive half of each core were obtained using the Geotek X-Y digital imaging system (Geoscan II). This system collects digital images with three line-scan charge-coupled device arrays (1024 pixels each). The image resolution is controlled by the height of the camera and width of the core. The standard configuration for the Geoscan II produces a 100 pixels per centimeter (ppcm) resolution downcore, but for Expedition 310 cores this was increased in order for the line scans to be used more effectively for core description. Thus, settings used were 200 ppcm downcore resolution and 130 ppcm crosscore resolution, with the exception of the first cores scanned (Cores 310-M00023A-1R through 12R), where a resolution of 100 ppcm downcore and 83 ppcm crosscore were used. Synchronization and track control are better than 0.02 mm. The dynamic range is 8 bits for all three channels. The framestore card has 48 MB of onboard random access memory (RAM) for the acquisition of images with an ISA interface card for personal computers. The system was calibrated at the start of each day using color and gray scale. Output from the digital imaging system includes a Windows bitmap (.BMP) file and a compressed (.JPEG) file. The bitmap file contains the original data with no compressional algorithms applied. These were processed using a height reduction of 56% in order to obtain the correct image proportions. All cores were imaged using an aperture setting of f/4.7. Diffuse color reflectance spectrophotometryArchive halves were typically measured at 5 cm intervals using a handheld Minolta spectrophotometer (model CM-2600d). Interval spacing was adjusted appropriately depending on the nature of the core. Black and white calibration of the spectrophotometer was performed every 24 h. Prior to measurement, the core surface was covered with clear plastic wrap to maintain a clean spectrometer window. Spectrophotometric analysis produced three types of data:
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. Moisture and densityMoisture and density (MAD; bulk density, dry density, grain density, water content, porosity, and void ratio) were determined from measurements of the wet and dry mass of core plugs and dry volume. Discrete samples were taken from the working half of split cores, typically at an interval of one sample per section, where core recovery allowed. These samples were dried in an oven at 105° ± 5°C for 24 h followed by cooling to room temperature in a dessicator. Samples were weighed to a precision of 0.001 g using an electronic balance to determine the dry sediment mass (Mdry). Dry volume (Vdry) was determined using a Quantachrome pentapycnometer (helium-displacement pycnometer) with a precision of 0.02 cm3 capable of measuring five samples per run. The volume measurements were repeated a maximum of five times, or until the last three measurements exhibited <0.02% standard deviation, with a purge time of 1 min. A reference volume was included within each sample set and rotated sequentially among the cells to check for instrument drift and systematic error. Salt precipitated in sample pores during the drying process is included in the Mdry and Vdry values. Procedures for the determination of MAD comply with the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM International, 1990). Samples were water-saturated in a vacuum for 24 h and their wet mass (Mwet) measured on an electronic balance to a precision of 0.1 g. The mass of the evaporated water (Mwater) and the salt (Msalt) in the sample are given by Mwater = Mwet – Mdry, and Msalt = Mwater [s/(1 – s)], where s = the assumed seawater salinity (0.035) and corresponds to a pore water density (ρpw) of 1.024 g/cm3 and a salt density (ρsalt) of 2.257 g/cm3. The corrected mass of pore water (Mpw), volume of pore water (Vpw), mass of solids excluding salt (Msolid), volume of salt (Vsalt), volume of solids excluding salt (Vsolid), and the wet volume (Vwet) are, respectively, Mpw = Mwater + Msalt = Mwater/(1 – s), Vpw = Mpw/ρpw, Msolid = Mdry – Msalt, Vsalt = Msalt/ρsalt, Vsolid = Vdry – Vsalt = Vdry – Msalt/ρsalt, and Vwet = Vsolid + Vpw. For all sediment samples, water content (w) is expressed as the ratio of the mass of pore water to the wet sediment (total) mass: w = Mpw/Mwet. Wet bulk density (ρ), dry bulk density (ρd), sediment grain density (ρg), and porosity (ϕ) are calculated from ρw = Mwet/Vwet, ρd = Msolid/Vwet, ρg = Msolid/Vsolid, and ϕ = Vpw/Vwet. Thermal conductivityThermal 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, as it is significantly more conductive than the unconsolidated sediments that it is measuring. Cores were brought into the laboratory and allowed to equilibrate to room temperature over a 12 h period. Thermal conductivity was measured by inserting the needle probe into the sediment subsequent to removal of either the top or bottom end cap. Generally, thermal conductivity (k) is calculated from the following: k(t) = (q/4π) × {[ln(t2) – ln(t1)]/[T(t2) – T(t1)]}, where
The correct choice of t1 and t2 is complex, and commonly thermal conductivity is calculated from the maximum interval (t1, 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 (SAM), employed by the TK04 software, is fitted to the heating curve for all of the time intervals where 20 ≤ t1 ≤ 40, 45 ≤ t2 ≤ 80, and t2 – t1 > 25. In a good measurement, several hundred time intervals along the heating curve can be matched. The best solution is the one that most closely corresponds to the theoretical curve, and this is the output thermal conductivity. Ten measuring cycles were automatically performed at each sampling location and, when obtained, the closest three were used to calculate an average thermal conductivity. Thermal conductivity measurements were taken where material was suitable, in soft sediments, into which the TK04 needles could be inserted without risk of damage. Compressional wave velocityMeasurements of P-wave velocity were made using a P-wave sensor (PWS3) on a modified Hamilton frame velocimeter (Boyce, 1976). Calibration of the system was performed in accordance with Blum et al. (1997). The separation between transducers was calibrated with four polycarbonate standards with varying thicknesses (10–40 mm). The delay time was determined by a linear regression of traveltime versus thickness (15.4–57.5 mm) of aluminum standards. The PWS3 system uses a vertically oriented transducer pair capable of measuring sample cubes or cylinders. P-wave velocity measurements (VP) were measured on discrete samples where cylindrical core plugs could be drilled from the reefal material, typically at a frequency of one per section. An acoustic signal of 500 kHz was transmitted and received by the two transducers. Analog to digital transformation of the signal allowed the seismic signal to be displayed on a digital oscilloscope with the first-arrival waveform automatically picked and velocity calculated. To improve the coupling between the transducer and sample, distilled water was applied to the transducer/receiver heads. First-arrival waveforms from samples where the signal was weak were manually picked. Measurements were performed first on samples dried in the oven for 24 h followed by measurement of the water-saturated sample. 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. |
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