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doi:10.2204/iodp.proc.347.102.2015

Physical properties

The primary objective of the petrophysical program was to collect high-resolution physical properties data that will enable

• Characterization of lithostratigraphic units and formation properties,
• Facilitation of hole-to-hole correlation,
• Retrieval and construction of complete composite stratigraphic sections together with lithologic and sedimentological descriptions for each hole, and
• Provision of data for the construction of synthetic seismograms and investigation of the characteristics of major seismic reflectors.

Offshore, the physical properties program involved collecting high-resolution, nondestructive measurements on whole-round cores using a Geotek MSCL. The standard MSCL is equipped with four sensors that sequentially 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 through the core (see “Gamma density”).

The second MSCL system used offshore was a rapid magnetic susceptibility MSCL system “Fast-track” utilizing two offset Bartington loop sensors. This facilitated timely stratigraphic correlation and provided a means to correlate the microbiology holes to the paleoceanographic holes at the same site. The Fast-track MSCL provided the only opportunity to obtain a complete record of petrophysical measurements from the microbiology cores recovered at specific sites, as the cores were rapidly subsampled offshore to prevent microbial degradation.

Onshore, the physical properties program included natural gamma ray (NGR) measurements (measured on whole-round cores prior to the OSP), digital line scan imaging, and color reflectance (measured on split cores during the OSP). Lower resolution measurements of thermal conductivity were also performed on unsplit (whole-round) cores using a full-space needle probe prior to the OSP. During the OSP, separate subsamples for discrete P-wave velocity and moisture and density (MAD) analyses were acquired at an approximate resolution of one per core section (about every 1.5 m) from the working halves of the split core. P-wave velocities were typically not performed on shallow core sections because of the lack of sediment consolidation. If possible, P-wave velocities were measured on samples taken from the same depth interval as the MAD measurements (i.e., from adjacent stratigraphic levels).

Offshore petrophysical measurements: standard MSCL

The MSCL is equipped with four primary sensors mounted on an automated track that sequentially measure gamma density, P-wave velocity, NCR, and magnetic susceptibility. Two additional secondary sensors allow these primary measurements to be corrected for core diameter and temperature. Automated measurements were taken on whole-round core sections after they had equilibrated to ambient room temperature with the MSCL set up in horizontal mode. Core sections shorter than 15 cm were not included in the logging process. Core catcher (CC) intervals were measured if they were longer than 15 cm.

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 for P-wave velocity and gamma density measurements. In sections where the core liner is not filled or the core is insufficiently saturated, the measurement quality is compromised. In terms of sensor precision, gamma density and MS sensors are affected by the duration of each point measurement. P-wave velocity, NCR, and MS are affected by room and core temperature. To attain optimal resolution of data during Expedition 347, a measurement sampling interval of 2 cm was chosen for all sensors, based on the amount of core and time available. A decision was made to change the measurement sampling interval to 1 cm at Sites M0061 and M0062 (Ångermanälven River estuary) to characterize the varved sediments recovered at a higher resolution.

A full calibration of the MSCL sensors 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 the full calibration values (Table T3). Checks on the full calibration were then performed approximately once every 6 h to consistently monitor the machine and sensor performance. The calibration check involved logging three calibration reference pieces (a magnetic susceptibility standard, a core liner filled with distilled water, and a core liner filled with 17.5 g/L salinity fluid [NaCl]) and comparing the results to the values derived during the full calibration. This allowed the performance of the four primary sensors to be assessed quickly and consistently; if the calibration check values departed from the acceptable range, a full calibration was performed (Table T3).

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 to estimate bulk density. The degree of attenuation is proportional to the electron density in the gamma path (Rider, 2006). 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 collimated gamma beam with primary photon energies of 0.662 MeV. Two collimators are available (2.5 and 5 mm). The 5 mm diameter collimator was used throughout MSCL measurement operations during Expedition 347. The standard sampling interval was set at 2 cm (except at Sites M0061 and M0062, where a 1 cm interval was used) with count time set to 10 s (the same as the magnetic susceptibility sensor). The resolution with this setup is 0.5 cm.

Calibration of the system for Expedition 347 was completed using a stepped water/aluminum density standard (provided by Geotek). Initial full calibration was performed using a standard core liner (~0.3 m length) containing a stepped aluminum calibration piece centered inside the liner, which was filled with distilled water (“wet calibration”). Gamma counts were taken for 60 s through each of the five aluminum steps of known thicknesses (6, 5, 4, 3, and 2 cm). In addition, the gamma count of the liner filled with only distilled water was recorded. Regular (every 6 h) calibration checks were conducted during logging using the distilled water calibration piece and compared to the full calibration values. All data were handled using the processing parameters from these wet calibrations. Dry calibrations (air/aluminum) were also conducted and 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 with P-waves passing through the core horizontally (in whole-core set up). A compressional wave pulse centered on a frequency of 230 kHz was transmitted through the core. The P-wave transducers also function as displacement transducers, monitoring the small variations of the outside diameter of the liner over which the traveltime was measured. These variations are ultimately used in processing the gamma density, P-wave velocity, and magnetic susceptibility data sets. Standard measurement spacing was set at 2 cm (except at Sites M0061 and M0062, where a 1 cm interval was used), as for all other sensors.

Initial calibration was performed using a core liner filled with distilled water (~0.3 m in length), measured at a known temperature. The calibration was repeated as necessary when checks revealed a departure from the acceptable initial calibration value. Calibration checks were made by logging the distilled water–filled calibration piece at regular intervals during the core logging process (one calibration check every 6 h).

P-wave measurement is critically affected by the quality of the core. Poor data results occur where undersized core causes separation between the core material and core liner. Also, if the cores are gaseous or insufficiently water saturated, then an optimal propagation of P-waves will not be achieved through the core. During Expedition 347, whenever the core had obvious insufficient water saturation and/or contact with the liner was poor, the quality of the P-wave data was considered to be unreliable and a note was appended to the comments column in the MSCL spreadsheet.

Noncontact electrical resistivity

Electrical resistivity of sediment cores was measured using the NCR sensor. The measurement is based on a technique using two sets of coils, allowing for comparison of readings from the core (one set of coils) with readings taken in air (second set of coils). The transmitter coil induces a high-frequency magnetic field in the core, which induces electrical currents in the core that are proportional to the core’s conductivity (inversely proportional to resistivity). The very small magnetic fields regenerated by these electrical currents are measured by a receiver coil and normalized with the separate set of identical coils operating in air. The spatial resolution of this measurement is ~2 cm. The measurement interval selected was 2 cm (except at Sites M0061 and M0062, where a 1 cm interval was used), the same as for the other sensors.

Initial calibration was performed using six standard core liner sections (~0.3 m in length) containing water of known but varying salinity. Five standards were made up to concentrations (NaCl) of 35, 17.5, 8.75, 3.5, 1.75, and 0.35 g/L. The sixth standard was filled with distilled water. Calibration checks were undertaken every 6 h and consisted of logging a standard piece of core liner filled with 17.5 g/L saline fluid. The calibration procedure was repeated as necessary whenever calibration checks indicated an unacceptable difference from the initial calibration values.

Resistivity measurements indicate the amount and kind of pore fluid in the core sediment. Because these values are integrated over a depth of several centimeters, measurements taken near the ends of a core section are automatically removed. Furthermore, any trends in values toward these deleted values should be used with caution.

Magnetic susceptibility

Whole-core magnetic susceptibility was measured on the MSCL using a Bartington MS2 meter coupled to a MS2C sensor coil. The loop sensor has an internal diameter of 80 mm, corresponding to a coil diameter of 88 mm. The loop used for Expedition 347 was a standard loop operating at a frequency of 565 Hz, which means no correction factor need be applied to the data. The MS2 system operates on two fixed sensitivity levels ×0.1 and ×1, corresponding to 10 s and 1 s sampling integration periods, respectively. For Expedition 347, all cores were measured using the 10 s (×0.1) setting. Magnetic susceptibility measurements were made at a sampling interval of 2 cm (except at Sites M0061 and M0062, where a 1 cm interval was used). The sensor automatically zeroes and takes a free air reading at the start and end of each run 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 754 × 10^–5 SI (598 × 10^–6 cgs). This calibration piece was centered within a core liner, and calibration checks were carried out approximately every 6 h to check reliability of the sensor.

Quality assurance and quality control

Two quality assurance/quality control (QA/QC) checks were carried out, one during the offshore phase of the expedition and one before the OSP. During the offshore phase, QA/QC involved core quality description, use of hard copy and electronic MSCL log sheets and calibration sheets, repeat MSCL logging of calibration standards (calibration checks) every 6 h, and repeat logging of selected cores as described below. Postoffshore systematic cross-checks of electronic calibration, data files, and processed data were made. The final data set was made available during the onshore phase both as raw and processed data.

In addition to sensor calibrations and calibration checks to ensure the accuracy and consistency of the sensors, a QA/QC measurement plan was in place during the offshore phase of the expedition. The reproducibility of the MSCL data was assessed by repeat logging a selection of core sections from each hole. Attempts were made to log core sections from roughly every 50 m in depth to avoid any bias. Typically, sections that were completely filled with sediment were selected to ensure accurate and representative measurements could be obtained from all sensors. A limited number of QA/QC cores were selected to monitor and document the influence of temperature change on the measurements achieved. In this case, each of the selected core sections were first logged “cold” and then relogged every 2 h until they were temperature equilibrated. The temperature of the sediment was measured with a thermometer inserted into the bottom of each of the chosen core sections.

In general, reproducibility was good. Repeat measurements were highly reproducible for gamma density and magnetic susceptibility, but there was often a variation in the NCR values and, occasionally, P-wave velocity. NCR variations sometimes existed between the measurements of individual core sections and particularly at the end of Sections 2 and 3. However, the 6 h calibration checks showed that all sensors were performing perfectly and were well within the acceptable limits of the full calibration parameters. In light of this, the first resistivity sensor was exchanged for the spare one to test a different sensor response. Results from both sensors were equivalent. It was therefore concluded that, as NCR measurements are extremely sensitive and can be adversely affected by temperature, repeat logging after chilling in the reefer could not yield identical results to the original data.

MSCL logsheets

Throughout the expedition, information was recorded on both hard copy and electronic MSCL logsheets in an attempt to ensure all information that affects the quality of the result was retained. QA/QC included

• Observation of core liner fill, fluid content, and cracks in the core;
• Top depths cross-checked with final DIS depths;
• Notes for sections where there were apparently sensor issues;
• Notes for those sections measured at a 1 cm sampling interval (instead of the standard 2 cm); and
• Notes on the presence of metal in any section noted and cross-checked with the DIS.

Deletion of values around core endcaps

Offshore, operators deleted from the processed files all data around endcaps that were clearly affected by these caps. As the amount of data affected varies for each sensor and the effect is gradational away from the end of the core, different analyzers may choose to delete various amounts of data. Therefore data were checked before the OSP for errors and consistency.

The “raw data” have no deleted values. For the “processed data,” endcaps and bad data points (e.g., from metal in the liner or cracks) were removed.

Fast-track MSCL

The Fast-track MSCL comprised two offset 90 mm magnetic susceptibility loops, each taking a measurement every 4 cm; in combination the offset loops rapidly provided data at 2 cm intervals. The Fast-track MSCL was employed during Expedition 347 to provide a rapid logging record of the microbiology cores immediately after they were curated, as the sediments needed to be sampled by the microbiologists as soon as possible after retrieval from the borehole. The Fast-track MSCL therefore provided the only opportunity to obtain a complete composite record of petrophysical measurements on the microbiology cores and greatly facilitated stratigraphic correlation between the cores recovered from individual holes at each site. This aided stratigraphic correlation, enhanced the coring strategy, and helped to optimize recovery both at each site and across all the sites in general.

Magnetic susceptibility

Whole-core magnetic susceptibility was measured on the Fast-track MSCL using two Bartington MS2 meters each coupled to a MS2C sensor coil. Both loop sensors had an internal diameter of 90 mm. The loops used for Expedition 347 were nonstandard loops operating at frequencies of 513 and 621 Hz, which require correction factors of 0.908 (513 Hz) and 1.099 (621 Hz) to be applied to the data. The MS2 system operates on a fixed sensitivity level of 5 s sampling integration periods. For Expedition 347, all cores were measured at a sampling interval of 4 cm, but with two sensors working offset, this resulted in a sampling resolution of 2 cm. The sensor automatically zeroes and takes a free air reading at the start and end of each run 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 sensors was checked using two calibration standards (made of impregnated resin) with bulk susceptibilities of 185 × 10^–5 SI (147 × 10^–6 cgs) and 767 × 10^–5 SI (610 × 10^–6 cgs), respectively. These calibration pieces were centered within a core liner, and calibration checks were carried out approximately every 6 h to check the reliability and consistency of the sensors.

Quality assurance and quality control

QA/QC checks were carried out during the offshore phase. QA/QC involved core quality description, use of hard copy and electronic Fast-track MSCL logsheets and calibration sheets, and regular calibration checks. This consisted of the repeat logging of both calibration standards every 6 h to monitor sensor precision and accuracy. The final data set was made available during the offshore phase as raw data.

Onshore petrophysical measurements

The OSP was held at the BCR from 22 January to 20 February 2014; preonshore measurements were performed between the end of November 2013 and the start of the OSP. Onshore petrophysical methods are described in the order in which they were performed. NGR and thermal conductivity were measured on whole-round cores in advance of the OSP (preonshore). Line scanning and color reflectance measurements were conducted on split-core sections, immediately after splitting, to record accurate colors prior to core desiccation and oxidation. Digital images of archive halves were made with a digital imaging system. Discrete color reflectance measurements of working halves were made with an MSCL system. P-wave and MAD measurements were conducted on discrete samples taken from the working-half cores. A helium gas pycnometer was used to measure the matrix volume of discrete samples at an approximate resolution of one sample per core section. Discrete P-wave velocity measurements were performed on semiconsolidated and consolidated samples with a Geotek DPW sensor.

Natural gamma radiation

NGR emissions of sediments are a function of the random and discrete decay of radioactive isotopes, predominantly those of ²³⁸U, ²³²Th, and ⁴⁰K and their radioactive daughter products (for example, ²⁰⁸Tl, ²²⁸Ac, and ²¹⁴Bi). These decays are measured through scintillation detectors housed in a shielded collector. For Expedition 347, a bismuth germanate (BGO) crystal was used as scintillation material. Total counts per second were measured by integration of all emission counts over the gamma ray energy range between 0 and 3 MeV. Measuring total counts gives a reasonable precision with relatively low counting times (minutes per sample) and is well suited for correlation with core and downhole wireline logging measurements. No corrections were made to NGR data from Expedition 347 to account for volume effects related to sediment incompletely filling the core liner.

Measurements were carried out at 4°–6°C in the refrigerated core repository at the MARUM–Center for Marine Environmental Sciences at the University of Bremen to ensure there was minimal drift in measurement over time. After the equipment became temperature equilibrated, a source of ¹³³Ba and a source of ⁶⁰Co were used for energy calibration of the spectrum. A zero reading of background NGR counts was taken before measurements began, and a calibrated clean sandstone background was also obtained. To obtain the clean sandstone background a small section of liner (~50 cm) was filled with clean, wet sand and the NGR spectra was acquired. This calibrated clean sandstone background allows for the ambient levels of background gamma radiation to be subtracted from the measurement, a method that was used during Expedition 347 preonshore NGR measurements. To track potential drift of the spectra a daily check was performed on standard U and K samples to ensure the K spectra peak was within the correct energy window. These data were recorded at the same time each day and are available on request.

NGR measurements began 6 weeks prior to the start of the OSP and were completed before the OSP began on 22 January 2014. A Geotek XYZ frame system was employed that allowed up to six core sections to be logged during a single run. The gamma ray detector has a measurement window of 7.5 cm, and the sampling interval was set at 9 cm to maximize the resolution of measurements in the time available while minimizing resampling of intervals. The count time at each sampling point was set at 2 min. This sampling interval and count time provided the highest resolution and best data quality possible within the time available to complete the core logging. For the shorter sections (<30 cm), measuring points were individually selected so that a greater density of data points could be acquired, and cores <15 cm in length were not measured. Where cracks >1 cm were present, the interval around the crack was omitted, and the top 5 cm and bottom 5 cm of each core section was discarded to minimize the error associated with the endcaps.

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 measured. Cores were measured at 4°–6°C in the refrigerated core repository at the MARUM–Center for Marine Environmental Sciences at the University of Bremen.

Thermal conductivity of unsplit cores from Expedition 347 was measured by inserting the needle probe into the sediment through a small hole drilled into the core liner parallel to the working half/archive half surface (splitting surface) to ensure minimum disturbance to either split-core section. Generally, thermal conductivity (k) is calculated from the following:

where

• T = temperature (K),
• q = heating power (W/m), and
• t₁, t₂ = time interval (normally 80 s duration) along the heating curve(s).

The optimal choice of t₁ and t₂ is difficult to determine; 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 (SAM) 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.

Thermal conductivity measurements were taken from one location within each core and sampled at a frequency of one per core, preferably from Section 2, where available, for selected holes at each site. If core quality was not appropriate, the nearest section to it was selected. Measurements were taken in soft sediments, into which the TK04 needles could be inserted without risk of damage.

The quality of thermal conductivity measurements was monitored as measurements were taken. Multiple measurements (up to 10 per thermal conductivity run) were taken at the selected depth within each core. Data quality was evaluated by the operator, with spurious or poor data owing to boundary effects and reflections (especially where there was not good contact between the core and the probe [due to cracking]) being removed prior to calculation of a mean thermal conductivity and standard deviation. Where possible, a minimum of four good quality measurements were chosen to calculate the mean thermal conductivity and a variance of less than ±5% was sought. Sometimes the SAM employed by the TK04 software could not calculate a best fit to the heating curve because of boundary effects or a lack of contact between needle and sediment.

Digital imaging

Digital linescan images of the split cores were obtained during the OSP using the Avaatech Superslit X-ray fluorescence (XRF) core scanner. The XRF scanner has an option for linescan camera and linear light source. The line scanner produces high-resolution color images and also outputs accurate color data in red-green-blue (RGB) and Commission Internationale d’Eclairage (CIE) “L” (lightness), “a” (green to red chromacity), and “b” (blue to yellow chromacity) units (L*a*b*) because of individual charge-coupled device (CCD) pixel calibration. The Linescan Program uses the Stemmer Common Vision Blox (CVB) platform to acquire and process color images.

The camera system contains a 3-CCD camera using 3 × 2048 pixels with beam-splitter and a manually controlled Pentax 50 mm lens. The image resolution is ~150 pixel/cm (70 µm/cm) in crosscore and downcore directions. With an exposure time of 5 ms, a scan speed of 125 mm/s was achieved. Added to this is initialization time and camera repositioning after a scan. The image coverage is ~13.5 cm crosscore and in the downcore direction a maximum of 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 were generated for each core section: a high-resolution bitmap file (.BMP), a compressed image file (.JPEG) (see SLABCORESCAN in “Supplementary material”), 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). Additional apertures were used when necessary but were always run in addition to the standard aperture used on all cores. Consistency of equipment settings was chosen over custom settings to ensure uniformity of the data set. Software features necessitated the length of line scan images at a couple of centimeters longer than the curated core length. 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. For Expedition 347, blue pieces of foam labeled “MBio” were used to fill voids created by the large number of microbiology samples that were taken during the offshore phase of Expedition 347 (see “Microbiology”) from the microbiology holes (e.g., M0059C and M0060B). The operator verified that the full core section had been imaged during the scan before the data were accepted.

Diffuse color reflectance spectrophotometry

Working halves of all split cores were measured during the OSP at a 4 cm sampling interval using a Minolta spectrophotometer (model CM-2600d) installed on a Geotek split-core MSCL system. Badly disturbed core sections or core sections shorter than 5 cm were not measured. Core sections with significantly high fluid content were omitted because of the risk of sensor damage. On this integrated MSCL system, the spectrophotometer moves vertically, interlocked with the P-wave equipment. For Expedition 347, color reflectance measurements were carried out separately, with all other MSCL sensors switched off. White calibration of the spectrophotometer was carried out once per day, and a calibration for zero was performed once per day on starting up the machine. Vertical adjustment of spectrophotometer and pusher adjustment to the reference point were performed before every run. The split calibration piece (stepped aluminum) for gamma density was used as the reference height of the spectrophotometer. 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:

• L*, a*, and b* values, where L* is a total reflectance index ranging from 0% to 100%, a* is green (–) to red (+) chromaticity, and b* is blue (–) to yellow (+) chromaticity;
• Munsell color values; and
• Intensity values for 31 contiguous 10 nm wide bands across the 400–700 nm interval of the visible light spectrum.

Measurement quality is affected by the degree and uniformity of sediment fill in the split liner, by cracks or other core disturbance, and by the smoothness of the clear plastic wrap over the core surface (e.g., no air bubbles). Comments for every section (e.g., cracks, holes and voids in sediment, and quality of the sediment surface) were appended to the spectrophotometer data to allow for better retrospective evaluation of data quality. When utilizing the spectrophotometric measurements, it is recommended that detailed examination of core photos/images and disturbance descriptions/tables be undertaken to filter out unreliable or spurious data.

P-wave velocity from discrete samples

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

P-wave analyses were performed with a Geotek P-wave logger for discrete samples (PWL-D), which consists of a mechanical section containing the transducers (between which the sample to be measured is placed), an electronic panel, and a laptop. Acoustic coupling is through solid neoprene surfaces (pads on the transducers) and is improved by applying downward pressure on the sample between transducers and 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, semiconsolidated or lithified core specimens. Unconsolidated samples are not suitable for measurement with the PWL-D because they tend to crumble or squash when placed between the transducers. The system allows measurements to be taken in the x-, y-, and z- directions of the core to study P-wave velocity anisotropy. To know the orientation, the cubes taken were marked on the surface with an arrow pointing to the top of the core.

The basic velocity equation is

where

• d = distance traveled through the material (m) and
• t = traveltime (s).

The PWL-D was calibrated several times a day using a standard of known length and P-wave velocity provided by Geotek. At the start of each set of samples measured, a calibration check was performed using the same standard. To monitor instrument drift, the velocity of the calibration piece was also recorded at the end of each measuring session.

Time delays subtracted to correct traveltime are delays related to the latency of transducers and electronics (t_delay) and delays 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_sample = velocity through sample (m/s),
• d_sample = 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 compressional pulse at ~230 kHz that propagates through the sample and is received by the receiver sensor. The received signal is processed through an analog to digital converter before appearing 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 wavelengths 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 347 to prepare the sample correctly to get good contact between the transducers. Preparation involved cutting the sample to ensure there were two flat, parallel surfaces to aid good acoustic coupling with the transducers, wetting the pads of the transducers, and applying downward pressure. P-wave velocity is also sensitive to temperature (Leroy, 1969) and increases with increasing temperature. Temperature was recorded during every measurement and was so uniform that no temperature corrections were applied.

P-wave velocity was measured on discrete samples taken from the working halves of split cores, at an average of one per section in semiconsolidated or consolidated sediments and adjacent to the MAD samples. Relatively shallow sections were typically not measured because of the lack of the requisite consolidation.

P-wave measurements were performed three times in each direction (x-, y-, and z-directions), and an average was calculated for each direction. The data files are in comma-separated value (CSV) format, containing a header with the core and sample identifier followed by measured data and calculated velocity. The waveform is recorded in two columns containing the time base and voltage changes.

Moisture and density

Moisture and density (MAD) properties (bulk density, dry density, grain density, water content, porosity, and void ratio) were derived from measurement of the wet and dry masses of core samples and their dry volumes. Discrete samples (~6 cm³) were acquired at an approximate resolution of one per core section from the working halves of split cores at vertically adjacent stratigraphic levels to the P-wave subsamples.

These wet samples were transferred into previously weighed 10 mL glass beakers and weighed to a precision of 0.001 g using an electronic balance to determine the wet mass (M_wet). Samples were next dried in a convection oven at 60° ± 5°C for a period of 24 h followed by cooling to room temperature in a desiccator for at least 2 h. Dry sediments were successively weighed to determine dry mass (M_dry).

The volume of dried sample was immediately analyzed using a Quantachrome pentapycnometer (helium-displacement pycnometer) with a precision of 0.02 cm³. This equipment allowed the simultaneous analysis of four samples and one standard (calibration spheres). Volume measurements were repeated a maximum of five times or until the last three measurements exhibited <0.01% standard deviation for Site M0059, with a purge time of 1 min. After this, to speed up the pycnometer process and as no difference could be observed between using 3 and 5 measurements, volume measurements were only repeated 3 times with a purge time of 1 min for the following sites. Volume measurements were averaged per sample (V_dry). Calibration spheres were successively cycled between different pycnometer cells for each run to check for accuracy, instrument drift, and systematic error.

The mass of the evaporated water (M_water) is given by

The volume of pore water (V_pw) is given by

where

• M_pw = mass of the pore water and
• ρ_w = pore water density (1.024 g/cm³).

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

• The mass of pore water (M_pw) is given by the mass of the evaporated water (M_water = M_pw);
• The mass of solids, including salt (M_solid), is given by the dried mass of the sample (M_dry = M_solid); and
• The volume of solids, including salt (V_solid), is given by the measured dry volume from the pycnometer (V_dry = V_solid).

The mass of the evaporated water (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 347 samples were selected from undisturbed intervals. However, it was not possible to ensure that all were completely uncontaminated by fluid introduced during the core collection, splitting, and sampling process.

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