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doi:10.2204/iodp.proc.313.102.2010

Physical properties

The primary objective of the petrophysical program was to collect high-resolution petrophysical property data that would

• Enable characterization of lithologic units and formation properties,

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

• Provide data for post-OSP construction of synthetic seismograms and investigation of the characteristics of major seismic reflectors.

Offshore, the petrophysics program involved collecting high-resolution, nondestructive measurements on whole cores using a Geotek MSCL. The MSCL was outfitted 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").

Onshore, petrophysical measurements included NGR (measured on whole-round cores prior to the OSP), digital line scan images, and color reflectance (split cores during the OSP). Lower resolution measurements of thermal conductivity on unsplit cores using a full-space needle probe prior to the OSP and on samples extracted from cores split and measured during the OSP for discrete P-wave velocities and moisture and density (MAD) were routinely performed manually. Discrete P-wave velocities were mostly measured at the same depth as MAD determinations at an approximate resolution of one per core section from the working half of split cores. A helium gas pycnometer was used to measure the volume (for density determinations) of discrete samples at an approximate resolution of one per core 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, all of which were used to calibrate the high-resolution, nondestructive measurements made offshore on whole cores with the MSCL.

Offshore petrophysical measurements

The MSCL has four primary measurement sensors mounted on an automated track that sequentially measure gamma density, P-wave velocity, NCR, and magnetic susceptibility. Two secondary measurement sensors are also present that allow these primary measurements to be corrected for core diameter and temperature. Whole-round core sections were measured with the MSCL set up in horizontal mode. Standard mode measurements were taken on all core sections except for cores below Core 313-M0029A-173R (see "P-wave velocity") after they were equilibrated to ambient temperature. Core catcher pieces were not included in the logging process.

Data quality is a function of both core quality and sensor precision. Optimal measurements for all sensors require a completely filled core liner and fully water-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 magnetic susceptibility sensors are affected by the duration of each point measurement. P-wave velocity, resistivity, and magnetic susceptibility measurements are affected by room and core temperature. The influence of room temperature is corrected for in postprocessing. To provide high-resolution data during Expedition 313, a downcore measurement sampling interval of 1 cm for all sensors was chosen on the basis of the amount of core and time, coupled with attaining optimal resolution of data.

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 (see below) (Table T3). Checks on the full calibration were then performed approximately once every five to six cores. The calibration check 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 performance of the four primary sensors to be assessed quickly for gamma density, P-wave velocity, electrical resistivity, and magnetic susceptibility. Only if the values departed from the acceptable range was a repeat full calibration 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. Gamma density is used to estimate bulk density. The degree of attenuation is proportional to the electron density in the gamma path (Rider, 2006; see 2008GEO.PDF in PHYSPROP in "Supplementary material"). 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 662 keV. Two collimators are available (2.5 and 5 mm). Measurement intervals were set at 1 cm with count time set at 10 s (the same as the magnetic susceptibility sensor) and the 5 mm collimator position selected in order to maximize gamma counts. The resolution with this setup is 0.5 cm.

Calibration of the system for Expedition 313 was completed using stepped water/aluminium density standards (provided by Geotek). Initial full calibration was performed using a standard core liner (~0.3 m length) containing a stepped aluminium calibration piece centered inside the liner, which was filled with water. Gamma counts were taken for 30 s through each of the five aluminium steps of known thicknesses (6, 5, 4, 3, and 2 cm). In addition, the gamma counts of a liner filled with only distilled water was recorded. Regular (every five to six cores) calibration checks were conducted during logging, using the distilled water calibration piece (used for P-wave velocity in the main calibration).

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 320 kHz was transmitted through the core. A pair of displacement transducers monitored the separation between the P-wave transducers so that small variations of the distance (liner diameter) over which the traveltime was measured could be corrected for. As for the other sensors, measurement spacing was set at 1 cm.

Initial calibration was performed using a distilled water–saturated standard core liner piece (~0.3 m length) at known temperature. This was repeated as necessary following checks by logging the calibration piece at regular intervals during the logging process (one calibration check every five to six cores).

This measurement is critically affected by the quality of the core. Poor data results occur where undersized core causes a separation between the core material and core liner and where the core is insufficiently water saturated to allow for optimal propagation of P-waves. During Expedition 313, whenever the core was clearly insufficiently water saturated and/or contact with the liner was poor, the quality of the P-wave data was considered to be unreliable and the data were either deleted from the final processed data set or a note was appended to the comments column in the MSCL spreadsheet. In some cores, the poor quality of core/lack of fluid in the liner resulted in "cycle skipping," where the automatic picker in the software cannot pick the first arrival of the signal and chooses a later waveform. This results in a constant offset in the data, some of which was corrected prior to the OSP (Fig. F13B).

A change in procedure was decided upon during measurements of Hole M0029A cores because of poor or no signal through the sediment. This was attributed to small bubbles in the fluid in the liner, which were thought to impede the passage of the signal through the sediment (see "Physical properties" in the "Site M0029" chapter). Tests indicated that results improved when measurements were made on cores cooled to ~4°C so that gases partially dissolved in the fluid. Cores below Core 313-M0029A-173R were measured in this manner.

Noncontact electrical resistivity

Electrical resistivity of sediment cores was measured using the noncontact resistivity sensor. The sensor operates by inducing a high-frequency magnetic field in the core from a transmitter coil, which in turn induces electrical currents in the core. These currents are inversely proportional to resistivity (Jackson et al., 2006). The small magnetic fields regenerated by these electrical currents are measured by a receiver coil and normalized with a separate set of identical coils operating in air. The spatial resolution of measurement is ~2 cm. The measurement interval selected was 1 cm, as for the other sensors.

Initial calibration was performed using six standard core liner sections (~0.3 m in length) containing water of varying but known salinity. The five standards were made up to concentrations of 35 (35,000 ppmw), 17.5, 8.75, 3.5, 1.75, and 0.35 g/L. This calibration procedure was repeated as necessary following regular checks by logging a piece of core liner filled with saline fluid from the middle of the anticipated salinity range (3.5 g/L) of the logged core.

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 not included. 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 313 is a nonstandard loop operating at a frequency of 513 Hz, which necessitates a correction factor (× 0.908) to be applied to the 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 mode used was adjusted according to the signal response from the cores in order to optimize the data quality. For all cores except a small interval in Hole M0027A, the ×0.1 setting was most appropriate. Measurements were made at a spacing 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 core liner, and calibration checks were carried out approximately every five to six cores to check reliability of the sensor.

Because of a sensor problem in Hole M0029A, the 80 mm loop was changed to a 120 mm loop with a standard operating frequency of 565 Hz for all cores deeper than Section 313-M0029A-120R-1. Caution should be taken with data in cores measured from some depths above the change because of occasional skips in the data resulting from the sensor malfunctioning. Extra care should also be used in analyzing magnetic susceptibility data from Hole M0029A subsequent to the change in procedure (see "P-wave velocity") because of variable unknown effects of colder temperatures on the electronics of the magnetic susceptibility loop.

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. A considerable number of core sections were relogged for QA/QC and will enable more comprehensive postexpedition analysis than has been carried out to date, especially relating to corrections using core liner fill estimations and the effect of measuring cores "cold." The final data set was made available during the onshore phase as raw, processed, and filtered data.

Repeat logging for QA/QC

The reproducibility of the MSCL data was assessed by repeat logging a selection of core sections from each hole while offshore (Fig. F13; Table T4). Attempts were made to log core sections from roughly every 50 m in depth to avoid any bias. Typically sections with good liner-filling were selected to ensure measurements could be obtained from all sensors. A few cores were selected for QA/QC to document the influence of core thickness and drilling mud filling the core liner. When procedural or method changes occurred, such as changing to a larger magnetic susceptibility loop size and measuring cores at a colder temperature, additional cores were relogged to enable evaluation of the effect of the change and to test for consistency of the measurements.

In general, reproducibility is good. Gamma density is very consistent, with deviations <3% (Fig. F13A; Table T4). P-wave velocity, resistivity, and magnetic susceptibility have deviations between 10% and 40% (Fig. F13B, F13C, F13D; Table T4).

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 2 cm sampling interval (instead of the standard 1 cm);

• A column for mud type used (cross-checked with Operations Log);

• Notes on the presence of metal in any section noted and cross-checked with the DIS;

• Notes where cores were measured cold; and

• Notes as to which magnetic susceptibility loop was used.

Core quality and liner fluid fill

In an attempt to allow improvement of the MSCL data set in the future, the percentage of liner fill and the percentage of drill fluid in the liner were estimated and noted on the MSCL logsheets. In particular, electrical resistivity is affected by core liner saturation with drill fluid. Most short sections have low core liner saturation. In general, all Section 3s (generally short sections) are removed from the plotted results in the site chapters.

For gamma density data, it is clear that the tops of cores and ends of cores often have greatly reduced densities. From core inspection, this problem is a result of fractured/disturbed core top and bottom. In case of short Section 2s or 3s, the problem seems persistent, especially if short sections contain hard cemented sediments. The most obvious of these data were removed during the logging process offshore. Other than this, no attempt was made to correct for this, except through numerical high pass filtering (in the results). Future analyses could choose to filter out any sections shorter than a certain length (~20 cm) or to check whether the values of a short section are lower than the average of the core.

Deletion of values around core end-caps

Offshore, operators deleted from the processed files all data around end-caps that were clearly affected by these caps, although the decision was to be conservative if any operator was unsure. 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. For this reason, a column was included in the final spreadsheet listing all values within 10 cm of an endcap.

QA/QC before OSP

MSCL calibration sheets were checked for errors and consistency before the OSP and compiled into a summary file that is accessed for data processing of all raw data files.

MSCL calibration summary file

A MSCL Calibration Summary File was created that included

• A compilation of all the processing parameters for each of the calibrations and

• A calculation of all processing parameters cross-checked between electronic and hard copies.

Reprocessing of raw MSCL data

After correction of calibration files and the calibration summary file, all raw MSCL data files and raw QA/QC data from the offshore part were reprocessed. This involved P-wave velocity and resistivity in the merged MSCL excel sheet. The final data set was made available for the OSP as raw, processed, and filtered data.

Onshore petrophysical measurements

Onshore petrophysical methods are described in the order in which they were done. NGR and thermal conductivity required measurement on whole cores, and 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 record accurate colors before the core dried. The remaining measurements (discrete P-wave and MAD) were conducted on discrete samples taken from the split cores and were done last.

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 (see 2008GEO.PDF in PHYSPROP in "Supplementary 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 log measurements used for core depth corrections. No corrections were made to NGR data from Expedition 313 to account for volume effects related to sediment incompletely filling the core liner.

NGR measurements began 6 weeks prior to the start of the OSP and continued into the second week of the OSP. 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. For the shortest sections, measuring points were individually selected. The count time at each sampling point was set at 3 min. This sampling interval and count time provided the highest resolution and best data quality possible within the time available to complete core logging. Where appropriate, sampling intervals on short cores were manually set so that a greater density of data points could be acquired.

Measurements were carried out at ~5°C in the refrigerated core repository at the Center for Marine Environmental Sciences (MARUM) at the University of Bremen to ensure there was no 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 measurement began, followed by measurement of the background when a core of similar density in a liner was in the machine. This allowed for the correction of data at ambient levels of gamma radiation. These measurements were repeated at the completion of logging. The Geotek XYZ software was used for measuring background NGR and calibration and for standard measurements.

Fourteen core sections across the three holes were relogged with the NGR to assess the repeatability of the results. Generally speaking, the trends observed in the two data sets were very similar. However, original values were usually greater than their QA/QC counterparts (9% higher, on average). There is no correlation between the number of counts and the difference in values between the original and repeated values.

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.

Thermal conductivity of unsplit cores from Expedition 313 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 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 at one location within a core (all cores from Expedition 313 were <3 m), and core sections were sampled at a frequency of one per every five cores within Section 1 where available. If core quality was not appropriate for measurements in the fifth core, the nearest section to it was selected. 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. Twelve additional measurements were taken at the request of an expedition geochemist.

The quality of thermal conductivity measurements was monitored as measurements were taken. Multiple measurements (up to 99 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.

Digital imaging

Digital line scan images of the split cores were obtained during the OSP using the Avaatech Superslit X-ray fluorescence (XRF) core scanner in operation at MARUM. The linescanner is 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 line scan 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) Lab (lightness) a (green to red chromacity) b (blue to yellow chromacity) units (L*a*b*) because of individual charge-coupled device (CCD) pixel calibration. The Line Scan program uses the Stemmer Common Vision Blox (CVB) platform to acquire and process color images.

The camera system contains a CCD camera using 3 × 2048 pixels with beam-splitter and a manually controlled Pentax 50 mm lens. The image resolution is ~150 pixels/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 a maximum of 153 cm downcore.

After every split core was imaged, a color/gray chart was measured with the same settings as those used for the core. Three output files were 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 line scan 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 data set. Software features necessitated the length of line scan images at a couple of centimeters longer than the curated core length. For Expedition 313, 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. Numeric data files were corrected to the same length as the modified .BMP picture files. Sections longer than ~154 cm extend beyond the length of the scanner and thus were scanned twice (opposite ends at the top of the scanner). The images were carefully merged in Adobe Photoshop. 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 the data was accepted.

Diffuse color reflectance spectrophotometry

Archive halves of all split cores were measured during the OSP at a 2.5 cm sampling interval using a Minolta spectrophotometer (model CM-2600d) installed on the MSCL system. Where the split cores were exceptionally underfilled or disturbed, the measurement interval was changed to 10 cm. In certain instances where core sections contained a lot of fluid, the sections were not measured because of the risk that fluid might come into direct contact with the sensor. On this integrated system, the spectrophotometer moves vertically up and down, interlocked with the P-wave equipment. For Expedition 313, color reflectance measurements were carried out separately with all other sensors switched off. 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. Vertical adjustment of spectrophotometer and pusher adjustment to the reference point were performed before every run. The split calibration piece (stepped aluminium) 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 core disturbance, and by the smoothness of the clear plastic wrap over the core surface. Comments for every section were uploaded to the database along with the 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 in order to filter out unreliable or spurious data.

P-wave velocity from discrete samples

P-wave velocity in material removed from a split core can be 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, 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 velocity was measured during the Expedition 313 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 PC. The two transducers also contain receivers. 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 or lithified core specimens. Unconsolidated 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 (s).

The PWL-D was calibrated at the start of each set of samples measured. Calibration involved the 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 measuring session.

Time delays subtracted to correct traveltime are (1) delays related to the transducers and electronics (t_delay) and (2) 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 travel time (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; this determination can be adjusted by the operator. 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. This value represents a delay that 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 313 to prepare the sample correctly in order to get good contact between the transducers. Preparation involved cutting the sample to ensure there were two flat, parallel surfaces to aid in good acoustic coupling with the transducers; surfaces were wetted, and downward pressure was applied. 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 needed to be applied. The P-wave system was calibrated before every measuring session.

P-wave measurements were made on discrete samples taken one per section on average, in most cases at the same depth or near the MAD sample. It was not possible to measure all samples. This was because some samples were not well consolidated and crumbled when placed between the transducers. P-wave measurement was performed three times for each sample to check for consistency. No averaging was made. The data files are in comma-separated value (CSV) format, containing a header with the core and sample ID followed by measured data and calculated velocity. The waveform is recorded in two columns containing the time base and voltage changes.

A batch of samples was remeasured on different days in order to check the repeatability of the data and ascertain the validity of the calibration. Discrete P-wave results were shown to be consistent when measuring these same samples at different times.

Moisture 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. Together with sonic velocity, density measurements are used 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. Density measurements also reflect lithologic variations.

Discrete samples of <10 cm³ were taken from working-half sections and transferred into previously weighed 10 mL glass beakers. Samples were taken at an interval of one sample per section where core quality allowed. An additional 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 60° ± 5°C for a period of 48 h followed by cooling to room temperature in a desiccator. 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 the last three 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 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

• pw = 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 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 313 samples were selected from undisturbed intervals. However, it was not possible to ensure that all were completely uncontaminated by fluid introduced during the core splitting and sampling process. QA/QC assessment included measuring samples taken at the same depth ("different sample, same day") and remeasuring selected samples at different times ("same sample, different day"). The repeated measurements differed by ±2% to ±8% (Table T5). Despite any uncertainties, there is reasonable correspondence between gamma density (MSCL) and wet bulk density (see "Physical properties" in the "Site M0027" chapter).

Porosity values derived from MAD measurements may be underestimated—in particular, sand units by ~10% and perhaps up to 19%, based on measurements after resaturation of dried samples. This is as a result of the high permeability of these sediments, which allow the cores to drain off fluid. Finer grained cores are less permeable and less susceptible to such draining, and as such, the porosity estimates are more accurate.

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