Physical properties were measured on unsplit cores and on the undisturbed parts of split cores. The MST was used for nondestructive measurements of wet bulk density, compressional wave velocity, magnetic susceptibility, and natural gamma radiation in unsplit cores. Thermal conductivity measurements were also conducted on unsplit sediment cores and split rock cores. Undrained shear strength and/or unconfined compressive strength tests were performed on split sediment cores, and three-directional compressional wave velocities were measured on both sediment and rock cores. Portions of split cores that were undisturbed by drilling and sampling, gas expansion, bioturbation, cracking, and large voids were used to obtain specimens for index properties measurements (wet bulk density, grain density, dry bulk density, water content, void ratio, and porosity).
Physical properties measurements during Leg 180 were used to obtain (1) near-continuous records for hole-to-hole correlation, construction of complete stratigraphic sequences, and downhole log calibration; (2) sediment properties related to composition and consolidation history, such as porosity, natural gamma radiation, magnetic susceptibility, and shear strength to help constrain the location of unconformities, sediment fracturing, and fluid migration and expulsion; (3) estimates of rock and fracture properties in order to constrain paleo- and contemporary-fault movements, stress orientation, and fluid migration and expulsion; and (4) data for the calculation of synthetic seismograms, such as compressional wave velocity and bulk density, and for the calculation of local heat flow (i.e., thermal conductivity). A graphical representation of the physical properties procedures adopted for Leg 180 is flow-charted in Figure F14.
All physical properties measurements were conducted after the cores had equilibrated to ambient room temperature (i.e., 22º-24ºC) ~2-4 hr after recovery. The first measurement station was the MST, which combines four sensors on an automated track to measure magnetic susceptibility, bulk density, compressional wave velocity, and natural gamma- ray emission on whole-core sections. The respective sensors are the magnetic susceptibility meter (MSM), the gamma-ray attenuation porosity evaluator (GRAPE), the P-wave logger (PWL), and the natural gamma-ray (NGR) detector. The MST measurement intervals and periods for each 1.5-m core section were selected so that the physical properties could be accurately characterized in 12-14 min without hindering the flow of core processing in the laboratory. Sediment cores were split into archive and working halves immediately following thermal conductivity measurements on the whole sediment core. However, the rock cores were split prior to thermal measurements and each measurement was conducted on the half core.
The working half of each unconsolidated sediment and rock section was used for further physical properties measurements. For the unconsolidated sediment sections, P-wave velocity, undrained vane shear strength and/or compressive strength, and water content and grain density were measured. For the rock sections, P-wave velocity, water content, and grain density were measured. Water content and grain density were used to calculate bulk density, porosity, and related parameters. A summary of each of the physical properties measurement procedures for Leg 180 is outlined in the paragraphs below. Refer also to the physical properties handbook (Blum, 1997) for a description of the physical principles underlying the sampling methods.
Magnetic susceptibility, bulk density, and natural gamma-ray emission were generally measured on all cores indiscriminate of collection method, (i.e., APC, XCB, and RCB). The P-wave velocities were measured on all APC-cored intervals and some XCB- and RCB-cored intervals. Measuring P-wave velocities on XCB and RCB cores is not usually recommended because of the likelihood of discontinuous coring and/or a loss of coupling between the liner and the core. However, high frequency of P-wave measurements on XCB and RCB cores was deemed necessary for the earlier sites during Leg 180 to provide a backup data set in the event that logging could not be conducted in unstable boreholes.
In order to collect measurements, individual, unsplit core sections were placed on the MST, which automatically moves the core section through the four sensors on a fiberglass track. The MST data are not always continuous as a function of depth because of the removal of whole-round sections immediately after coring. For those sections cored through XCB and RCB methods, the presence of overcompacted, discontinuous core (biscuits) was noted and subsequently cross-correlated with core descriptions. Upon encountering highly fractured hard rock intervals, whether to carry out MST measurements was decided on a core-by-core basis.
Magnetic susceptibility is used mostly as a proxy for changes in composition that can be linked to depositional cycles. The high precision and sensitivity of susceptibility logs makes this measurement extremely useful for core-to-core and core-downhole log correlation. The magnetic susceptibility was measured with a Bartington meter MS2 using an 80-mm internal diameter sensor loop (88-mm coil diameter) operating at a frequency of 565 Hz and an AF of 80 A·m-1 (0.1 mT) and set on SI units. The sensitivity range was set to the low sensitivity setting (1.0 Hz). The sample period and interval were set to 4 s and 4 cm, respectively. The raw mean value of the measurements was calculated and stored automatically in units of 10-5 SI. The quality of these results degrades in XCB and RCB sections, where the core may be undersized and/or disturbed. Nevertheless, the general downhole trends are useful for stratigraphic correlations. The MS2 meter measures relative susceptibilities, which need to be corrected for volume variations. For core (d) and coil (D) diameters of 66 and 88 mm, respectively, the corresponding correction factor for d/D is 1.48 (Blum, 1997; page 38). During data reduction, it is necessary to convert the relative susceptibility to the volume-normalized magnetic susceptibility by multiplying by 1/1.48 or 0.68. All magnetic susceptibility data presented in the "Physical Properties" figures have been volume normalized.
Bulk density was estimated for unsplit core sections as they passed through the GRAPE, using a sampling period of 4 s every 4 cm on the MST. The gamma-ray source was 137Cs. For each site the GRAPE bulk densities and the bulk densities measured on discrete samples were compared for repeatability.
The P-wave velocity was measured at 4-cm intervals for 4-s periods with the high-resolution P-wave logger (PWL) mounted on the MST. The PWL measures P-wave velocity across the unsplit core sections. To determine the P-wave velocity, the PWL transmits 500-kHz compressional wave pulses through the core at a frequency of 1 kHz. The transmitting and receiving transducers are aligned perpendicular to the core axis while a pair of displacement transducers monitor the separation between the compressional wave transducers. Variations in the outer diameter of the liner do not degrade the accuracy of the velocities, but the unconsolidated sediment or rock core must completely fill the liner for the PWL to provide accurate results.
Natural gamma-ray emissions are a function of the random and discrete decay of radioactive atoms and are measured with scintillating detectors as outlined by Hoppie et al. (1994). During Leg 180, NGR emissions were measured using observation periods suitable for the predicted radioactivity of the recovered core. For those cores believed to contain low radioactive (i.e., carbonate-rich) sediments, NGR was measured for 28 s per each 14-cm length of core, and for non-carbonate cores, NGR was measured for 20 s per each 10-cm length of core. If the carbonate content could not be predicted, the longer sampling period was selected as a default. The NGR calibration was performed at the beginning of the leg, and sample standards were run for instrument accuracy between each site.
Thermal conductivity was measured during Leg 180 using the TK04 system described by Blum (1997). This systems employs a single-needle probe (von Herzen and Maxwell, 1959) heated continuously in "full-space configuration" for soft sediments and in "half-space configuration" for hard rock. Under conditions of moderate to full recovery, thermal conductivity measurements were conducted at a minimum frequency of one every other section and at increased frequencies when time allowed. When recovery was limited, a minimum of one thermal conductivity measurement per core was obtained.
Full-core unconsolidated sediment sections were measured for thermal conductivity using a full-space single-probe TeKa (Berlin) TK04 unit. An aperture was drilled in the outer core liner and the 2-mm temperature probe was inserted into the working half of the core section. Half-core rock specimens were measured for thermal conductivity using the half-space configuration. The needle probe was secured onto the flat surface of the half core. Good coupling with the needle probes was ensured by flattening and smoothing the core surface with carbide grit sandpaper. The samples and needles were then immersed in seawater. This procedure has been used since ODP Leg 140 (Shipboard Scientific Party, 1992c), with the TK04 unit first being used for hard-rock thermal conductivity measurements during ODP Leg 169 (Shipboard Scientific Party, 1998b).
At the beginning of each half-space and full-space measurement, temperatures in the samples were monitored automatically, without applying a heater current until the background thermal drift was determined to be less than 0.04ºC/min. The heater circuit was then closed and the temperature increase in the probe was recorded. This technique proved highly sensitive to small variations in ambient temperature. To account for the sensitivity, core samples and monitor needles were equilibrated to a constant temperature by immersion in a seawater bath for 15 min to 1 hr before measurements. Immersion in seawater kept the samples saturated, improved the thermal contact between the needle and the sample, and reduced thermal drift during the tests.
The reported thermal conductivity measurement for each sample was the average of three repeated measurements for the full-space method and four repetitions for the half-space method. Under conditions where it was necessary to expedite core processing, the thermal conductivity repetitions were truncated after the second measurement if the first two measurements differed by 1% or less.
Data are reported in W·m-1·ºC-1 with a stated error of about 5%. Choice of measurement interval and assessment of thermal stability are automatic with the TK04 meter, which does not require shipboard calibration.
Moisture and density (MAD) measurements (water content, wet and dry bulk density, and grain density) were routinely measured using ~10-cm3 specimens from the split cores. Other related properties, such as porosity and void ratio, were calculated from phase-relation equations. Samples for MAD measurements were collected at a frequency of one per section. However, the sampling frequency was either increased as needed to characterize all significant lithologies throughout the cores or decreased for homogeneous sections. In XCB cores, which frequently showed a biscuiting type of disturbance, particular care was taken to sample undisturbed parts of the core and to avoid drilling slurry. Immediately after the samples were collected, wet sediment mass (Mwet) was measured. Dry sediment mass (Mdry) and dry sediment volume (Vdry) were determined after the samples had dried in a convection oven for 24 hr at a temperature of 105º ± 5ºC. Wet sediment volume (Vwet) was not measured.
The calculation of index properties was derived from both pre-determined parameters and measurements collected during the MAD procedure. Below is a discussion of the parameters and equations used to calculate the index properties.
The mass of salt (Msalt) in the sample is given by:
where s is the assumed
saltwater salinity (0.035) corresponding to a pore-water density (
pw)
of 1.024 g·cm-3 and a salt density (salt)
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), and volume of solids excluding salt
(Vsolid) are, respectively
pw,
salt,
and
salt,
where Mdry and Vdry are the dry mass and volume, respectively.
Wet water content (Wwet) is expressed as the ratio of the mass of pore water to the wet sediment (total) mass, and the dry water content (Wdry) is the ratio of the mass of pore water to the mass of solids (excluding salt; ASTM Standard D 2216-80; ASTM, 1980):
and
Wet bulk density (wet)
was calculated from
wet
= Mwet/Vwet.
Dry bulk density (dry)
is used to estimate the mass accumulation rate for a given depth interval and is
defined by
dry
= Mdry/Vdry.
Sediment grain (solid)
density (solid)
was calculated from
solid
= Msolid/Vsolid.
Porosity (
)
and void ratio (e) were determined assuming that all original sediment
voids (Vvoids) were filled with pore water. Porosity and
void ratio were, respectively, calculated from
= Vvoids/Vwet
and
The chosen method for compressional wave velocity measurements (VP) was dependent on the degree of sediment consolidation. For unconsolidated sediments, the PWS1 and PWS2 insertion probe system was used to measure the transverse and longitudinal (i.e., along the core axis) P-wave velocity. In semilithified sediments and rock cores, the PWS3 contact probe system, described by Boyce (1976), was employed. The hard-rock measurements were made on ~10-cm3 cubes cut perpendicular to the axis of the core at room temperature and pressure.
The PWS1 and PWS2 probe system calculates P-wave velocity based on a fixed distance and measured traveltime. Anisotropy was calculated using the following equation:
where VPt is the transverse compressional wave velocity and VPl is the longitudinal velocity. The velocity meter was calibrated by measuring VP in water.
In addition to traveltime, the PWS3 system measures variable sample thickness with a digital micrometer. Measurements were generally taken once per section, with more measurements taken in sections characterized by varying lithology. In cores that were too consolidated for the PWS1 and PWS2 insertion probes, but too soft or friable to cut into cubes, the PWS3 system was used to measure P-wave velocity in the x direction. In hard rock, cubes were oriented, cut, and then rotated so that the PSW3 could measure velocities in the X, Y, and Z directions.
Undrained shear strength (Su) was estimated using a motorized miniature vane shear apparatus that was inserted into soft sediment and rotated until the sediment failed, following the ASTM D 4648-87 procedure (ASTM, 1987). Difference in rotational strain between the top and bottom of the vane shear spring is measured digitally and the peak shear strength is recorded. Shear strength measurements by this apparatus are reliable up to a threshold of 100-150 kPa.
A pocket penetrometer was used to measure unconfined compressive strength in stiffer sediments. The penetrometer is a small, flat-footed, cylindrical probe that is pushed ~6.5 mm vertically into the split-core surface; the measured resistance (in kg·cm-2) is the unconfined compressive strength, or 2 Su. The values of unconfined compression were converted to values of Su and reported in units of kPa. The maximum strength that can be measured with the pocket penetrometer is 225 kPa. Both vane shear and pocket penetrometer measurements were performed on each section in regions where the Su values ranged between 100 and 150 kPa.
