The purposes of physical properties measurements during Leg 178 were (1) to provide near-continuous records for hole-to-hole correlation, the construction of complete stratigraphic sequences, and core-to-downhole log ties; and (2) to provide estimates of sediment properties, which can be used to reconstruct glacial and interglacial/proglacial depositional processes.
The physical properties measurements were made only after the cores had equilibrated to ambient temperature, 3-4 hr after recovery. The first measurement station was the multisensor track (MST), which combines four sensors on an automated track to measure magnetic susceptibility, bulk density (by gamma-ray attenuation), P-wave velocity, and natural gamma-ray emission, on whole-core sections. The maximum run time for a section was 11 min, 24 s. Next, thermal conductivity was measured on whole-core sections at sites where downhole temperature measurements were taken.
The cores were then split, and P-wave velocity and shear strength were measured on the working half. Moisture content and density measurements were made on samples taken either one per section or three per core. Most of the methods are described in detail in the Physical Properties Handbook (Blum, 1997), and a summary of procedures and details of the resolution of these measurements is given below.
Magnetic susceptibility was usually measured with a Bartington meter MS1 using an 8-cm loop and the low-sensitivity setting (1.0 Hz). Sample periods were 2 s, and sample intervals were set at 2 cm. The raw mean value of the measurements was calculated and stored automatically. These values can be converted to volume-normalized magnetic susceptibilities by multiplying by ~0.7 x 10-5 (Blum, 1997).
In addition, magnetic susceptibility measurements were taken at 1-cm intervals using the Bartington magnetic sensor probe on several split cores from Sites 1095 and 1096 with the aim of obtaining a more detailed susceptibility record to correlate with previously studied piston cores from the same sediments (Pudsey and Camerlenghi, 1998).
Bulk density was estimated for whole-round core sections using a sampling period of 2 s every 2 cm on the MST. The gamma-ray source was 137Ce. The calibration was based on aluminum standards of different thickness mounted in a water-filled core liner (Blum, 1997). The calibration takes into consideration (1) the higher Compton scattering in water, (2) the changes in GRAPE densities resulting from different count rates for calibrations using uncombined pure standards, compared with sediments in which the solid and liquid components are mixed pervasively (Weber et al., 1997), and (3) correction for the core liner. For each site, the GRAPE bulk densities and the bulk densities measured on separate samples were compared.
P-wave velocity was measured over 2 s in steps of 2 cm on whole-round core sections, orthogonal to the core axis, with the P-wave logger (PWL) mounted on the MST.
Natural gamma-ray emissions were measured over 15 s for every 15 cm length of the core. Calibration was performed at the beginning of the leg, and sample standards were measured at the end of every site. Background radiation, determined with a water core, was ~3 counts per second (cps). Background radiation has not been subtracted from data presented in the site chapters. The total counts were useful for definition of some lithologic trends.
Thermal conductivity was measured during Leg 178 when required for geothermal heat flow determination, using the TK04 system described by Blum (1997) and available on board ship since ODP Leg 168. It employs a single-needle probe (Von Herzen and Maxwell, 1959) heated continuously, in full-space configuration for soft sediments (via a hole in the liner of an unsplit core), and in half-space configuration (embedded in clear plastic in good thermal contact with a flat surface) for hard rock. Thermal contact for the half-space method is made using Type 120 Thermal Joint Compound (Wakefield Engineering, Wakefield MA); the measured sections are marked with a yellow caution label.
Data are reported in W/(m·K), with a stated error of ~5%. Choice of measurement interval and assessment of thermal stability are automatic with the TK04. In contrast with the system it replaced, it requires no calibration beyond a simple check against a Macor standard.
The JOIDES Resolution carries three tools for downhole temperature measurements, two of which were used during Leg 178. The Adara temperature tool, a thermistor and its associated control, measurement, and memory circuitry, is built into an APC cutting shoe and may be used provided the formation is not too hard. Normally its use requires that, after the APC has fired, it remains motionless in the sediment for at least 8 min to record the temperature decline toward ambient values after frictional heating during corer penetration has dissipated. The thermal decay curve is modeled to determine in situ temperature. It is inadvisable to use this tool within the top 30 m of a hole because the sediment at shallow depth may be insufficiently consolidated to prevent bit movement over this 8-min period, causing additional frictional heating and disturbing the thermal decay. During Leg 178, the Adara temperature tool was also used to obtain a bottom-water temperature by pausing for 10 min above the mudline before firing the first core. In this mode, the core cutter is slightly recessed within the outer bit for the measurement period, but it equilibrates rapidly with seawater.
The second tool is the Davis-Villinger probe (Davis et al., 1997), considerably more rugged than the Adara temperature tool and capable of use in more indurated sediments. It latches into the base of the bottom-hole assembly (BHA) so that a narrow cone, which contains two thermistors and projects 1.4 m ahead of an XCB or RCB bit, may be pushed into the underlying sediments. It needs a separate wireline trip between cores for its operation, which also includes a 5- to 10-min period in the sediments. It is therefore more time consuming to use than the Adara temperature tool. Temperature determination is as for Adara temperature tool data.
Moisture and density (MAD) measurements were made on ~8-cm3 samples taken from the working half of cores as required for ODP shipboard procedures (gravimetric/volumetric determinations of water content, bulk density, grain (solid) density, and related properties such as porosity, void ratio, and dry density). The measured parameters are initial wet bulk mass, then dry mass and volume after drying the samples in a convection oven for 24 hr at 105ºC. Masses were measured with an electronic balance (precision = ±0.01 g), and volumes with a helium-displacement pycnometer (precision = ~±0.02 cm3).
Samples for MAD measurements were taken at either one per core section or three per core. All significant lithologies throughout the cores were sampled. In XCB cores, which commonly showed a biscuiting type of disturbance, particular care was taken to sample undisturbed parts of the core and to avoid the drilling slurry.
The choice of method for compressional wave velocity measurements (Vp) depended upon the degree of consolidation of the sediments. Two P-wave transducer pairs (PWS1 and PWS2), located so as to examine the same sediment interval, were inserted along and orthogonal to the core axis in sections of soft sediment, to examine sediment anisotropy. Transducer spacings were 69.5 (PWS1) and 34.8 mm (PWS2). In harder sediments, transverse P-wave velocity was measured using the PWS3 contact probe system (a modified Hamilton Frame). Measurements were made across the split-core section, through transducer contact with the sediment on top and the core liner on bottom. All signals were digitized by an oscilloscope, and the raw waveform was saved to a computer using either 500 or 1000 points.
The PWS1 and PWS2 split-core velocimeter calculates velocity based on a fixed distance and measured traveltime. Anisotropy is then calculated by the difference between these horizontal and vertical velocities 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 using a digital micrometer that is zeroed periodically. The calibration procedures for both the PWL and PWS3 followed Blum (1997). Measurements were taken once per section and at lithologic changes.
Undrained shear strength (Su) was estimated using a vane that is inserted into soft sediment and rotated until the sediment fails. The apparatus is a motorized miniature vane shear, following the ASTM D 4648-87 procedure (ASTM, 1987). The difference in rotational strain between the top and bottom of a spring is measured digitally.
Because there are potential sources of error using the motorized vane shear device at Su >100-150 kPa, the pocket penetrometer was used when Su values were >100 kPa. The pocket penetrometer is a flat-footed, cylindrical probe that is pushed 6.4 mm deep into the split-core surface. The resulting resistance is the unconfined compressive strength, or 2 Su. The maximum Su that can be measured with the pocket penetrometer is 220 kPa.