At Site 1176, laboratory measurements were made to provide a downhole profile of physical properties within a slope basin overlying accreted sediments. With the exception of extremely short (<50 cm) sections, all cores were initially passed through the MST before being split. Gamma-ray attenuation (GRA) and magnetic susceptibility measurements were taken at 4-cm intervals with 2-s acquisition times for all cores. Natural gamma ray (NGR) was counted every 20 cm for 20-s intervals.
Moisture and density samples were selected from undisturbed core at regularly spaced intervals of one per section. Measurements of dry volume and wet and dry mass were uploaded to the Janus database and were used to calculate water content, bulk density, grain density, porosity, void ratio, and dry bulk density. P-wave velocities were measured on split cores or discrete samples at a frequency of two to three per core. Measurements were taken in three directions when core conditions permitted.
Undrained shear strength measurements were made near the P-wave core measurement locations from between the mudline and 170 mbsf, at which point XCB coring began and the cores became too stiff for insertion of the vane shear device. Conductivity measurements were taken at least once per core. Raw data and calculated physical properties are available from the Janus database for all MST, moisture and density, thermal conductivity, velocity, and shear strength measurements (see the "Related Leg Data" contents list). Because electrical conductivity data are not currently available from the database, they are included in Tables T17 and T18.
Sediment bulk density was determined by both the GRA method on unsplit cores and the mass/volume method ("index properties") on discrete samples (see "Physical Properties" in the "Explanatory Notes" chapter). The GRA density data and the bulk densities determined by the mass/volume method are generally in good agreement for APC cores (Fig. F20A, F20B). Below ~170 mbsf, the small diameter of XCB cores results in GRA bulk densities that are 0.1-0.2 g/cm3 lower than those determined from discrete samples. Both moisture and density measured on discrete samples and GRA density measurements show similar downhole trends. The GRA density measurements are more closely spaced than moisture and density samples and show detailed bulk density variations within lithostratigraphic Unit I (upper slope-basin facies) that probably reflect lithologic variations. Both methods show shifts to higher bulk densities at ~60 mbsf (from ~1.55 above to ~1.65 g/cm3 below) and ~220 mbsf (from ~1.75 above to ~1.85 g/cm3 below).
Grain densities determined from dry mass and volume measurements are slightly lower in Unit I (average of 2.66 g/cm3) than in Unit III (accretionary prism) (average of 2.71 g/cm3), whereas values are scattered within Unit II (middle slope-basin) (Fig. F20C). In Units I and II, porosities decrease gradually with depth, from ~65%-70% at the mudline to 55%-60% at 220 mbsf. Porosities decrease by 3%-5% at the upper boundary of Unit III then decrease gradually with depth to ~300 mbsf. Below 300 mbsf, porosities remain relatively constant at between 40% and 45%.
Undrained shear strength measurements were made using a miniature automated vane shear (AVS) and were conducted exclusively in fine-grained silty clays. Below 170 mbsf (Core 190-1176A-19H), samples were sufficiently indurated that insertion of the AVS caused fracturing of the sediments. Shear strengths increase gradually downhole from ~10 kPa at the seafloor to values approaching ~80 kPa at 170 mbsf (Fig. F21). Scatter in the data increases between 50 and 110 mbsf as a result of fracturing of sediment and opening of fractures at the tips of the AVS vanes during some measurements. For this reason, actual sediment strength is probably best reflected by the highest measured values.
Thermal conductivity was measured using a needle probe that was inserted into the unsplit core for a full-space conductivity measurement. In Units I and II, thermal conductivities generally range between 1.0 and 1.1 W/(m·°C) (Fig. F22A). The few thermal conductivity values obtained below this depth are significantly higher than those above ~220 mbsf. A conductive heat flow of 56 mW/m2 was defined by shipboard thermal conductivity and downhole temperature measurements to 248 mbsf (see "In Situ Temperature and Pressure Measurements") (Fig. F22B).
In APC cores, P-wave velocities were measured using the P-wave sensors 1 and 2 (PWS1 and PWS2) insertion probe system along the core axis (z-axis) and across the core axis (y-axis), respectively. The PWS3 contact probe system was used to measure P-wave velocities across the core liner (x-axis) (Fig. F23A). In XCB cores, sample cubes were cut and measurements were performed in all three directions using the PWS3 contact probe system. Acoustic impedance was computed as the product of bulk density and velocity along the z-axis. Bulk densities were obtained from moisture and density samples, and values were used only when acquired within 20 cm of the P-wave velocity measurements. When cubes were cut, moisture and density samples were generally taken adjacent to P-wave velocity samples.
Few velocity measurements could be obtained along the axis of APC cores because of gas expansion cracks, but measurements along the x- or y-axes of APC cores could be obtained in almost all cores (Fig. F23A). An increase in velocity from 1520 to 1620 m/s is observed between 57 and 65 mbsf. This increase correlates with an increase in bulk density and may correspond to a laterally continuous reflector on the seismic section. Velocities acquired between this level and ~180 mbsf display some scatter and do not readily correlate with specific seismic reflectors. The transition from Unit II to Unit III is marked by a strong gradient in velocity, with an increase from 1630 to 1830 m/s over a 20-m interval. This transition is also well marked on the impedance plot and thus may correspond to a seismic reflector (Fig. F23B). In Unit III, recovery was poor and measurements were infrequent. Unlike Site 1175, there is no velocity increase in the lower part of the hole and velocities at 400 mbsf are comparatively low, between 1700 and 1800 m/s.
Measurements were made on APC cores with a four-needle 30-kHz electrode array. On XCB cores, conductivity was measured on the same sample cubes used for P-wave measurements with a two-electrode 30-kHz system. Electrical conductivity and formation factor (see "Physical Properties" in the "Explanatory Notes" chapter) measured on the sample cubes are given in Table T18. For needle-probe measurements, only the apparent formation factor is given. The formation factor displays the same sharp boundaries as porosity and P-wave velocity at ~60 and 200-220 mbsf. The formation factor ranges from 2.3 to 3 between 0 and 60 mbsf and from 3 to 4.5 between 60 and 220 mbsf and increases to ~5.5 at 220 mbsf (Fig. F24). The formation factor in Unit III could only be measured at the very top of the unit and near the bottom of Hole 1176A in the few layers that were recovered. These samples are poorly sorted sand/silt/clay mixtures and have anisotropic properties with a bedding-parallel formation factor of ~5 and a bedding-transverse formation factor of ~6 (Table T17).
Volumetric magnetic susceptibilities were measured in all recovered cores from Site 1176 (Fig. F25). Uncorrected values of magnetic susceptibility from the Janus database were used. Magnetic susceptibility values show no obvious downhole trend and generally fall between 10 × 10-5 and 40 × 10-5 SI with little scatter. Several magnetic susceptibility peaks have values as high as 300 × 10-5 SI. Between 220 and 250 mbsf, magnetic susceptibility values increase slightly from 10 × 10-5-30 × 10-5 to 50 × 10-5-120 × 10-5 SI.
NGR results are presented in counts per second (cps) (Fig. F26). The background scatter, produced by Compton scattering, photoelectric absorption, and pair production, was measured at the beginning (6.39 cps) and subtracted from the measured gamma-ray values. In general, NGR counts are low and are consequently likely to be affected by the short counting interval and by porosity variations. Overall, NGR data show considerable scatter between 15 and 30 cps. A slight decrease of NGR values is observed at 60 and 180 mbsf.
Porosities decrease gradually with depth in the upper slope-basin facies (mudline to ~200 mbsf). Between the mudline and 50 mbsf, a slight decrease in bulk density is observed. Within this interval, zones of increasing and decreasing bulk density are observed. This observation may be related to slump structures. No clear changes in bulk density or porosity occur at the boundary between the upper and middle slope-basin facies (Unit I/II boundary). In the middle slope-basin facies (Unit II), there are no obvious porosity changes. Changes in moisture and density correlate with the boundary between the middle slope-basin and accretionary prism facies (Unit II/III boundary). Porosity decreases from 56% to 48%-54% at 225 mbsf. Velocity and formation factor increase at the top of Unit III. Between 225 and 300 mbsf, porosity decreases rapidly with depth, reaching ~40%-47%. Porosity does not change significantly between 300 mbsf and the last measurement at 405 mbsf.