At Site 1175, laboratory measurements were made to provide a downhole profile of physical properties within a basin located immediately upslope from an active OOST fault. With the exception of short sections (<50 cm), all cores were initially passed through the MST before being split. Gamma-ray attentuation (GRA) and magnetic susceptibility measurements were taken at 4-cm intervals with 2-s acquisition times for all cores. P-wave velocity logger (PWL) measurements were taken at 4-cm intervals with 2-s acquisition times for APC cores. No PWL measurements were taken on XCB cores. Natural gamma ray (NGR) was counted every 20 cm for 20-s intervals. Voids and cracking caused by gas expansion degraded MST measurements and were noted in cores between 14 and 200 mbsf. Biscuiting in XCB cores also degraded measurements.
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.
Shear strength measurements were made near the P-wave core measurement locations from the mudline to 205 mbsf, at which point XCB coring began and the cores became too stiff for insertion of the vane shear device. Resistivity measurements were taken at least once per core. Raw data and calculated physical properties data 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 T19 and T20.
Sediment bulk density was determined by both the GRA method on unsplit cores and the mass/volume ("index properties") method 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. F24A, F24B). Below ~205 mbsf, the small diameter of XCB cores results in GRA bulk densities that are, on average, 0.1-0.2 g/cm3 lower than those determined from discrete samples. Both moisture and density measurements on discrete samples and GRA density measurements show similar downhole trends. Between the mudline and 100 mbsf, intervals of increasing and decreasing bulk density probably reflect lithologic variations.
Grain densities determined from dry mass and volume measurements are essentially constant downhole, with a mean value of 2.69 g/cm3 (Fig. F24C). Porosity decreases gradually with depth, from ~62%-70% at the mudline to 53%-60% at 225 mbsf. Between 225 and 435 mbsf, porosity decreases with depth more rapidly than in the upper 225 m to values of 38%-47% by 400 mbsf. The change in the porosity-depth gradient at 225 mbsf correlates with the lithostratigraphic boundary between the upper slope-basin facies at 224.75 mbsf (see "Lithostratigraphy"). There is no clear change in porosity, bulk density, or grain density at 301 mbsf, the boundary between the middle and lower slope-basin facies.
Undrained shear strength measurements were made using a miniature automated vane shear (AVS) and were conducted exclusively in fine-grained silty clays. Shear strengths increase gradually downhole from <15 kPa at the seafloor to values approaching ~100 kPa at 205 mbsf (Fig. F25). Scatter in the data increases below ~145 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 one of two methods depending on core condition. For shallow, nonindurated samples, a needle probe was inserted into the unsplit core for a full-space conductivity measurement. For two samples from below 390 mbsf, insertion of the needle caused fracturing, so a half-space method was used on split cores. Because of poor core recovery and condition, no measurements were made below 397 mbsf. Thermal conductivities range from 0.67 to 1.51 W/(m·°C). (Fig. F26A). Between the mudline and ~300 mbsf, thermal conductivities remain near 1.0 W/(m·°C), ranging from 0.67 to 1.18 W/(m·°C) with no distinct trend. Apparent low thermal conductivities and scatter between 50 and 100 mbsf may be caused by gas expansion in the cores. Thermal conductivities increase below ~300 mbsf to a maximum of ~1.5 W/(m·°C) at 397 mbsf. Measurements are sparse below 350 mbsf.
A conductive heat flow of ~54 mW/m2 was defined by shipboard thermal conductivities and downhole temperature measurements to 273 mbsf (see "In Situ Temperature and Pressure Measurements"). Using this estimated heat flow and measured shipboard thermal conductivities, projected downhole temperatures reach ~22°C at 400 mbsf (Fig. F26B).
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. F27A). In XCB cores, sample cubes were cut and measurements in all three directions were performed 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 P-wave velocity measurements. When cubes were cut, moisture and density samples were generally taken adjacent to P-wave velocity samples.
As at previous sites, few velocity measurements could be obtained along the axis of APC cores because of expansion cracks. Measurements along the x- and y-axes of APC cores cluster around 1550 m/s between the mudline and 90 mbsf (Fig. F27A). Attenuation was high along the x- and y-axes below 90 mbsf, and no reliable measurements could be obtained in the lower part of Unit I. Velocities measured on sample cubes in Units II and III follow a nearly constant velocity-depth gradient from 1600 m/s at 225 mbsf to ~2000 m/s at 400 mbsf. Two excursions from this constant gradient are observed at 285 mbsf and below 400 mbsf, respectively. The upper excursion corresponds to the first sandy layers that were sufficiently lithified to be cut with the saw. These layers also have a high acoustic impedance (Fig. F27B). The lower excursion is represented by only one data point and probably reflects the heterogeneity of the sediment in the lower part of Hole 1175A, where recovery was poor.
Measurements were made on APC cores with a four-needle 30-kHz frequency electrode array. On XCB cores, conductivity was measured on the same sample cubes used for P-wave measurements with a two-electrode 30-kHz frequency system.
Electrical conductivity and formation factor (see "Physical Properties" in the "Explanatory Notes" chapter) measured on sample cubes are given in Table T20. For needle-probe measurements, only the apparent formation factor is given. As at other sites, the formation factor roughly follows the changes in porosity with depth. However, there is no marked discontinuity in the formation factor profile (Fig. F28); notably, the decrease in porosity below the thick ash layers at ~100 mbsf is not reflected clearly in the formation factor trend. The higher measurement scatter between 50 and 70 mbsf may be caused by sediment heterogeneity as it coincides with the presence of turbidites and thin ash layers. However, the scatter observed between 140 and 160 mbsf is probably an artifact as it corresponds to a zone where cores were affected by cracks. These cracks are preferentially oriented along the x-y plane and may contribute to the higher formation factor measured along the core with the needle probe. In Units II and III, the formation factor increases steadily with depth from an average of 4.5 at 225 mbsf to ~7 at 400 mbsf. The sediment displays only weak bedding-parallel parting but develops some anisotropy with burial. The vertical anisotropy averages <20% at the bottom of Hole 1175A.
Site 1175 volumetric magnetic susceptibilities were measured on unsplit cores by the MST (Fig. F29). Uncorrected values of magnetic susceptibility from the Janus database were used. Magnetic susceptibility values show no obvious downhole trend and generally fall between 10 and 50 × 10-5 SI. Several peaks of magnetic susceptibility have values as high as 300 × 10-5 SI. Magnetic susceptibility data increase slightly from 10 - 50 × 10-5 SI at 340 mbsf to 50 - 100 × 10-5 SI at 420 mbsf.
NGR results are presented in counts per second (cps) (Fig. F30). 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 40 cps. A slight decrease in NGR values is observed at 100 and 220 mbsf.
Porosities within the upper slope-basin facies (Unit I) are characterized by high variability and decrease slightly with depth from 62%-70% at the mudline to 61%-68% at ~100 mbsf. This porosity profile may reflect a combination of the rapid deposition and disrupted nature of sediments deposited by slope-failure processes (see "Lithostratigraphy"). Porosity decreases abruptly at ~100 mbsf to values of 57%-61% and then decreases gradually to 220 mbsf at the depth of the transition between the upper and middle slope-basin facies (Unit I/II boundary). Below 220 mbsf (within the middle and lower slope-basin facies), porosity decreases more rapidly with depth than in the upper slope-basin facies, reaching values of 38%-47% by 400 mbsf. The rapid decrease in porosity below 220 mbsf coincides with increasing P-wave velocity. There is no clear change in porosity, bulk density, or grain density at the depth of the middle slope-basin/lower slope-basin facies boundary (301 mbsf, Unit II/III boundary). A spike of high velocity and impedance values 20 m above this transition may correspond to a seismic reflector. The depth of this spike coincides with the depth of the upper unconformity on the depth-converted seismic profile.