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doi:10.2204/iodp.proc.322.103.2010

Physical properties

The goal of physical property measurements in Hole C0011B was to provide high-resolution data on the bulk physical properties and their downhole variations. Measurements included natural gamma radiation, gamma ray attenuation (GRA) density, electrical resistivity, magnetic susceptibility, thermal conductivity, moisture and density (MAD) properties (bulk and grain density, porosity, and water content), and P-wave velocity (see "Physical properties" in the "Methods" chapter). All physical property measurements were made after cores had been imaged by X-ray CT and had equilibrated to room temperature (~20°C). Whole-round multisensor core logger (MSCL-W) data were collected on whole-round sections to define natural gamma radiation, GRA density, noncontact resistivity, magnetic susceptibility, and P-wave velocity. Thermal conductivity was measured using either a full-space needle probe method on whole-round cores or a half-space line source method on working halves of cores. The half-space method was used on sediments below 365 m CSF because the needle probe could not be inserted into lithified sediment. MAD analyses were performed on discrete samples collected from the working halves and cluster samples taken adjacent to whole-round samples. Sample cubes cut from working halves were used to measure P-wave velocity and electrical resistivity in the x-, y-, and z-directions (see Fig. F17 in the "Methods" chapter).

MSCL-W

GRA density

GRA density generally and gradually increases with depth through lithologic Units II, III, and IV (Fig. F46). Calculated density values range from 0.86 to 0.95 g/cm³ over the cored interval. Subtle increases and decreases in density occur throughout Unit II and a slightly higher density unit occurs from 620 to 638 m CSF. High-density intervals exist in Unit IV and in the lower section of Unit III. These GRA density data are not representative of formation bulk density because the cores are smaller than the inner diameter of the core liner. The presence of air around the core creates low-density artifacts.

Natural gamma radiation

Natural gamma radiation increases downhole through Unit II and into Unit III and then remains approximately constant through Units III, IV, and V with the exception of the upper portion of Unit IV (Fig. F46). Natural gamma radiation averages 22.6 counts per second (cps) in the upper portion of Unit II, 33.7 cps in the lower portion of Unit II, 44.6 cps in Unit III, 43.5 cps in Unit IV, and 40.7 cps in Unit V. These variations are consistent with LWD data that are interpreted to represent decreasing sand content with depth through Units II and III (Fig. F47). Natural gamma radiation also shows a step increase from Units II to III consistent with a lithologic change downhole to sediments dominated by hemipelagic mudstone. Unit IV begins with an increase in natural gamma radiation followed by a decrease to a near-constant average value. Natural gamma radiation in Unit V has a decreasing trend. These data proved valuable for identifying correlatable intervals between Holes C0011A and C0011B (see "Logging and core-log-seismic integration") and linking the gamma ray response to lithologic character (see "Lithology").

Magnetic susceptibility

Magnetic susceptibility appears to decrease downhole, ranging from 8.9 × 10^–5 CMS to 1393 × 10^–5 CMS within the upper portion of Unit II and 3.0 × 10^–5 to 806.0 × 10^–5 CMS within the lower portion of Unit II, whereas the maximum magnetic susceptibility within Unit III is 127.5 × 10^–5 CMS (Fig. F46). Spikes of high magnetic susceptibility in Unit II correlate with tuffaceous and volcaniclastic sandstones (Figs. F2, F3, F4), thus relating magnetic susceptibility to grain size and sediment composition. Two distinct magnetic susceptibility zones exist within Unit III: above 616 m CSF resistivity is variable with a slight downhole increase; however, a small step decrease occurs at 616 m CSF, and through the rest of Unit III, magnetic susceptibility is constant (Fig. F46). This shift in magnetic susceptibility coincides with a change in the rate of hemipelagic sediment accumulation from 7.7 to 3.9 cm/k.y. (Fig. F16) and may also correspond to a change in clay mineralogy. Unit IV begins with a peak and an increase in magnetic susceptibility, below which it decreases to levels similar to those measured in the lower portion of Unit III (Fig. F46).

Electrical resistivity

Electrical resistivity is generally low, with >90% of the values throughout Hole C0011B <5 Ωm, but locally high-resistivity layers, up to 60.4 Ωm, are present (Fig. F46). Throughout Hole C0011B, resistivity varies greatly; at times variations correlate with LWD data from Hole C0011A and provide ties from cores to logs (see "Logging and core-log-seismic integration"). Usually variations in electrical resistivity correlate with variations in natural gamma radiation, providing additional information on lithologic variability (Fig. F46).

P-wave velocity

Anomalously low P-wave velocity data were recorded throughout Hole C0011B with the MSCL-W P-wave system. This likely results from insufficient transmission of compressional waves between transducers. This was caused by cores that across sediment cores, which did not completely fill the core liner.

MAD measurements

MAD measurements in Hole C0011B provide a detailed characterization of grain density, bulk density, porosity, void ratio, and water content. All MAD data are provided in Tables T12 and T13 and are summarized below.

Bulk and grain density

Bulk density of mudstone and sandstone discrete samples increases downhole as anticipated from increasing overburden and normal consolidation (Tables T12, T13). Mudstone bulk density ranges from 1.54 to 2.55 g/cm³, whereas sandstone bulk density ranges from 1.39 to 2.19 g/cm³ (Fig. F48A). Mudstone and sandstone bulk density increases slightly with depth in Unit II (Fig. F48A). The Unit II/III transition has a small but abrupt local increase in bulk density followed by a gradual increase in bulk density through lithologic Unit IV. Grain density has an average of 2.67 and 2.61 g/cm³ for mudstone and sandstone, respectively. The spread in grain density is unusually large, 2.02–2.99 g/cm³ for mudstone grains and 2.11–2.99 g/cm³ for sandstone grains; however, average values are consistent and representative of siliciclastic grain densities. Some of the bulk and grain density scatter is probably due to faulty pycnometer measurements, and some of the bulk density scatter is due to drilling disturbance.

Porosity

Calculated porosity values decrease with depth as anticipated from normal consolidation (Fig. F48B). To characterize the porosity behavior, we employ a porosity-effective stress model (e.g., Rubey and Hubbert, 1959):

,

where

• ϕ_o = reference porosity,

• β = bulk compressibility, and

• σ′_v = effective vertical stress determined by integrating the bulk density data and assuming hydrostatic fluid pressure.

A regression of the mudstone porosity-effective stress data yields bulk compressibility of 0.073 1/MPa and reference porosity of 0.61. In general, the porosity model matches observations (Fig. F48B). Lithologic Unit II above 440 m CSF has slightly higher porosity than the modeled porosity. Possible causes for this deviation are higher sand content and/or the presence of cement, both of which decrease compressibility. Observed and modeled porosities are in general agreement from 440 to 720 m CSF and below 740 m CSF. From 720 to 740 m CSF the model overpredicts the measured porosity, suggesting either overconsolidation of the interval or locally enhanced compression.

The interpretation of subdued porosity loss above 440 m CSF, followed by a transition to normal consolidation below is similar to trends observed at Sites 1173 (at 343 mbsf) and 1177 (at 400 mbsf). The interpreted transition in Hole C0011B is far less clear than at Sites 1173 and 1177. This apparent transition is harder to identify because of several possible influences: (1) mudstones in Hole C0011B have lower compressibility than those at Sites 1173 and 1177 (Spinelli et al., 2007), (2) the transition occurs deeper in Hole C0011B, and (3) data from Hole C0011B have greater scatter. Because the mudstones in Hole C0011B are stiffer and experienced a higher stress prior to the apparent preservation of porosity, we expect the porosity shift to be less pronounced than if the transition occurred in shallower, more compressible sediments (e.g., Sites 1173 and 1177).

Sandstone porosity at Site C0011 does not exhibit a consistent trend but ranges from 0.30 to 0.78 (Fig. F48B; Table T13). The porosity of sand-rich units is harder to evaluate accurately with standard MAD methods because of enhanced drainage and evaporation.

Shear strength

No shear strength measurements were made below 340 m CSF because the sediment is too stiff for reliable measurements.

Anisotropy of P-wave velocity and electrical resistivity

P-wave velocity and electrical resistivity were measured on discrete cube samples cut from working halves. Measurements were made in three directions orthogonal to the x-, y-, and z-axes of the core reference (see Fig. F17 in the "Methods" chapter). For both P-wave velocity and electrical resistivity, the horizontal-plane anisotropy (a_VPhor, a_ρhor) and vertical-plane anisotropy (a_VPvert, a_ρvert) were calculated (see "Physical properties" in the "Methods" chapter).

Velocity is highly varied from 1600 to 2300 m/s (Table T14). There are no definitive trends between 340 and 440 m CSF. Below 440 m CSF, velocity gradually increases with depth (Figs. F48C, F49). Within lithologic Unit III, two velocity trends exist: velocity increases from the top of Unit III to 575 m CSF, and the velocity gradient is constant from 575 m CSF to the base of Unit III. Velocity then increases through Unit IV to Unit V; however, the data have significant scatter.

Vertical-plane anisotropy of velocity (a_VPvert) (see "Physical properties" in the "Methods" chapter) is generally positive, indicating average horizontal velocity is greater than vertical velocity (Fig. F49). A transition from no vertical-plane anisotropy (isotropic) to positive vertical-plane anisotropy may exist near 440 m CSF; however, significant scatter in the data obscures the trend. Horizontal-plane anisotropy (a_VPhor) varies widely between –10% and 7%, and it fluctuates around 0% from Unit II through Unit V. Velocity data tend to have less scatter with increasing depth, which is perhaps related to more competent sediments facilitating increased sample integrity. In contrast, velocity anisotropy data appear to suffer significantly from core disturbance (e.g., fluid-filled microfractures); scatter in the P-wave anisotropy data likely reflects variable degrees (and/or orientation) of drilling disturbance.

A velocity-porosity relationship is developed with measured P-wave velocity and nearest neighbor (<66 cm offset) porosity data (Fig. F50). For comparison to the observations, empirical relationships for P-wave velocity versus porosity from (1) global compilations for "normal" and "high" consolidation sediment (Erickson and Jarrard, 1998) and (2) Shikoku Basin sediments (Hoffman and Tobin, 2004) are plotted. The global compilations provide loose upper and lower bounds for the expected relationship between velocity and porosity. The trend reported for Shikoku Basin sediments is

where

• V_P = P-wave velocity,

• ϕ = porosity,

• v_sh = shale fraction (0.32), and

• X₁ = tanh[40(ϕ – ϕ_c)] – |tanh[40(ϕ – ϕ_c)]|,

where critical porosity (ϕ_c) is 0.295 (Hoffman and Tobin, 2004). Many of the values from Hole C0011B lie within the velocity model bounds (Fig. F50). Some of the samples, however, are faster than predicted by the velocity model, especially for samples with porosity >55%.

Electrical resistivity measured on cube samples varies from 2.0 to 12.0 Ωm (Table T15; Fig. F51). In most cases, vertical resistivity is greater than horizontal (see "Physical properties" in the "Methods" chapter), with anisotropy becoming more negative downhole. In Unit II electrical resistivity is approximately constant in the vertical plane and decreases slightly with depth in the horizontal plane (Fig. F51); however, the data have significant scatter. Similar to the P-wave velocity measurements, electrical resistivity determinations are particularly sensitive to the amount of disturbance and orientation of drilling-induced, fluid-filled microfractures. Additionally, electrical resistivity is very sensitive to sample saturation and evaporation of water. In Units III–IV, resistivity appears to increase slightly with depth, likely resulting from decreased porosity with depth. Scatter in the data obscures any definitive trend. The general downhole trend of resistivity anisotropy becoming more negative is probably controlled by the alignment and flattening of pores caused by consolidation and grain reorientation.

Thermal conductivity

Thermal conductivity measurements were conducted with the needle probe on whole-round samples (<365 m CSF) and with the half-space probe on split core samples (>365 m CSF). Thermal conductivity values vary between 0.98 and 1.77 W/(m·K) (Table T16; Fig. F48D). Thermal conductivity of sand-rich layers ranges from 1.11 to 1.75 W/(m·K). In Unit II, thermal conductivity slightly increases with depth. From the lower portion of Unit II through the upper ~50 m of Unit III, thermal conductivity increases steeply in response to porosity loss. In the lower portion of Unit III, thermal conductivity slightly decreases with depth; this may result from an increasing number of drilling-induced cracks in the core. Thermal conductivity increases in Units IV and V but has significant scatter. The correlation between thermal conductivity and porosity is shown in Figure F52. The relationship for Site C0011 sediments is consistent with sediment grain thermal conductivity ranging from 2.1 to 3.4 W/(m·K) and centered at 2.6 W/(m·K) (a typical value for hemipelagic sediment).

Comparison with Site 1177

The location of Site C0011 is physiographically similar to the location of Site 1177. Both are ~20 km seaward of the Nankai Trough and >100 km off the axis of the Kinan seamount chain. We compare some of the general trends in sediment physical properties between the two sites. At Site 1177, the upper Shikoku Basin facies has nearly constant porosity, and P-wave velocity gradually increases with depth through this interval (Fig. F53). In the ~100 m below the upper Shikoku Basin facies at Site 1177, porosity decreases with depth dramatically and P-wave velocity increases with depth along a steeper trend than in the upper Shikoku Basin facies. P-wave anisotropy is nearly zero in the upper Shikoku Basin facies. In the lower Shikoku Basin facies, P-waves travel faster in the horizontal direction than vertical. At ~440 m CSF in Hole C0011B, transitions in sediment physical properties, such as those observed at Site 1177, are less obvious (Figs. F48, F53). However, drilling-induced core disturbance at Site C0011 obscures details in trends (see "Core quality and physical properties"). There appear to be shifts in trends of porosity, P-wave velocity, and P-wave velocity anisotropy consistent with those at Site 1177 (Fig. F53). The general trends are consistent with shallower sediment having an open framework of randomly oriented sediment grains. Consolidation of deeper sediment has caused porosity loss and horizontal alignment of platey, clay-sized particles. The consolidation trends of mudstones, however, are different between Hole C0011B and Site 1177. Bulk compressibility (β = 0.073 1/MPa) for mudstone in Hole C0011B is ~80% of the bulk compressibility at Site 1177. This difference in compressibility may relate to variations in grain shape, grain size distribution, mineralogy, and/or grain fabric between sites.

Core quality and physical properties

Multiple drilling and coring processes affected core quality in a negative way, which had adverse effects on data quality from bulk physical property measurements. Decreased core diameter from overwashing during RCB coring produced cores that did not fill the core liner, resulting in a significant amount of air and water between the core and the liner. This resulted in calculated values that are not representative of in situ conditions (e.g., low GRA density); measurements are made assuming the core liner is filled with sediment. Advanced processing, integrating X-ray CT images for absolute core diameter, could be applied to MSCL-W data to account for the smaller core diameter. In addition, a large number of cores had microscale cracks and fractures induced by drilling, coring, and/or the core recovery process. Some of these features were distinct on the X-ray CT images and in half-round cores (Fig. F54). While physical property sampling made efforts to avoid regions that had visibly identifiable cracks, the data indicate that disturbance is pervasive through most of the cored interval in Hole C0011B. This disturbance is evidenced by the high variability in MAD bulk density and porosity, which have a broader-than-normal (Hole C0011B versus Site 1177) variability based on the general mud-dominated sediments (Fig. F53). The significant variation in velocity measured on cube samples, in comparison to Site 1177 (Fig. F53), also suggests microcracks yielding low velocity on samples. The thermal conductivity decrease with decreasing porosity (Fig. F48) also confirms small, water-filled fractures in competent material, resulting in a measured thermal conductivity that is lower than in situ thermal conductivity. Side-by-side comparison of split core photographs and X-ray CT images (Fig. F54) shows that not all macroscopic fractures are imaged by X-ray CT (assuming the core splitting process introduces few macroscopic fractures relative to the coring process). Similarly, not all cracks are identifiable in hand samples. Thus, whole-round samples are likely more impacted by drilling disturbance than is evident from X-ray CT images. The presence of small fractures in whole-round samples could introduce artificial anisotropy in permeability and seismic velocity or they could act as planes of weakness in deformation tests. These potential artifacts must be considered in postexpedition geotechnical studies. Even in the presence of disturbance effects, it is possible to interpret general downhole trends and interunit trends. Detailed interpretation of localized changes in physical properties is less reliable because of coring-related disturbance.

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