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doi:10.2204/iodp.proc.322.104.2010

Physical properties

The goal of physical property measurements was to define bulk physical properties and their downhole variations in Hole C0012A. 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 computed tomography and 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 130 m CSF where the needle probe could not be inserted into whole-round cores. MAD analyses were performed on discrete samples collected from the working halves and cluster samples taken adjacent to whole-round samples. Undrained shear strength measurements were made above 95 m CSF. Below 115 m CSF, where sediment was competent enough to cut samples from working halves, P-wave velocity and electrical resistivity were measured in the x-, y-, and z-directions (see Fig. F17 in the "Methods" chapter) on discrete cube samples.

MSCL-W

Electrical resistivity

Electrical resistivity increases downhole in Units I–V (Fig. F41). Average values are 0.85 Ωm in Unit I, 1.46 Ωm in Unit II, 1.92 Ωm in Unit III, 2.70 Ωm in Unit IV, and 4.13 Ωm in Unit V. Electrical resistivity then decreases abruptly to an average of 3.85 Ωm in Unit VI. The increasing resistivity through Units I–IV is likely controlled by porosity loss (Fig. F42) during burial and consolidation. Maximum resistivity values of 59.7 Ωm are nearly identical to those observed in Hole C0011B.

Natural gamma radiation

Natural gamma radiation also increases downhole through Unit IV, averaging 27.3 counts per second (cps) in Unit I, 32.2 cps in Unit II, 36.9 cps in Unit III, and 39.3 cps in Unit IV (Fig. F41). Natural gamma radiation then decreases to average values of 35.1 cps in Unit V and 29.3 cps in Unit VI. The shift to lower natural gamma radiation within Unit V occurs at ~480 m CSF. This corresponds to an increase in magnetic susceptibility (Fig. F41), suggesting a bulk compositional change. Natural gamma radiation again decreases in Unit VII.

Magnetic susceptibility

Magnetic susceptibility averages 33.8 × 10^–5 corrected volume magnetic susceptibility (CMS) in Unit I, then increases within Unit II, with an average of 101.6 × 10^–5 CMS with peaks reaching 1445 × 10^–5 CMS. Local high magnetic susceptibility values occur within volcaniclastic sandstones (see "Lithology," Fig. F2). Magnetic susceptibility increases with depth in Unit III until ~270 m CSF, below which it decreases to an average of 14.7 × 10^–5 CMS. A similar decrease within Unit III was observed in Hole C0011B at 575 m CSF, providing a surface within Unit III for correlation between sites. Magnetic susceptibility is constant in Unit IV and the upper half of Unit V. At ~480 m CSF, average magnetic susceptibility increases to 59.2 × 10^–5 CMS for the lower section of Unit V and Unit VI. This correlates with a decrease in natural gamma radiation (Fig. F41), and both of these changes correlate with the appearance of sandstone beds containing abundant iron-rich minerals (see "Lithology"). The basalt of Unit VII is characterized by high magnetic susceptibility.

GRA density

GRA density is relatively uniform over Units I–VI and ranges from 0.82 to 0.97 g/cm³, with an average of 0.94 g/cm³ (Fig. F41). As observed in Hole C0011B, the presence of air between cores and core liners resulted in anomalously low values of both GRA density and P-wave velocity.

MAD measurements

MAD measurements of discrete samples from Hole C0012A provide a detailed characterization of grain density, bulk density, porosity, void ratio, and water content. All MAD data are provided in Tables T14 and T15 and are summarized below.

Bulk and grain density

Bulk density in Hole C0012A increases downhole from 1.45 g/cm³ at the seafloor to 2.10 g/cm³ by 530 m CSF (Fig. F42A). Within lithologic Unit I, bulk density shows a minor increase from 100 to 136 m CSF. Bulk density then increases consistently through Units II, III, and IV. The Unit IV/V boundary has a notable decrease (~0.12 g/cm³) in bulk density. Bulk density then continues to increase with depth through Unit V. Grain density throughout the sedimentary section (lithologic Units I–VI) has high variability ranging between 2.1 and 3.0 g/cm³ with few outliers. Some of this scatter could be due to inaccurate pycnometer measurements. Some of the bulk density scatter was likely the result of coring disturbance.

Porosity

MAD-determined porosity decreases downhole through the sedimentary section in Hole C0012A (lithologic Units I–VI) (Fig. F42B). Seafloor porosity is 0.70, and porosity decreases to 0.35 by 530 m CSF. Similar to bulk density, small deviations from the overall trend are present. For example, the lithologic Unit IV/V boundary (418 m CSF) displays an increase in porosity from 0.38 to 0.45. A subtle porosity increase may also exist near 125 m CSF, but there are scatter in the data.

To characterize the general 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 0.15 1/MPa and reference porosity of 0.65. This bulk compressibility is twice that observed in Hole C0011A (see "Physical properties" in the "Site C0011" chapter). The source of the difference in compressibility requires future analyses. The hemipelagic mudstones from the two sites are similar in bulk mineral composition (see "Lithology"), and thus, they should have similar compression behavior.

The porosity model provides a baseline behavior for sediments in Hole C0012A. Deviations from the model can provide insights to lithologic variation, mineralogy, or grain fabric differences. Our model trend predicts slightly lower porosity than observed in Unit I (Fig. F42B). This mismatch is most pronounced from ~120 to 136 m CSF, which may indicate greater stiffness in these sediments. This stiffness could be controlled by grain shape, sediment composition, or the presence of intergranular cement. Lithologic Unit III generally has slightly lower porosity than predicted; these sediments may have been exposed to higher stresses (e.g., erosion of previous overburden) and are mildly overconsolidated or they may have greater compressibility than the bounding sediments. A thin interval of strata near the top of the Unit V boundary has higher porosity than the background trend. This high-porosity zone correlates with a zone of decreased compressional velocity and lower thermal conductivity (Fig. F42). This interval could represent an in situ high-porosity zone within the volcaniclastic-rich deposits. Further shore-based analyses are required to establish the origin of this high-porosity/low-velocity/low-thermal conductivity zone.

Shear strength

Vane shear and penetrometer measurements constrain undrained shear strength in lithologic Unit I above 95 m CSF (Fig. F43; Table T16). In this interval, plastic deformation was observed during shear strength measurements; below 95 m CSF, brittle deformation occurred when shear strength measurements were attempted, so no data were recorded. The observed shear strength was low (21–30 kPa) near the seafloor. From 50 to 95 m CSF, shear strength was highly variable, ranging from values similar to observations at the seafloor to ~100 kPa. The scatter in the data does not indicate any trend with depth or lithology.

Anisotropy of P-wave velocity and electrical resistivity

P-wave velocity and electrical resistivity were measured on discrete cube samples cut from working halves. Sample cubes were cut once the sediment was competent enough for cutting, which occurred at 115 m CSF. 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), allowing determination of the horizontal-plane anisotropy (a_VPhor, a_ρhor) and vertical-plane anisotropy (a_VPvert, a_ρvert) of P-wave velocity and electrical resistivity (see "Physical properties" in the "Methods" chapter).

Compressional wave velocity (V_P) increases downhole consistent with the increase in bulk density and the decrease in porosity (Fig. F42). Average compressional velocity is 1600 m/s near the base of lithologic Unit I and increases to 2100 m/s in Unit V. From 500 m CSF into lithologic Unit VI, average velocity decreases. Two samples had V_P that exceeded 2200 m/s; these samples were collected in proximity to densely lithified calcareous claystones (see "Lithology"). Velocity in lithologic Unit VII (basalt) greatly exceeds the overlying sedimentary section (Fig. F42C). P-wave velocities of basalt samples ranged from 3083 to 4767 m/s (Table T17). Within these data, one sample (322-C0012A-54R-2, 24.0 cm) was distinctly faster than the others.

Vertical-plane anisotropy (a_VPvert) has significant scatter in Hole C0012A that may be related to sample disturbance; however, general trends are discernable. Sediment velocity is slightly positive at the base of lithologic Unit I, from which point anisotropy increases (horizontal velocity greater than vertical velocity) with depth through Units II–VI (Fig. F44; Table T17). This increase correlates with increasing bulk density and decreasing porosity and thus likely reflects preferential grain orientation and enhanced grain contacts from consolidation. At ~450 m CSF, a cluster of samples has vertical velocity exceeding horizontal (negative V_P anisotropy). These samples are from sandstone layers or fine-grained sediment adjacent to sandstone layers. Variable grain shape and structure may have influenced grain orientation and reorientation during burial, producing this response. Alternately, a change in depositional process (e.g., turbidity current versus hemipelagic settling) could have produced a different initial fabric. P-wave velocity anisotropy in the basalt was generally positive with a maximum of 5.84%. The fastest specimen (Sample 322-C0012A-54R-2, 24.0 cm) was the only basalt specimen with a negative vertical-plane anisotropy of velocity (Fig. F44).

A velocity-porosity relation is developed with measured P-wave velocity and nearest neighbor (<66 cm offset) porosity data (Fig. F45). For comparison to the observations, empirical relations 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 (Fig. F45). The global compilations provide loose upper and lower bounds for the expected relationship between velocity and porosity. The curve 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). Most of the velocity observations in Hole C0012A lie above the Shikoku Basin trend but within the bounds of the global compilation (Fig. F45). This trend suggests that for a given porosity, sediments in Hole C0012A are faster than those at Site 1173. This is similar to our observations of data from Hole C0011B (Fig. F44) and suggests that the bulk and/or shear moduli of the sediments is higher at Sites C0011 and C0012 in comparison to Site 1173. The nature of grain contacts and fabric could contribute to these differences between Shikoku Basin sites.

Electrical resistivity measured on cube samples varies from 2.0 to 8.0 Ωm (Table T18; Fig. F46). In most cases, vertical resistivity is greater than horizontal (negative anisotropy), with anisotropy becoming more negative downhole. The increase in resistivity with depth through Units I and II is steeper than in the underlying units. The increase in anisotropy with depth appears fairly constant through all lithologic units. Scatter in the data (likely resulting from variability in the number and orientation of drilling-induced, fluid-filled microfractures) obscures any definitive trend in resistivity anisotropy.

Thermal conductivity

Thermal conductivity measurements were conducted on whole-round cores (<105 m CSF) with the full-space needle probe and on split core samples (>130 m CSF) with the half-space method. Thermal conductivity values vary between 0.98 and 1.47 W/(m·K) in mud(stone) and between 1.19 and 2.10 W/(m·K) in sand(stone) (Table T19; Fig. F42D). Units I and II have increasing thermal conductivity with depth. Thermal conductivity in Unit III increases until 290 m CSF, where a step decrease occurs. This decrease coincides with a marked decrease in core quality (smaller biscuits and more fractures below 290 m CSF). The change in core quality may reflect a lithologic change; the rate of porosity loss may also change at 290 m CSF, potentially suggesting a change in compression and/or pore structure. Thermal conductivity then increases to the base of Unit IV. The top of Unit V is marked by a second decrease in thermal conductivity, which correlates with a porosity increase (Fig. F42). Sand(stone) in Unit V has a higher thermal conductivity than mud(stone). The correlation between thermal conductivity and porosity is shown in Fig. F47. The relationship is consistent with thermal conductivity of the sediment grains ranging from 2.1 to 3.4 W/(m·K) and centered at 2.6 W/(m·K).

Comparison with Site C0011 and Sites 1173 and 1177

We compare some of the general trends in sediment physical properties between Sites C0012, C0011, 1173, and 1177. Site C0012 has an interval where porosity may not decrease steadily with depth. Similar trends existed at Sites 1173, 1177, and C0011 (Figs. F47, F42B). This zone is shallower (<136 m CSF) at Site C0012 than it is at the other sites. Below this zone, porosity decreases along a normal consolidation trend at each site. The interval of near-constant porosity and the transition to normal consolidation are much more pronounced (i.e., larger porosity shifts) at Sites 1173 and 1177. The trends are less obvious at Sites C0011 and C0012. Each of the sites has increasing P-wave velocity and P-wave anisotropy with depth through the normal consolidation zone (Figs. F47, F44). With the exception of a small region in lithologic Unit V in Hole C0012A, all sites have positive velocity anisotropy, indicating P-waves travel faster in the horizontal direction than in the vertical direction. Overall, these general porosity and velocity trends are indicative of shallow sediments having an open framework of randomly oriented particles. As consolidation progresses during burial, porosity loss occurs, platey, clay-sized particles align, and preferential fabrics develop. While the general consolidation trends of mudstones between sites are similar, there are distinct differences in compressibility. Specifically, the sediments at Sites C0012 and 1173 are more compressible than sediments at Site C0011. These differences, for what appear to be similar hemipelagic mudstones, are difficult to explain. They could be influenced by minor changes in depositional environment and processes, variations in stress history, and/or bias and distribution in sampling or disturbance. These differences could prove important for pore shape changes and flow pathways, and thus warrant further research, which will be facilitated by shore-based geotechnical experiments.

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. Hole C0012A suffered from many of the same problems observed in Hole C0011A (see "Core quality and physical properties" in "Physical properties" in the "Site C0011" chapter): decreased core diameter from overwashing, microscale cracks and fractures, decreasing thermal conductivity with decreasing porosity, large scatter in MAD measurements, and scatter in velocity and velocity anisotropy (Figs. F47, F41, F42, F44). Although disturbance existed, Hole C0012A had intervals of good core recovery where data trends are cleaner than those observed in Hole C0011A. With depth, however, core quality degraded and disturbance artifacts increased. These disturbance effects do not preclude interpretation of general trends in the physical properties; however, they should be carefully considered as additional research, including shore-based geotechnical experiments, continues.

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