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doi:10.2204/iodp.proc.333.104.2012

Physical properties

The goal of physical property measurements in Holes C0011C and C0011D was to provide high-resolution data on the bulk physical properties and their downhole variations. 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 to define natural gamma radiation, gamma ray attenuation (GRA) density, noncontact resistivity, magnetic susceptibility, and P-wave velocity. P-wave velocity data from the MSCL-W were of poor quality and will not be discussed. Thermal conductivity was measured using the full-space needle probe method on whole-round cores or the half-space line source method on working halves of cores. The half-space method was used on sediments below 311 m core depth below seafloor (CSF) because the sediments below this depth were too hard to allow insertion of the needle probe. Penetrometer, vane shear, and electrical resistivity measurements were made shortly after the core was split, and moisture and density (MAD) analyses were performed on discrete samples collected from either the working halves of split cores or cluster samples taken adjacent to whole-round samples. Penetrometer and vane shear measurements were not conducted below 163 m CSF because the sediments became too stiff to allow measurement. Discrete resistivity and P-wave velocity measurements in the x-, y-, and z-directions were performed on sample cubes cut from working halves of cores below 215 m CSF.

MSCL-W

MSCL-W data were collected on whole-round sections to define GRA density, electrical resistivity, magnetic susceptibility, and natural gamma radiation. Figure F30 shows the results of MSCL-W measurements on cores, which are summarized and correlated with Expedition 322 logging units (dashed lines) using LWD data from Hole C0011A, and lithologic units from Expedition 333. GRA density, electrical resistivity, and natural gamma radiation all increase slightly with depth, whereas magnetic susceptibility decreases with depth. The variation patterns of GRA density, electrical resistivity, magnetic susceptibility, and natural gamma radiation generally show good agreement with each logging unit.

GRA density

GRA density was calculated by gamma ray attenuation measured using the MSCL-W. Although measurement errors exist in GRA density data because of the presence of air between core and core liner, in general, GRA density tends to reflect the characteristic of each lithologic unit (Fig. F30). Density slightly decreases from the seafloor to 250 m CSF. However, GRA density abruptly increases at 250 m CSF, coinciding with the logging Unit 1/2 boundary.

Magnetic susceptibility

Magnetic susceptibility generally and gradually decreases with depth (Fig. F30). From the seafloor to 80 m CSF, magnetic susceptibility shows little variation; however, it decreases from 80 to 200 m CSF. After a small step increase of magnetic susceptibility between 200 and 230 m CSF, magnetic susceptibility tends to decrease slightly downhole, with the exception of spikes of high magnetic susceptibility from 337 to 410 m CSF. These high-value spikes of magnetic susceptibility correlate with tuffaceous sandstones (see lithology in Fig. F3).

Natural gamma radiation

Natural gamma radiation is plotted with gamma ray data from LWD in Figure F30. Although natural gamma radiation measured by MSCL-W is systematically lower than gamma ray data from LWD, the variation patterns are similar. Natural gamma radiation gradually increases with depth to ~200 m CSF. At 250 and 480 m CSF (near the logging Unit 1/2 and 2/3 boundaries), natural gamma radiation shows large step increases. Many intervals of low natural gamma radiation are found between 337 and 440 m CSF where tuffaceous sand layers exist. This variation is also consistent with LWD data.

Electrical resistivity

Electrical resistivity generally increases with depth (Fig. F30). From the top of Site C0011 to 250 m CSF, a low-resistivity section is observed. Then, resistivity abruptly increases at 250 CSF where porosity decreases. From 337 m CSF, where tuffaceous sand layers were cored, resistivity gradually increases downhole with strong local variation.

Moisture and density measurements

MAD measurements on discrete samples from Site C0011 provide a detailed characterization of bulk density and porosity. All MAD data from Expedition 333 are provided in Table T9, and results from Expeditions 322 and 333 are summarized below.

From the surface to ~50 m CSF, bulk density generally increases and porosity decreases downhole as expected for progressive burial (Fig. F31). This trend reverses between ~50 and ~80 m CSF, and then porosity remains relatively constant until ~240 m CSF. A sharp increase in bulk density (decrease in porosity) occurs between ~240 and 270 m CSF, after which a steady compaction trend continues to the base of the drilled interval.

The absence of bulk density and porosity variation between 80 and 240 m CSF is anomalous. Similar behavior was observed at ODP Site 1173, the reference site for the Muroto transect, and Site 1177, the reference site for the Ashizuri transect (Shipboard Scientific Party, 2001b, 2001c) (Fig. F32). Spinelli et al. (2007) attributed the Site 1173 anomalously high porosities at shallow depths to cementation provided by opal-CT and the abrupt porosity decline to dissolution of these cements as temperature exceeded 55°C.

Below the depth of rapid porosity decline at Site C0011, the porosity depth profiles at Sites C0011, 1173, and 1177 are remarkably similar (Fig. F32).

Strength measurements

The strength of soft sediments was measured using an analog vane shear device (Wykeham Farrance, model WF23544) and a pocket penetrometer (Geotest E284B) in the working half of the core from 0 to 162 m CSF (Fig. F33). Beneath this depth, high sediment strength did not allow measurements with these devices. Both vane shear and penetrometer data sets are highly scattered, particularly below 40 m CSF.

P-wave velocity and anisotropy

P-wave velocity was 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. The horizontal-plane anisotropy (α_VPhor, α_ρhor) and vertical-plane anisotropy (α_VPvert, α_ρvert) were calculated (see “Physical properties” in the “Methods” chapter [Expedition 333 Scientists, 2012]).

P-wave velocity in Hole C0011D varies from 1557 to 2333 m/s and increases with depth (Table T10). The high velocities near the base of lithologic Unit I correspond to volcaniclastic sands in this interval. The values measured in this expedition match the values measured in Expedition 322 in the zone of overlap from 340 to 380 m CSF.

Vertical-plane anisotropy of velocity (α_VPvert) (see “Physical properties” in the “Methods” chapter [Expedition 333 Scientists, 2012]) is slightly >0, indicating average horizontal velocity is greater than vertical velocity (Fig. F34). This is the expected trend in transversely isotropic sediments because acoustic waves propagate more slowly perpendicular to bedding. The anisotropy values match those measured during Expedition 322 (Expedition 322 Scientists, 2010) in the zone of overlap but exhibit less scatter. This is probably due to better core recovery in this interval during this expedition relative to Expedition 322.

A velocity-porosity relationship is developed with measured P-wave velocity and nearest neighbor (<66 cm offset) porosity data (Fig. F35). 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). The P-wave velocity values measured in Hole C0011D generally fall inside the envelope defined by these trends, which matches the trend from Expedition 322. There are some outliers from Hole C0011D with anomalously high porosity. These points correspond to the lowest part of lithologic Subunit IA (see “Moisture and density measurements”), which has high velocity relative to that which would be expected from these velocity-porosity models. Some additional high-velocity points (>1800 m/s) correspond to sands near the base of lithologic Unit I, illustrating that differences in lithology may cause some points to lie outside the bounds of these velocity-porosity trends.

Electrical resistivity and anisotropy

In soft sediments, from 0 to 210 m CSF, the resistivity was measured with a four-pin, 2 kHz Wenner array. These measurements are consistent with the baseline values provided by the MSCL, with lower scatter. Resistivity generally increases with depth as a result of pore volume loss. Here, resistivity increases from 0.8 to 0.95 ± 0.15 Ωm between 0 and 20 m CSF then remains stable at 0.95 ± 0.15 Ωm until 80 m CSF (Fig. F36). Resistivity slightly decreases to 0.85 ± 0.05 Ωm between 80 and 120 m CSF, which is consistent with the higher porosities at these depths. Then resistivity increases to 0.95 ± 0.15 Ωm again and remains at this value to 220 m CSF.

Electrical resistivity measured on cube samples varies from 0.6 to 2.6 Ωm (Table T11; Fig. F36, F37). Measurements from discrete samples show continuity with data from the Wenner array. Anisotropy was calculated from resistivity measurements made in the x-, y-, and z-directions. In most cases, vertical resistivity is greater than horizontal (see “Physical properties” in the “Methods” chapter [Expedition 333 Scientists, 2012]), with vertical anisotropy becoming more negative downhole. The anisotropy values from this expedition match well with those measured in Hole C0011B during Expedition 322 (Expedition 322 Scientists, 2010) and exhibit the same trend with depth (Fig. F37). Measured resistivity values from this expedition increase slightly downhole but exhibit little variation. There is an abrupt increase in resistivity at the lithologic Unit I/II boundary. This could be due to the greater sand abundance in Unit II causing resistivity to increase or differences in data quality between this expedition and Expedition 322 because of the use of a different probe setup (see the “Methods” chapter [Expedition 333 Scientists, 2012]). Core recovery and core quality during Expedition 322 were generally poorer than during this expedition, and because resistivity measurements are strongly affected by coring disturbances such as coring-induced fractures, this may also account for the difference in data between the two expeditions. 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.

The relationship between porosity and resistivity is illustrated by a log-log plot of porosity and resistivity (Fig. F38); resistivity was matched with nearest neighbor porosity values (within 30 cm) from MAD measurements. The points in the zone of anomalously high porosity from 80 to 240 m CSF do not fit the general trend from the rest of the data. This could hypothetically be due to the presence of a cementing phase (Ellis and Singer, 2007) or a change in clay mineralogy (Henry, 1997).

Thermal conductivity and heat flow

Nine in situ temperatures were measured from 22.5 to 184.5 m CSF using the advanced piston corer temperature tool (APCT-3) (Fig. F39). Temperature tends to increase linearly with depth. The average geothermal gradient determined by a least-squares regression on the nine temperature measurements is ~91.3°C/km, which is much higher than the average thermal gradient (~63°C/km) of Hole C0018A. The mean seafloor temperature obtained from the intercept of a least squares regression on the nine temperature measurements is ~1.7°C.

To estimate heat flow, we made 585 thermal conductivity measurements on core samples from Hole C0011D. Measurements were made with a full-space needle probe in most cases and a half-space line source if the needle probe could not be used because a core sample was too hard. Thermal conductivity ranges from 0.62 to 1.41 W/(m·K) (Fig. F40). Thermal conductivity with depth tends to be constant to 250 m CSF and then abruptly increases at 250 m CSF, which coincides with a logging boundary. For clastic sediments, the main controlling factors on thermal conductivity are porosity and quartz content. At Site C0011, thermal conductivity appears inversely correlated with porosity.

We estimated heat flow by multiplying the average thermal gradient by the harmonic mean of thermal conductivity (~0.98 W/[m·K]) estimated in the interval where temperatures were measured. Estimated heat flow is ~89.5 mW/m² in Hole C0011D, which is much higher than heat flow (~62 mW/m²) in Hole C0018A.

To estimate temperatures from the seafloor to the basement, we used the Bullard method, which requires surface heat flow and thermal conductivity data (Fig. F41). Based on seismic reflection data, basement depth is estimated to be 1050 mbsf (Expedition 322 Scientists, 2010). Because we were not able to obtain complete cores from Hole C0011D, thermal conductivities determined from measurements on samples from Expedition 322 Hole C0011B were used in temperature estimates (Expedition 322 Scientists, 2010). A thermal conductivity of 1.35 mW/m² was assumed for the undrilled sediments between the base of Hole C0011B and basement. Temperature at the sediment/basement interface is estimated to be ~79°C.

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