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doi:10.2204/iodp.proc.313.105.2010

Physical properties

The focus of this section is to provide a description of the petrophysical data sets collected on cores from Hole M0029A and their significant characteristics and variations, as well as concentrating in more detail on a number of interesting aspects. Some measurements display variation primarily associated with depositional changes, whereas other measurements are controlled more by postdepositional factors such as the degree of cementation, porosity, and the type of interstitial water. In addition, the combined integration of the core and logging petrophysical data sets allows calibration of core data with in situ borehole properties and provides an assessment of the precise depth from which core was collected in the borehole.

Whole-core measurements were acquired on a multisensor core logger (MSCL): gamma density, P-wave velocity, noncontact electrical resistivity, and magnetic susceptibility. Thermal conductivity and natural gamma radiation (NGR) measurements were taken on whole cores prior to the commencement of the Onshore Science Party (see "Physical properties" in the "Methods" chapter). Density, porosity, and P-wave velocity were measured and calculated from, on average, one sample in every core section. Figure F30 provides an overview of the data acquired. A synthesis of all downhole and physical property data from this site can be found at the end of "Downhole measurements."

Density and porosity

Gamma density and wet bulk density

Gamma density in Hole M0029A is more variable than in Hole M0027A but less variable than in Hole M0028A. Outliers outside the 1.5 to 2.9 g/cm³ interval are not considered below (Figs. F31, F32). Wet bulk density calculated from wet and dry weight and dry volume of samples from cores varies between 1.65 and 2.25 g/cm³, with a few high values from the sampled cemented beds (2.7 and 2.9 g/cm³).

Repeat measurements with resaturation of sediment before measurement indicate that the density of samples may be overestimated by up to 5%. Despite this uncertainty, there is fair correspondence between gamma density and wet bulk density; however, there is greater scatter than is found Holes M0027A and M0028A. Detailed comparison of gamma density and wet bulk density for Hole M0029A shows wet bulk density measurements that were probably entered for the wrong core section in the data files. Based on written data sheets, it was possible to correct some of these errors. For gamma density, tops and ends of cores often show reduced densities. This problem is a result of fractured/disturbed core extremities. In the case of short sections, the problem seems persistent. No attempt has been made here to correct for this, except through numerical high-pass filtering. It is recommended in the future that short Section 2s or 3s be removed in the gamma density data set if values are lower than the average of the core.

Density varies downhole with intervals of lower densities (~1.85 g/cm³) and higher densities (~2.1 g/cm³) (Fig. F31). These variations broadly correspond to different bulk lithologies: respectively, clay- to silt-rich intervals and sand, glauconite, or cemented intervals, as in Holes M0027A and M0028A. These variations are more frequent and show higher amplitude from 600 m to the base of the hole, enriched with coarse-grained glauconitic quartz sand horizons. This correspondence to lithology is particularly clear when investigated in combination with porosity (see below).

Calculated porosity

Calculated porosity, here referred to as porosity, is derived from the difference between wet and dry weights and dry volume of selected sediment samples (see "Physical properties" in the "Methods" chapter). Some core liners had little drill fluid in them. As a result, sediments with high permeability may have been drained of water during the storage and handling. Porosity may therefore be underestimated, particularly that of sands. Based on repeated measurements, we estimate that the porosity of sands may be underestimated by ~10%, perhaps up to 19%. Clays and silt sediments have relatively little permeability and are not expected to have drained as much. Calculated porosity values vary from 27% to 58%.

Stratigraphically, porosity varies with intervals of low porosity (~40%) and intervals with high porosity (~45%–55%). This variation corresponds to bulk lithology (Fig. F31). Clay- to silt-rich sediments in general have higher porosity than sand, glauconite, or cemented sediments.

The porosity of clay and silt intervals decreases with depth. The same tendency is observable for sand-rich intervals. Porosity reductions broadly follow the exponential decrease in porosity of sands, silts, and clays that has been observed from a number of passive margins and derived analytically (Sclater and Christi, 1980; Bahr et al., 2001). High-amplitude, high-frequency variations are common in the lower part of the hole from 620 m downhole and are characterized by coarse-grained quartz glauconitic intervals.

Combined density and porosity as a stratigraphic tool

As in Hole M0027A, there is a significant inverse variation between porosity and density in samples from Hole M0029A (Fig. F33). The different lithologies of Hole M0029A fall close to a mixing line between rock (zero porosity and density of 2.7 g/cm³) and saltwater (100% porosity and density of 1.027 g/cm³). Sand sediments and the majority of clay and silt samples plot close to the mixing line between rock and saltwater. Glauconite-rich sediments clearly plot above the mixing line at higher densities, most likely influenced by mature glauconite with a density of 2.9 g/cm³ as opposed to ~2.65 for quartz. A significant portion of the silt and clay samples plot below the line and have relatively reduced densities, which could be interpreted as organic matter and other low-density minerals (Rider, 2006).

Downhole, there are differences between gamma density and density measured on samples. Gamma density corresponds to the main lithology variation in the hole and is a bit more variable than bulk wet density (Fig. F34A, F34B, F34C, F34D). There is little separation between density and porosity curves when plotted on inverted scales.

A number of packages of gradual overall increasing density and decreasing porosity can be delineated based on the average "log motif" of gamma density and sample-based density and porosity (Fig. F34A, F34B, F34C, F34D). In lithostratigraphic Unit I and Subunit IIA above 430 mbsf, the packages in general are separated by an abrupt density decrease and a porosity increase stratigraphically uphole. These packages correspond to large-scale coarsening-upward successions ending in abrupt fining (horizontal arrows; Fig. F34A, F34B) (see "Paleontology"). Subpackages of coarsening uphole sediments can be seen. In Subunits IIB and IIC, thicker fining-upward packages are present (Fig. F34B, F34C, F34D). Below Unit II, the log motif is inverted compared to that in the top of Unit I and II. An abrupt density increase and porosity decrease are followed by a more gradual density decrease and porosity increase, stratigraphically uphole. This pattern mimics fining-upward successions.

Significant boundaries based on the averaged log motif can be identified at several places. Many of the abrupt density shifts correspond to surfaces or intervals with a shift in other petrophysical parameters and wireline logs (Table T11). Such surfaces often correspond to important surfaces, as determined by sedimentology and stratigraphic correlators (see "Lithostratigraphy" and "Stratigraphic correlation"). Intervals with abundant organic phytoclasts seem to have densities reduced relative to porosity.

P-wave velocity

P-wave velocity was measured on both the MSCL and discrete samples. Additionally, sonic logs were obtained between 720 and 404 m wireline log depth below seafloor (WSF) (see "Downhole measurements") (Fig. F35). These three sets of velocity measurements generally correlate well with each other, as well as with density data. Core recovery in the upper ~320 mbsf was sparse in the unconsolidated sediments of Unit I, and suitable samples for discrete velocity measurements were correspondingly few. MSCL velocity measurements are also mostly lacking between ~400 and ~620 mbsf (see "Physical properties" in the "Methods" chapter), except for the occasional particularly consolidated intervals. The lack of accurate MSCL velocity measurements in that section of the hole is thought to be due to the presence of gas (methane) resulting in small bubbles forming in the core liner fluid and preventing signal transmission between the transducers. Confirmation of this awaits postexpedition analysis of the headspace gas collected.

The highest discrete velocities obtained were on cemented intervals or concretions, for example at 343 mbsf in the siltstone of Unit II (just below Subunit IIA₂), where a velocity of 5600 m/s corresponds to a clear increase in MSCL density, MSCL velocity, and conductivity logs (Fig. F35). With the exception of this siltstone bed, MSCL velocities are low (<1800 m/s) throughout Unit II (as are most of the discrete velocity measurements) and in Unit I (where obtained). Velocities are slightly higher in Subunit IIA₂, where the change in lithology from sands to silts and clays enabled more data to be collected. Around 479 mbsf there is a claystone interval just above a gap in core recovery. At the base of this level, MSCL density and all velocities display a clear increase. The sonic log indicates that the maximum peak is within the gap. Around 602 mbsf (Subunit IID₁), velocities increase in the sonic log with correspondingly higher core velocities (where obtained), especially in the center of this section at ~612 mbsf. In the glauconite-rich coarse cemented intervals toward the base of Subunit IID₂ (see "Lithostratigraphy"), velocities increase. Between ~668 and 674 mbsf, a further increase in velocity is apparent (up to 3500 m/s in the samples measured). Discrete velocities from ~574 mbsf (in Subunit VB) are fairly constant and not high (<2500 m/s), although more variation is evident in both MSCL and sonic velocity logs. Only discrete velocities and very few MSCL velocities were acquired from 728 mbsf to the base of the hole; they display variation with a particularly high value measured on a cemented nodule bed at 734 mbsf.

Magnetic susceptibility

MSCL magnetic susceptibility was measured at 1 cm intervals and illustrates high-resolution changes in lithology related to variations in the magnetic minerals and/or their sizes. The 5 cm resolution downhole trends associated with lithological units and stratigraphic surfaces are described in "Downhole measurements." Core measurements and log measurements correlate extremely well, although absolute values are different.

Electrical resistivity

As in Hole M0027A, electrical resistivity is affected by core liner saturation with drilling fluids. Short Section 3s are not considered here because of their low core liner saturation, although the removal of this effect has been attempted by high-pass filtering (Figs. F31, F34).

Resistivity is highly variable throughout the hole (Figs. F34, F35), from low values of ~0.3 Ωm to high values of 6 Ωm. High resistivity is encountered in a number of cemented levels. Above ~280 mbsf, resistivity is ~1 Ωm. Below ~280 mbsf, resistivity is ~0.7 Ωm with thin intervals of higher resistivity in cemented levels. Variability is high, between ~400 and 535 mbsf and below 630 mbsf, respectively, from the lower part of Subunit IIA through the upper part of Subunit IIC and from the lower part of Subunit IID to the base of the hole.

In the latter interval, the larger resistivity variations are bounded by cemented beds (see correlation with sonic log in Fig. F35) and possibly pyrite-rich horizons that sharply differ from the smooth increase in chlorinity with depth.

Digital linescans and color reflectance

All cores from Hole M0029A were measured using an X-ray fluorescence (XRF) digital linescan system with data recorded at a resolution of 0.068 mm as both images and red-green-blue (RGB) values down the core centerline. All suitable cores were also scanned for color reflectance. These data may reflect changes in such factors as glauconite or iron content by analysis of a specific wavelength or a*/b* ratios. The quality of the core section shows a significant effect on the scattering of the data (see "Physical properties" in the "Methods" chapter).

Thermal conductivity

The few thermal conductivity measurements downhole to 300 mbsf (Fig. F36) show high values (>2 W/[m·K]) in sand with seawater chlorinity levels and low values (1.5–1.8 W/[m·K]) in clays with low-chlorinity water (see "Geochemistry"). Below 300 mbsf, variations are smooth and decrease downhole. Clay layers (380–460 and 530–600 mbsf) show low and subconstant thermal conductivity values. Thermal conductivity increases in glauconitic intervals, possibly in relation to changes in porosity. In contrast to Holes M0027A and M0028A, thermal conductivity does not clearly correlate with chlorinity (see "Downhole measurements").

Natural gamma radiation and core-log correlation

All cores were measured for NGR (Fig. F35). These data are equivalent to the downhole total gamma ray log; although absolute values are different, trends are parallel. Gamma ray trends and characteristics are described in "Downhole measurements." Correlations between the MSCL-NGR and downhole total gamma ray (TGR) are excellent, with only minor (<1 m and usually <0.6 m) vertical offsets in any interval. The match is better where there is both good core recovery and distinctive peaks in both data sets.

The correlation between core depths and logging data can be evaluated using several petrophysical data sets, notably NGR measurements, but magnetic susceptibility is also very useful. Toward the base of the hole, we applied a depth correction to several cores (see "Stratigraphic correlation"). Magnetic susceptibility measurements on both core and logs are also useful at this site in assessing correlation.

Petrophysical surfaces and intervals

Important surfaces and intervals are identified based on pronounced or sharp changes in trends in one or more of the petrophysical values. Many abrupt shifts correspond to surfaces or intervals with trend changes in wireline logs. A preliminary summary of the most important surfaces or intervals is given in Table T11. Some surfaces and intervals correspond to important surfaces, as determined from sedimentology and/or seismic data (see "Lithostratigraphy" and "Stratigraphic correlation").

A synthesis of petrophysical data together with downhole logs and derived quantities of each lithostratigraphic unit are presented at the end of "Downhole measurements."

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