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Physical properties

The primary aim of this section is to provide a description of the petrophysical data sets collected on cores from Hole M0027A, including their significant characteristics and variations. The secondary aim is to discuss in more detail a number of interesting aspects. Some measurements display variation primarily associated with depositional sedimentological changes, whereas other measurements are controlled by more postdepositional factors, such as the degree of cementation and the type of interstitial water. In addition, combined integration of 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 MSCL: gamma ray density, P-wave velocity, noncontact electrical resistivity, and magnetic susceptibility (see "Physical properties" in the "Methods" chapter). Thermal conductivity and natural gamma radiation (NGR) measurements were taken on whole cores prior to the commencement of the OSP. Density, porosity, and P-wave velocity were measured and calculated from, on average, one discrete sample in every core section. Figure F46 provides an overview of the data acquired.

Density and porosity

Gamma density and wet bulk density

Gamma density varies from 1.55 to 2.4 g/cm3 in cores from Hole M0027A (Fig. F47). Wet bulk density calculated from wet and dry weight and dry volume of samples from cores varies between 1.55 and 2.5 g/cm3. In general, there is good correspondence between gamma density and wet bulk density, with a slight tendency for higher estimates of density from gamma density (Fig. F48). Based on repeat measurements, it is estimated that wet bulk density of sands is overestimated by 5%.

Density varies downhole with intervals of lower densities (~1.85 g/cm3) and higher densities (~2.05 g/cm3) (Fig. F47). These variations broadly correspond to different lithologies: respectively, clay- to silt-rich intervals and sand, glauconite, or cemented intervals. This correspondence to lithologies is particularly clear when investigated in combination with porosity (see "Combined density and porosity as a stratigraphic tool").

Calculated porosity

Calculated porosity, here referred to as porosity, is derived from the difference between the wet and dry weights and the 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 storage and handling. Porosity, particularly that of sands, may therefore be underestimated. Based on repeat measurements, we estimate that porosity of sands was probably underestimated by 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 20% to 65%.

Stratigraphically, porosity varies, with intervals of low porosity (~35%) and intervals of high porosity (~50%–60%) corresponding to lithology variations (Fig. F47). Clay- to silt-rich sediments in general have high porosities, and sand, glauconitic, or cemented sediments have low porosities. Such differences are in contrast to deeply buried sediments in general (Bahr et al., 2001).

The top 200 m of Hole M0027A is characterized by reduced sediment porosities. Coring techniques in these less consolidated intervals included hydraulic piston coring and extended nose coring (see "Operations"). This may have resulted in dewatering and thus reduced calculated porosity. Also, loose sands may have drained more during storage and handling, resulting in artificially low porosities.

Below ~200 mbsf the porosity of the clay and silt intervals decreases with depth. The same tendency is observed for sand-rich intervals. This porosity reduction is a result of compaction caused by the lithostatic load. Porosity reductions broadly follow the exponential decrease in porosity of sands, silts, and clays that have been observed from a number of passive margins and derived analytically (Sclater and Christi, 1980; Bahr et al., 2001). At depths shallower than ~700 mbsf, the exponential porosity reduction is nearly linear (Bahr et al., 2001).

Combined density and porosity as a stratigraphic tool

When viewed in conjunction, wireline logs of density and porosity are very useful for detecting different lithologies in deeply buried sedimentary successions (Rider, 2006). Even at shallow depths in Hole M0027A there is a significant inverse variation between porosity and density of the samples (Fig. F49). The different lithologies of Hole M0027A fall close to a mixing line between rock and saltwater. Rock has zero porosity and a density of 2.7 g/cm3. Saltwater has 100% porosity and a density of 1.024 g/cm3. Sand, glauconite sediments, and the majority of clay and silt samples plot above or close to the line. A significant portion of the silt and clay samples plot below the line with reduced densities. This could result from the presence of organic matter that causes the density to be reduced comparatively more than if the decrease was only caused by a porosity increase (Rider, 2006).

Downhole, the separation between density and porosity curves varies from intervals with lower density and higher porosity to zones with higher density and lower porosity (Fig. F50A, F50B, F50C, F50D). These variations correspond to the main lithology variation in the hole, as shown in the crossplot (Fig. F49). In this context, it is worth noting that at these shallow depths the porosity of clay and silt is still relatively higher than that of sands. As a result, the "separation" of porosity and density curves are not fully expressed, as in deeply buried shale and sand successions (cf. Rider, 2006).

In a sequence stratigraphic context, as determined by sedimentology, transgressive and maximum flooding intervals seem to be characterized by a rapid reduction in density and an increase in porosity (Fig. F50C). Intervals with abundant organic phytoclasts seem to have densities reduced relative to porosity. Overall, density increases and porosity decreases in coarsening-upward prograding successions (Fig. F50).

P-wave velocity

Velocity measurements on whole cores can be correlated well with density measurements for the majority of Hole M0027A, although in places there is considerable scatter in the velocity data due to the variable liner fill and saturation of cores (see "Physical properties" in the "Methods" chapter). Major trends in P-wave velocity also correlate with major trends in the sonic log. There are some offsets observed in the MSCL data that are absent in the downhole log, often between successive sections or cores. Some of these offsets may relate to the degree of liner fill and the suitability of some of the whole cores for measurement (see "Physical properties" in the "Methods" chapter). Velocity trends are described more extensively in "Downhole measurements."

It was not possible to measure any discrete samples for velocity in the upper part of the hole above 195 mbsf or between 325 and 415 mbsf because of the unconsolidated nature of the core. In the clays between 195 and 208 mbsf, velocities are consistent and low. More variable values are present in the more variable grain-sized sands and silts in Units II–V. In Unit IV, there are fewer velocities attained on the MSCL because of more variable liner fill. In the sandstone around 315 mbsf, velocity measured on discrete samples is higher than that of the downhole sonic log. Higher values in both MSCL velocities and discrete velocities are apparent at the cemented levels at the Unit V/VI boundary. On average, velocity increases slightly with depth for both discrete samples and whole-core measurements. MSCL velocities increase at the Unit II/III boundary (236 mbsf), consistent with the increase observed in the downhole sonic log. Velocities on discrete samples were measurable again from 415 mbsf in the silts and display some variation with grain size.

Downhole from ~468 mbsf, velocities increase in both discrete samples and MSCL measurements as the succession becomes a glauconitic sand in Units V and VI. Velocities remain high and vary with cementation from here to the base of the hole.

Magnetic susceptibility

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. Core measurements and log measurements correlate extremely well, although absolute values are different. Hole M0027A trends associated with lithological units are more extensively described in "Downhole measurements."

Electrical resistivity

Resistivity is affected by lithology and cementation but also pore fluid and pore water salinity, as well as core liner saturation. In general, there appeared to be a decrease in the amount of drill fluid in core liners down a recovered core. As a result, the last section of every core, generally a short section, artificially has very high resistivity and was removed from analysis. Most very short sections have low core liner saturation. Removal of this effect has been attempted by high-pass filtering (Fig. F47). Resistivity values are low in some poorly recovered sands. Despite the uncertainties, resistivity measurements downhole have interesting variations related to changes in salinity of formation water and lithology, minerals, and organic matter.

Resistivity is highly variable from low values of 0.2 Ωm to high values of 16 Ωm. Notable high resistivity occurs at 21–29, 176–189, 265–267, 413–425, and, less certainly, 354–355 mbsf. The lowest resistivity is registered at 32–49 mbsf, sporadically between 94 and 164 mbsf, and between 600 and 623 mbsf. Values are low (0.8 Ωm) between 344 and 410 mbsf with a short interval of intermediate values at 353–361.5 mbsf.

The high resistivity of silts at 413–425 mbsf overlain by low-resistivity sands is an interesting feature, with the upper limit of silts corresponding to the transition from low to high chlorinity (see further description for the rest of the hole in "Conductivity logs").

At centimeter-scale resolution, the correlation between lithology and the high-pass filtered resistivity is variable and is related to variable amounts of low-resistivity minerals, high-resistivity lignite, and the degree of cementation (Fig. F50A, F50B).

Digital linescans and color reflectance

All cores from Hole M0027A were imaged using an X-ray fluorescence (XRF) digital linescan system equipped with a charge-coupled device (CCD) camera using 3 × 2048 pixels with a beam-splitter and a manually controlled Pentax 50 mm lens. Data were recorded at a resolution of 0.068 mm as both images and red-green-blue (RGB) values. All suitable cores were also scanned for color reflectance. These data may reflect changes in glauconite content (green wavelengths) or iron content (seen in a*/b* ratios). The varying suitability of the core section for color reflectance measurements was observed to have a significant effect on the scatter present within the data set (see "Physical properties" in the "Methods" chapter). Some effort was made to correlate color reflectance changes as measured by the Minolta with other measurements and observations, but detailed filtering and comparisons were deemed to fall within the domain of postexpedition research.

Thermal conductivity

Thermal conductivity data vary downhole and can be divided loosely into four intervals. In the upper ~150 m of the hole, there is considerable variation in thermal conductivity in the unconsolidated sands, although values are generally high, with a departure to a lower value around 80 mbsf. Between ~175 and ~375 mbsf, thermal conductivity decreases quite sharply to a low around 275 mbsf before increasing again. From 375 to 425 mbsf is another decrease. At ~425 mbsf, conductivity varies but generally stays low to the base of the hole. There is a correlation between thermal conductivity and chlorinity. However, the transition from low to high values tends to be sharper in the geochemical data than in the thermal conductivity data, possibly because of some diffusion processes occurring in the core between drilling and measurement subsequent to transportation and storage. This may have resulted in some mixing of fluid from upper and lower levels prior to thermal conductivity measurement, whereas geochemical data were derived from pore water extracted immediately after coring. For most of Hole M0027A, thermal conductivity and pore water chlorinity trends are parallel (see "Downhole measurements"). The highest values are between 353 and 412 mbsf in a saline interval. Values here may be linked to higher values in the quartz-rich sands.

Natural gamma radiation and core-log correlation

All cores were measured for NGR with the primary purpose of core-log correlation (see below). Data collected are equivalent to the TGR downhole log. Although absolute values are different, trends are the same. Gamma ray trends and characteristics are extensively described in "Downhole measurements." In general, correlation between core and log data sets is excellent, with only minor (<1 m and usually <0.6 m) vertical offsets in any interval (e.g., Fig. F51). The correlation data set is a most straightforward match where there is both good core recovery and distinctive peaks in both data sets.

Correlation between core and logging data is evaluated primarily using NGR, but several petrophysical data sets can be used. Toward the top of the hole, the correlation process enabled a systematic error in top core depth to be identified and corrected in Cores 313-M0027A-25R through 58R (see "Stratigraphic correlation").