Previous   |    Next

doi:10.2204/iodp.proc.302.104.2006

Petrophysics

Laboratory and in situ measurements were performed to characterize downhole variations in the physical properties of sediments on the Lomonosov Ridge. Petrophysical measurements included (1) downhole wireline logging, (2) nondestructive whole- and split-core measurements performed with the Geotek MSCL, and (3) discrete measurements carried out on both whole and split cores. Wireline logging provided in situ measurements of NGR, P-wave velocity, and resistivity (FMS). These data complement nondestructive whole-core determinations of bulk density, compressional P-wave velocity, electrical RES, MS, and NGR. In addition to whole-core measurements, split-core MSCL measurements of P-wave velocity, MS, and density were made on selected sections. Digital line scanning was performed on all split cores, as well as discrete color reflectance measurements. Other discrete measurements included needle point probe thermal conductivity, moisture and density (MAD) properties (bulk density, porosity, water content, and grain density) and shear strength (Torvane, pocket penetrometer, and fall cone). For a full description of petrophysical measurement methods, refer to “Petrophysics” in the “Methods” chapter.

Data quality and overview

Downhole logs were acquired in Hole M0004B from below the pipe at 65 mbsf to the bottom of the hole at 218 mbsf, providing data through lithostratigraphic Subunits 1/3 to 1/6. The tool string included the FMS, BHC tool, NGT, and the SGT (for a full description of the logging tools see “Petrophysics” in the “Methods” chapter). Two successful passes were made with the wireline depth to seafloor, determined using the step increase in the SGT and NGT data, set at 1291 mbrf. This compares favorably with the drillers depth of 1289.7 mbrf.

Caliper logs from the FMS provide a method for assessing borehole conditions. Caliper logs from both passes are presented in Figure F20. The outside diameter of the bit was 9½ inches; it can be seen that for much of the formation the hole diameter is under gauge. Narrowing of the hole occurs between 75 and 90 mbsf, at 155 mbsf, and again between 180 and 184 mbsf. The caliper logs indicate that borehole conditions are good, and given the narrow borehole diameter, the FMS pads should have made contact with the borehole wall for the entire length of the logged section. Nowhere is the borehole washed out to a degree where it would adversely affect tool response.

Because of the ice pack, wireline heave compensation was not required. The depth match between the logging passes is good, generally less than ±1 m. The largest offset is 2.6 m at ~155 mbsf. This offset has been removed by depth-matching the passes. It should be noted that the microresistivity images provided by the FMS have not yet been used to aid any interpretation. This is because they have been obscured in large part by the selected drill bit that marked the wall of the borehole (Fig. F21). Further reprocessing and detailed core-log integration could improve the quality of these images.

Offshore, all undisturbed cores >14 cm in length were run on the MSCL except for Core 302-M0004C-6X, which was cored without using a liner and subsequently was too thick to fit through the MS loops. This core was logged at the BCR using the split core configuration of the Geotek MSCL for measurements of density, P-wave velocity, and point-source MS. The split core MSCL records sediment thickness during logging and uses this value to correct density and velocity measurements.

Even after the automated thickness correction was applied to Core 302-M0004C-6X, split-core density measurements remain positively correlated with sediment thickness (Fig. F22). Furthermore, the higher density values in Section 302-M0004C-6X-1 are not supported by MAD measurements. The positive correlation between density and core thickness and the divergence of logged values from those acquired through MAD measurements suggests that the displacement transducers on the split core system were not calibrated. Corrections to the logged values were obtained by detrending the density data using the slope of the density versus core thickness regression. Further refinement can be achieved by correcting the density for the offset between MAD-derived results and logged results, a procedure that should also be applied to whole-core measurements.

Split-core logging of P-wave velocity was also performed on Cores 302-M0002A-5X, 15X, 21X, 23X, 24X, 27X, 30X, 37X, 38X, 40X, 42X, and 44X. Similar corrections for P-wave velocity on split cores were attempted, and an integrated data set of split-core, whole-core, and logging velocity is planned. Finally, susceptibility values obtained using the Bartington MS2F point sensor on the split-core system are not corrected for volume effects. Bartington recommends multiplying the point sensor values by 2 to acquire susceptibility values comparable to those obtained using the loop sensor.

In general, core quality was good during APC operations in Holes M0003A and M0004C. However, the majority of the material recovered from the Lomonosov Ridge was cored with an APC or extended core barrel shoe on a traditional, rotating extended core barrel. The rotating barrel caused undercutting through many of the more lithified intervals and resulted in significant core disturbance through the softer sediments. These disturbance effects are especially pronounced in the shallower intervals (Subunits 1/2 and 1/3), where less consolidated material is found.

MAD data collected during both the offshore and onshore phases of ACEX included bulk density, porosity, water content, and grain density. Comparison of the bulk density from MAD measurements with those calculated on the MSCL reveals close agreement through most of the section (Fig. F23; Tables T34, T35). Corrections to the MSCL-derived density can be made on a core-by-core basis using MAD measurements.

The most likely source for errors in offshore MAD measurements lies in the sample weighing process using the marine analytical balance. Intense shaking and vibration associated with icebreaking reduced the accuracy and precision of the balance. Further measurement errors in offshore data are likely due to the sampling procedure and sediment texture. For example, the four samples taken from the interval between 243 and 250 mbsf all show a lower bulk density than the MSCL data. These are from an interval that is categorized as a mud-bearing biosiliceous ooze and consists mainly of diatoms (see “Lithostratigraphy”). The dry ooze easily disintegrates into small particles of millimeter to centimeter size. For this reason, the constant-volume samples were difficult to fill properly, causing the anomalously low densities. Density values acquired using the MSCL may also be slightly biased through the biosiliceous interval, as the calibration technique of the MSCL GRA-derived bulk density assumes that the sediment measured is quartz-based.

Downhole variations in all petrophysical properties highlight a number of prominent changes that correlate well with observed seismic reflectors (see Jakobsson et al., this volume). Composite profiles of MSCL data are described in “Stratigraphic correlation.” In this section, results from all petrophysical measurements are presented for each hole in mbsf depths (Figs. F24, F25, F26, F27) and described by the main lithostratigraphic units (1–4). For more detailed sedimentological descriptions see “Lithostratigraphy.”

Lithostratigraphic Unit 1: 0–219 mbsf

Broadly speaking, this unit is a silty clay becoming coarser toward the bottom of the section (see “Lithostratigraphy”). The increase in the P-wave velocity downhole log from the pipe (65 mbsf) to 195 mbsf occurs without any appreciable change in bulk density, reflecting this coarsening trend (Fig. F28).

Lithologically, the unit was divided into a number of subunits (see “Lithostratigraphy”) that correlate with changes in color reflectance data (Fig. F29, F30, F31, F32) but do not necessarily match distinct changes within the other petrophysics data sets. The continuous core and downhole logs reveal significant variation throughout the unit, occurring at decimeter and larger (depth) scales.

The upper ~20 mbsf cored in Holes M0002A, M0003A, and M0004C and MSCL data sets exhibit a first-order increase in both density and velocity that presumably results from normal consolidation processes. Well-defined, in-phase, decimeter-scale variations in density, velocity, susceptibility, and NGR are present. High values of a* and b* chromaticity represent dark brown color banding of silty clay layers in the upper ~20 mbsf.

A distinct color change occurs at the boundary between lithostratigraphic Subunits 1/2 and 1/3 (Fig. F29, F31, F32). The existence of this color change as a syndepositional feature at all sites remains uncertain, and it may prove to be a geochemical front found at different stratigraphic intervals (Fig. F32). Much of the variability below 20 mbsf in the MSCL data may be tied to drilling-induced disturbances and needs to be carefully edited prior to use by referring to the disturbance table constructed while the cores were being split at the BCR (see “Core Descriptions;” Table T24).

At 45 mbsf, there is a noticeable decrease in the MAD-derived porosity that is mirrored by a slight increase in bulk density and very little change in the grain density (Fig. F23). This shift is associated with an increase in the consolidation ratio (see discussion below). Between 70 and 100 mbsf, cored only in Hole M0002A, there appears to be a reduction in variability in MSCL data, but poor recovery through this interval makes it difficult to decipher the extent of this change. This is the first interval for which downhole logging data are available, and it is characterized by a small increase in the baseline gamma radiation measurements, reflecting a potential increase in clay content (potassium contribution) (Fig. F24). Downhole velocity shows a small increase from the pipe to ~75 mbsf where it fluctuates around a baseline value of 1550 m/s to ~95 mbsf (Fig. F28).

Below 100 mbsf, the downhole gamma radiation logging cyclicity shows a number of frequencies (meter and larger depth scale) varying across a relatively constant baseline of 80–85 gAPI (Fig. F33). A shift in wireline velocity from ~1550 to ~1650 m/s occurs between 99 and 101 mbsf. It appears in both passes of the BHC, with the caliper logs indicating that the hole is in good condition, and is interpreted to be a true high-velocity layer that may prove useful as a tie to the seismic stratigraphy. This feature was not recovered in Hole M0002A. Another distinct change in the character and magnitude of the P-wave velocity trace occurs at 136 mbsf and is not accompanied by any noticeable change in the gamma radiation log (Figs. F33, F28). This suggests that the change in velocity is not driven by a change in clay mineral composition. The velocity contrast at this depth is of sufficient magnitude to produce a seismic reflector and to provide a tie to the seismic stratigraphy.

At ~153 mbsf, the borehole rapidly becomes under gauge (Fig. F20) and this is closely associated with a peak in downhole gamma radiation and drop in velocity. The decrease in velocity suggests a more porous, perhaps less consolidated, layer. This interval is assumed to represent swelling clays closing the borehole. Changes in the clay composition should be addressed by more detailed postcruise analysis, but a change in color reflectance (b*) at this same depth seems to support this interpretation (Fig. F28).

A slight but noticeable drop in all MSCL logs occurs in Core 302-M0002A-38X (~166 mbsf) (Fig. F24) and accompanies the transition from predominantly olive-green sediments into those characterized by a more yellowish to brown hue (see “Lithostratigraphy”). A change in the character and baseline value of the gamma radiation log is also apparent at this depth (Fig. F33). The borehole is again under gauge between ~180 and 185 mbsf, and this correlates with increasing gamma radiation values. Spectral gamma ray data point toward an increase in thorium content at this depth (Fig. F33). This section is not recovered in Hole M0002A. Below 185 mbsf, there is a rapid drop in the downhole gamma radiation log that displays low-amplitude meter-scale cyclicity downhole. A distinct brownish layer between 168 and ~192 mbsf in Subunit 1/4 is clearly identified in high a* and b* chromaticity values (Fig. F29).

One of the most prominent changes in MSCL data starts in Core 302-M0002A-44X at ~192 mbsf (Fig. F24), where a drop in susceptibility marks a sharp transition from the brown to a light greenish gray matrix material marking the boundary into Subunit 1/5 (see “Lithostratigraphy”). Over the next few meters, black banding in the cores correlates with a zone of large susceptibility spikes. Sharp peaks of high L* and low a* correspond to particular light gray and dark brown zebra-stripe color banding in Subunit 1/5 between 192.94 and 198.13 mbsf (Fig. F29).

A large drop in the MSCL P-wave velocity at ~198 mbsf (Fig. F24) marks the transition into Subunit 1/6, a silty clay to clayey silt with minor amounts of siliceous microfossils and enriched in both organic carbon and pyrite (see “Lithostratigraphy” and “Geochemistry”). This is reflected in MSCL data as the low-velocity, high-density unit extending between Cores 302-M0002A-47X and 51X. The high density originates from the presence of pyrite, and low velocity is likely associated with the increased organic carbon content of the sediment. A unique meter-long interval where density decreases from ~1.9 to 1.3 g/cm³ is captured in Section 302-M0002A-47X-2 and closely matches a darkening of the sediments. Below Core 302-M0002A-51X at ~220 mbsf, the density steps dramatically from 1.7 to 1.3 g/cm³ without a noticeable change in the P-wave velocity. This change accompanies a reduction in the pyrite concentration, as documented in the core descriptions, and a transition into the mud-bearing biosiliceous ooze of Unit 2. Core recovery through this interval (~195–205 mbsf) is such that the true nature of the boundary is not yet apparent. The downhole velocity log shows the boundary as a sharp increase in velocity at ~200 mbsf that drops in a series of cyclic fluctuations. Unfortunately, it is at this depth that downhole logging data terminate. Postcruise integration of core and logging data should aid in interpreting the exact nature of this critical transition.

Lithostratigraphic Unit 2: 220–318 mbsf

From the top of the unit to ~240 mbsf, all of the core physical properties vary around a constant baseline value. From 240 mbsf to the bottom of the unit, a gradual increase is apparent in the bulk density (Fig. F23). The increase in density through this unit may result from either normal consolidation processes or from a gradual downhole increase in the clay content. Discrete sample MAD grain density decreases sharply between 220 and 240 mbsf before increasing gradually to the bottom of the unit. The core NGR suggests a subtle increase in clay downhole, increasing more rapidly between ~285 mbsf and the bottom of the unit. MS shows little change through this unit.

Lithostratigraphic Unit 3: 318–404.75 mbsf

From the top of the unit to ~345 mbsf, there is little change apparent in any of the physical property data (Fig. F26). Below ~360 mbsf, the unit is characterized by higher bulk and grain density, which co-vary with porosity (bulk density fluctuates between 1.6 and 2.1 g/cm³). Core velocity also increases below 360 mbsf and is >1600 m/s. Magnetic susceptibility becomes significantly elevated below 370 mbsf (>1.5 × 10^–3 SI units). Throughout this interval, large amounts of pyrite are found in the cores. Very large peaks in susceptibility (>5 × 10^–3 SI) and density (>3 g/cm³) indicate the presence of clastic material that is probably of diagenetic origin.

Lithostratigraphic Unit 4: 404.75–427.63 mbsf

The cores recovered from this unit, documenting the transition through sandstone and mudstones and into the basement, were too short and disturbed to be run on the MSCL. NGR data were measured on a few short sections in the bottom of the hole but should be treated with caution.

Undrained shear strength

Measurements of the undrained shear strength (S_u) were performed offshore using either the pocket penetrometer or Torvane (Fig. F34; Table T36). Torvane measurements were restricted to the upper 40 mbsf, where sediments were soft enough to allow for the rotation of the shearing vane. For accurate strength measurements, the sample must be fairly cohesive and remain saturated during the test. Very stiff samples often crumble rather than shear, thereby underestimating the actual value. Torvane determinations of shear strength were made on cores from Holes M0003A, M0004A, and M0004C.

In Hole M0002A, the pocket penetrometer was used to measure the unconfined compressive strength of the sediments. Measurements made with the pocket penetrometer were converted from kilograms per square centimeter to kilopascals and then divided by 2, as the penetrometer measures unconfined compressive strength which is equal to twice the undrained shear strength in an ideal clay (Holtz and Kovacs, 1981). The maximum measurable strength was 245 kPa. Below 200 mbsf, clayey intervals were generally too stiff for this test method.

Onshore, the undrained shear strength was measured on split cores using a cone penetrometer (Fig. F34; Table T36) (for conversion formula see “Petrophysics” in the “Methods” chapter). The fall-cone device (Skempton and Bishop, 1950) provides a rapid and simple method for determination of undrained shear strength for undisturbed (as well as remolded) clays.

The consolidation ratio (CR), determined by dividing the undrained shear strength by the in situ effective stress (P′; assuming hydrostatic pressure), is an index for assessing the stress history of sediments. For normally consolidated clays, the consolidation index should be uniform with depth and fall between 0.2 and 0.3. The consolidation index (S_u/P′), calculated by assuming an average sediment bulk density of 1.85 g/cm³, is illustrated for the upper 260 mbsf in Figure F34.

There is generally close agreement in the consolidation index derived from shipboard and shore-based measurements. Between the seafloor and 40 mbsf, there is a decrease in the CR, beginning at the seafloor at ~0.5 and decreasing to ~0.05. An abrupt shift from 0.05 to 0.2 occurs at this depth. This shift coincides with the transition from Torvane to pocket penetrometer measurements but has also been recorded in the fall-cone data collected onshore. Although the consolidation ratio changes significantly here, it still remains within the range for normally consolidated marine sediments. This shift in the CR does not occur across any lithologic boundary, but is mirrored by a shift in the MAD-derived porosity (Fig. F23).

Measurements made with the fall cone become increasingly scattered below 130 mbsf and tend to fluctuate between 0.1 and 0.6. This variability is not captured in the lower-resolution cone penetrometer tests, suggesting that the variability is a measurement artifact resulting from the sediment strength exceeding the limit for the cone used during testing. Results from incremental load consolidation tests on samples taken from below this transition interval will offer a more accurate means for assessing the stress history of sediments in this interval.

Thermal conductivity

Thermal conductivity measurements were collected throughout lithostratigraphic Units 1 and 2 and document a clear a downhole trend (Table T37; Fig. F35). Conductivity values have not yet been corrected for in situ temperature conditions. Uncorrected conductivity initially increases from 1.07 to 1.35 W/m·K at ~50 mbsf. A slight decrease in the conductivity occurs below 50 mbsf; however, a limited number of measurements over the subsequent 20 m makes it difficult to assess the significance of this drop. Between 90 and 160 mbsf, thermal conductivity values remain around 1.4 W/m·K before they begin to fall toward the base of the unit. The single low value at 197 mbsf is a good measurement and was obtained in Subunit 1/5. Only one thermal conductivity measurement (0.739 W/m·K) was obtained in the biosiliceous ooze of Unit 2 at 275 mbsf.

In situ temperature measurements

Five in situ temperature measurements were made during coring operations in Holes M0004B and M0004C (Fig. F36). Three measurements made in Hole M0004B were performed using the BGS temperature tool, whereas two taken in Hole M0004C were made using the Adara temperature tool (Fig. F37). Frictional heating of the sediments during probe insertion is much less likely with the BGS temperature tool as evidenced by the temperature decay curves, and frictional heating of the tool does not appear to have raised the tool temperature above the in situ temperature (see “Petrophysics” in the “Methods” chapter for a description of the tools). Data for the BGS and Adara tools have not been processed to acquire the equilibrated in situ temperature, a process that should be completed postcruise. Equilibrium temperatures are generally calculated using an automated curve-fitting approximation technique (Davis et al., 1997). However, both the material and geometry of the tool influence the temperature decay process after insertion into the sediments. Postcruise processing of the BGS temperature measurements will need to address the differences in the tool's temperature decay response.

The mudline temperature was recorded on all runs and varied between tools. A preliminary attempt to normalize the in situ measurements was made by using the average Adara-determined mudline temperature and adjusting all in situ measurements to this baseline value. The results from all the runs and the applied correction factors are shown in Table T38. The average gradient calculated using all available data points is 43.2°C/km (Fig. F36).

The two deepest measurements were taken in Hole M0004B at depths of 60 and 100 mbsf using the BGS temperature tool. The values from these two runs are quite similar, with the measured temperature at 100 mbsf actually being less than that at 60 mbsf. Errors associated with in situ temperature measurements can arise from frictional heating if the tool is moved during the measurement process by either ship heave or rotation of the core barrel, or when a cooler temperature is measured, it may indicate that the tool was exposed to drilling fluid while it was supposed to be embedded in the seafloor. The measurement taken at 100 mbsf may be an example of the latter case. Data quality and equilibrated in situ temperatures will be reviewed postcruise.

Top of page   |    Previous   |    Next