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doi:10.2204/iodp.proc.313.105.2010

Downhole measurements

The focus in this section is to provide a description of the downhole data sets collected in Hole M0029A and their most significant characteristics. A few examples of log trends and boundaries where trends in MSCL core measurements are equivalent to borehole measurements are discussed in this section. Some variations depend mostly on lithology, whereas some are controlled by postdepositional factors such as the degree of cementation, diagenesis, and the type of interstitial water formation. Integration of core and logging petrophysical data allows calibration of core data with in situ borehole properties and provides an assessment of the precise depth below the seafloor. These correlations are briefly described in "Physical properties." At the end of the chapter, a synthesis of petrophysical data, downhole, and derived quantities are presented for each lithostratigraphic unit.

Downhole measurements in Hole M0029A

In Hole M0029A, 1923 m of wireline logging data was acquired, in addition to VSP measurements (see Tables T7 and T8 in the "Methods" chapter). Spectral gamma logs were acquired through the steel drill pipe down to 757 m WSF.

Two overlapping sections of open hole were logged in Hole M0029A, from 729 to 483 m WSF (lower section) and from 491 to 403 m WSF (middle section), as well as a small upper section between 348 and 322 m. In the lower interval in open hole, acoustic, conductivity, sonic, and magnetic susceptibility logs were run. In the middle interval, conductivity, magnetic susceptibility, and sonic logs were acquired, and in the upper section, open-hole spectral gamma and conductivity logs were collected. VSP measurements through the drill pipe were acquired at 3.05 m spacing from 753 to 207 m WSF and then on a second run from 226 to 13 m WSF. Figure F41 provides an overview of downhole measurements for Hole M0029A (see "Core-seismic-log synthesis").

In all figures, downhole geophysical data and images are plotted in meters below present-day seafloor based on the wireline scale (WSF), and, where appropriate, boundaries between lithostratigraphic units (Units I–VII; see "Lithostratigraphy") are indicated to facilitate comparison with other chapters.

Spectral gamma ray logs

Spectral gamma ray logs allow continuous characterization of the entire borehole because they mainly depend on lithology. As in Hole M0027A and M0028A, the TGR signal from the downhole probe and the NGR measurements run on whole cores correlate very well, in some cases allowing accurate repositioning of core depth below seafloor (see "Stratigraphic correlation").

Total gamma ray counts in Unit I (see "Lithostratigraphy") display high variability (Fig. F41). TGR values around 40 counts per second (cps) correspond to sand, whereas excursions to higher values (above 100 cps) correlate to clays, as observed in the recovered cores. From the top of Unit II to 600 m WSF, TGR values are around 170 cps, with several lows corresponding to coarser intervals (479.20, 490.90, and 497.62 m WSF) and two peaks around 560 and 565 m WSF corresponding to increased Th content. The lower part of Unit II is characterized by a drastic increase in K content related to glauconite-rich sand packages. These sands are also clearly evidenced by some peaks in magnetic susceptibility and higher density. Glauconite-rich sands are also observed in the units below, at the bottom of Unit V. The transition to the tan-colored clayey silt of Unit VI is not clearly expressed in the total gamma counts. However, as for Unit VI in Hole M0028A, this unit is characterized by a clear increase in Th content and low K and U.

The gamma ray record in Hole M0029A can be interpreted using the same guidelines used in Holes M0027A and M0028A ("Downhole measurements" in the "Site M0027" chapter and "Downhole measurements" in the "Site M0028" chapter), based on TGR (ASGRcgs) (Fig. F41); concentrations of K, U, and Th; and their ratios. TGR and elemental composition allow continuous lithological interpretation, which can be precisely tied to core observations and extended to unrecovered intervals.

Here we propose, as a preliminary attempt, an interpretation of the upper part of the hole (0–320 m WSF) (Fig. F42) where coring was sparse (see "Lithostratigraphy" and "Stratigraphic correlation" for comparison). The interpretation of log data suggests the presence of silt and clay in the upper part of Hole M0029A (0–53 m WSF) and mainly sand downhole, containing clay intervals at different levels. Several sharp contacts between sands and clays (43.5, 53, 154, 182, 220, 250, 287, and 293 m WSF) are observed. The sand may be glauconitic in the upper part of the hole, with glauconite content decreasing to 104.5 mbsf. The presence of glauconite is possibly detected in the 182–191.5 m WSF interval. Glauconite was not observed in the recovered cores (see "Lithostratigraphy"), but it might occur between 10 and 20 m WSF, as inferred from high magnetic susceptibility and a Th/K ratio <0.05. An XRD result at 7.28 mbsf supports this, as a relatively high amount of glauconite is indicated compared to cores from lower in the hole.

Variations in the Th/K ratio indicate that the glauconite content decreases in the sand near the clay intervals. This pattern is observed in the 0–100 m WSF interval (12, 43–53, 57.5, and 68 m WSF clayey intervals) (Fig. F42). In Figure F42, clay intervals are underlined in light gray and organic matter–rich intervals are underlined in dark gray. In clay intervals, Th/K ratios are close to 0.1, whereas in glauconitic sands, Th/K ratios are <0.05 and decrease on both sides (above and below) of clay layers (arrows in Fig. F42) and could be interpreted as resulting from diagenetic chemical diffusion processes at clay/sand boundaries.

Organic matter can be tentatively identified by peaks in U that do not encompass changes in K. It is noteworthy that organic matter content seems to increase downhole, with occurrences in sands at different levels. In Unit I, small peaks were encountered at 75, 83, 268–269, 274.5–278, 289, and 293–304 m WSF. A slight increase in organic matter occurs downhole in the 344–400 m WSF interval. Systematically higher values of U, compared to Th, from 470 m WSF downhole can be interpreted as better preservation of organic matter, especially in the glauconitic units. U content is particularly high at 560 and 564 mbsf, at the boundary between high- and lower density sediments (see "Physical properties"). From 600 to 700 m WSF, there is alternation of glauconitic sands and clays that are slightly depleted in Th and enriched in K and U.

One of the most striking features can be observed at the upper and lower boundaries of lithostratigraphic Unit VI (728.5–742 m WSF). Unit VI itself is characterized in the gamma ray record by depleted K and U and by increased Th when compared to the adjacent unit (Fig. F35). K and Th contents are highest in the center of the unit and decrease slightly toward the edges. Glauconite can be identified in correspondence of the upper and lower boundary by K and U peaks and low Th content.

Below 320 m WSF, lithologies inferred from the gamma ray logs are in good agreement with core observations (see "Lithostratigraphy"). Several sedimentary sequences can also be predicted from gradual changes in gamma ray measurements, coarsening either uphole (222–227, 267–287, 304–314 m WSF, and slightly between 344 and 400 mbsf) or downhole (231.5–236, 248–250, 287–293, and 314–318 m WSF).

Magnetic susceptibility logs

Magnetic susceptibility is commonly a clear indicator of lithological variations. In all holes, magnetic susceptibility signals from the wireline measurements (EM51 probe) and MSCL on whole cores correlate very well, in some cases allowing very accurate repositioning of core depth below seafloor. As the magnetic susceptibility wireline log in Hole M0029A begins at 404 m WSF in Subunit IIA (see "Lithostratigraphy"), further description of magnetic susceptibility throughout the borehole can be found in "Physical properties." In general, magnetic susceptibility trends correlate with glauconite occurrence throughout, with the exception of the tan clays of Unit VI (Figs. F41, F43). The magnetic susceptibility curve correlates also very well with K content (related to glauconite), density, and P-wave velocity. It anticorrelates with conductivity and, to a lower degree, with discrete porosity. In more detail, from the beginning of the lower logged interval at 404 m WSF, magnetic susceptibility is low and drastically increases at 602 m WSF (top of Subunit IID₁). Below this unit (Fig. F41), magnetic susceptibility fluctuates from very high values to intervals of lower values but always at a higher base level than that found above 602 m WSF. Lower values correspond to siltstone levels apparently poor in glauconite, whereas higher values tie to cemented glauconitic-rich intervals (also high velocities on the sonic and low conductivity values; see below). Consistently high magnetic susceptibility intervals are observed between 634 and 640, 666 and 674, and 698 and 705 m WSF, each of which represents glauconite estimates of at least 60% in part of each interval. From 720 m WSF to the end of the log at 730 m WSF, values are low with a slight increase downhole.

Acoustic image logs

The ABI40 was run at a resolution of 288 ppt in the lower part of Hole M0029A between 657 and 483 m WSF. Acoustic amplitude and traveltime images provide a gauge of the induration and texture of the borehole walls. Lithological changes and potential impedance contrasts can be identified in several places, despite the fact that the tool was not centralized in the hole. For example, lithologic unit boundaries are clearly evidenced on the acoustic images at 620.42 and 650.13 m WSF, correlating with the Subunit IID₁–IID₂ and Unit IV–V transitions, respectively. In Figure F43, a sharp contact is clearly observed on both the amplitude and acoustic images at the base of a submarine fan/apron corresponding to the lower part of Subunit IID₁ (see "Lithostratigraphy"). The transition between Subunits IIC and IID₁ is also marked by a change in borehole diameter that is more irregular and locally smaller in the very fine sandy silt and silt forming Subunit IIC. Slight changes in the amplitude image reflect locally some changes in lithologies, such as the occurrence of glauconite-rich sands between 607.52 and 608.45 m WSF (e.g., Core 313-M0029A-163R).

Sonic velocity logs

Sonic velocity logs were obtained from 720 to 404 m WSF in open hole (Subunit IIA₃ through Unit V). Velocity measurements from whole cores and discrete samples are described in "Physical properties" in the "Methods" chapter. All velocity measurement trends generally correlate relatively well with each other as well as with the core density data. In Hole M0029A, 2PSA P-wave velocities gradually increase from 404 to 602 m and become more irregular deeper, with alternating high and low values (Fig. F41). In more detail, sonic velocities are generally low (<1750 m/s) until 449 m WSF, where a small peak corresponds to a sand layer (interpreted as a sequence boundary; see "Lithostratigraphy"). Below this, velocities remain slightly higher (Subunit IIB₁) until decreasing over ~10 m to 479 m WSF, where there is a sharp increase in a siltstone horizon. The sonic log indicates that the maximum occurs within a core recovery gap. Velocities downhole of this gap are slightly variable but become more consistent around 502 m WSF as the lithology becomes finer grained. A major change occurs at 602 m WSF (Subunit IID₁) (Fig. F43) with correspondingly higher core velocities (where obtained). This interval from 602 to 643 m WSF is characterized by variable, often high (>2400 m/s) velocities correlating with more indurated intervals, especially in the glauconite-rich coarse cemented intervals toward the base of Subunit IID₂ (see "Lithostratigraphy"). Notable peaks occur at 612, 624, 640, and 634–642 m WSF (Fig. F43), generally fitting conductivity lows, increased densities, and impedances (Fig. F45). Between ~668 and 674 m WSF, a further increase in velocity is apparent (Fig. F45). Between ~695 and 707 m WSF, a broad raised velocity peak corresponds to an elevated K/Th ratio and higher magnetic susceptibility. Only discrete velocities and a very few MSCL velocities were acquired from 720 m WSF to the base of the hole (see "Physical properties").

Conductivity logs

Conductivity in the borehole is influenced by a variety of aspects, including lithology, pore water content, and salinity. In Hole M0029A where data were acquired, the conductivity signal globally increases toward 600 m WSF and becomes highly variable below (Fig. F41). It is generally anticorrelated with density core values (Fig. F44). A small interval was logged close to the top of Unit II from 334.37 to 347.87 m WSF. In this interval, conductivity shows drastic changes in the signal, passing from high values peaking from 344.60 to 344.16 m WSF to lower values below. This peak fits with a cemented fine-grained horizon possibly developed at the transition between saltier pore waters above and less salty pore water below (Fig. F36).

In the rest of the hole, from 403 m WSF downhole, conductivity increases relatively constantly to 601.20 m WSF, with several lows related to coarser intervals also evidenced by small peaks in the sonic curves (e.g., 479.20, 490.90, and 497.62 m WSF). At 601.20 m WSF, a drastic change is observed when entering a coarse-grained interval associated with a debris flow apron (Subunit IID; see "Lithostratigraphy"). In this interval, several conductivity lows correlate with glauconitic levels, evidenced by high values in the magnetic susceptibility signal, or with indurated intervals also evidenced by sonic peaks and density increase (Fig. F43). Conductivity then increases at the transition between Units II and III, when entering siltstones. A succession of high-value intervals (644–661, 675–682, and 709–715 m WSF) and low-value intervals characterize Units III–V (Fig. F45). The highest values correspond to siltstone levels poor in glauconite (low susceptibility), whereas low values tie to cemented glauconite-rich intervals (high magnetic susceptibility, high velocities on the sonic log, and high densities measured from whole cores).

In contrast to Holes M0027A and M0028A, there is no clear evidence for correlation between chlorinity variations from interstitial waters and conductivity variations in Hole M0029A (Fig. F36; see "Geochemistry"). However, the absolute values of conductivity are greater in this hole, which correlates with the significantly higher chlorinity values (Fig. F36). This correlation ties in with lower average resistivity values measured on the MSCL (see "Physical properties") compared to Sites M0027 and M0028.

Vertical seismic profiling

VSP data were acquired through pipe for the entire borehole (753 m WSF uphole) in two stages, but none were acquired in open hole at this site (Fig. F46). Downgoing waves were picked and time-depth relationships calculated. The resultant time-depth curve can be used to calibrate the time to depth of the seismic reflection profile.

Example of multilog interpretation

Petrophysical and downhole measurements can provide evidence for sequence boundaries. Figure F43 is a compilation of several parameters between 596 and 626 m WSF, containing two sedimentary transitions between lithologic Subunits IIC and IID₁ and Subunits IID₁ and IID₂ (see "Lithostratigraphy"). As evidenced from the figure, data measured on the whole cores (MSCL data) match the wireline signal relatively well. Some slight shifts in core depth are, however, evidenced locally by comparing magnetic susceptibility peaks from the downhole probe (EM51) and from the MSCL (e.g., 607.5–608.4 and 625.35 m WSF). A sharp change in the acoustic signals is clearly observed on the ABI40 images at 620.42 m WSF (wavy yellow line). It marks the abrupt transition between sandy siltstones below and glauconitic sands above (Section 313-M0029A-167R-2). This facies change was observed at 620.52 mbsf in the cores (see "Lithostratigraphy"). At the same depth, petrophysical data evidence an increase in density and a slight increase in resistivity when passing from Subunit IID₂ to Subunit IID₁. The magnetic susceptibility signal also increases at this transition, reflecting the presence of glauconite in the sands located above. These sands are 18 m thick (from 602.10 to 620.40 m WSF). They are locally characterized by high TGR counts (602.10–608.30, 612–617, and 618.6–625.35 m WSF), reflecting enrichment in glauconite content. These intervals are also associated with increased magnetic susceptibility. In some places, the sonic shows peaks (607, 611.9, and 612.9 m WSF) correlating with cemented levels associated with low conductivities. At the top of the sands (dotted yellow line), the total gamma count drastically decreases when entering the glauconite-free silt unit located above 602.10 m WSF. The absence of glauconite is also testified by an increased Th/K ratio. This transition is also associated with a decrease in density and a slight increase in resistivity.

Downhole log and physical property integration

This section combines results of logging and physical property measurements with the main characteristics of the lithostratigraphic units. A brief summary shows links between key petrophysical intervals (Table T11) and stratigraphic surfaces (Fig. F44A, F44B, F44C, F44D, F44E, F44F, F44G, F44H). All depths are given in meters below seafloor and ignore small differences that may exist between core (mbsf) and log (m WSF) depths. More details can be found in "Physical properties" or above. Numerical data are accessible online; see "Publisher's notes" for links to the database.

Lithostratigraphic Unit VII

Unit VII is characterized by relatively high TGR that corresponds to clayey silt to silt with two lows at 754 and 750 mbsf. It shows relatively high U content, possibly due to the presence of organic matter, and an increase in magnetic susceptibility at the top.

The transition to Unit VI is not clearly marked on the gamma ray logs. It fits a low in magnetic susceptibility, a low in density, and an increase in porosity, although the few measurement points do not allow clear characterization of this boundary.

Lithostratigraphic Unit VI

TGR in Unit VI shows an overall serrated shape, high Th content, and low amounts of K and U. Magnetic susceptibility is high and highly variable, reflecting the presence of magnetic minerals except glauconite, as indicated by the high Th/K ratio. The estimated clay volume is also high. Clay volumes and Th content increase at the base of the unit and decrease at the top. The lower part of this unit is different (from the unit lower boundary to 744 mbsf), as it is rich in K and U and depleted in Th and clay. According to the low Th/K ratio, this interval should be rich in glauconite, but the lack of magnetic susceptibility measurements prevent any conclusion. The above observations suggest that despite the poor core recovery, an important change occurs at 744 mbsf close to the m5.8 boundary. Chlorinity (Fig. F36) shows high values, reflecting high concentrations.

The transition to Unit V shows no change in TGR but an increase in K content; a sharp increase in magnetic susceptibility; and a decrease in Th, clay volume, and porosity. The transition is characterized by the appearance of glauconite in the sands of Unit V above.

Lithostratigraphic Unit V

TGR in Unit V is highly variable, serrated, and of medium value. Two large K-rich intervals at 708–695 and 674–666 mbsf and the succession of small peaks between 688 and 682 mbsf clearly correlate with high magnetic susceptibility, sonic velocity, and impedance values and low Th/K ratios, conductivity, and porosities. They reflect glauconite-rich sandy intervals. In contrast to Site M0027, the trend of conductivity in Unit V does not correlate with measured chlorinity, which is relatively constant and high in this interval (Figs. F36, F44). Conductivity is highly related to variations in porosity and magnetic susceptibility, suggesting that the electric signal is essentially carried by the matrix when passing through the formation and/or is very sensitive to variations in pore water volumes. However, in this unit, as well as in Units IV and III above, a line passing through the highest values of conductivity measured in the fine-grained intervals follows the chlorinity curve trend.

The transition to Unit IV is smooth. It is marked by an increase in K and Th contents, reaching a maximum a few meters above the boundary, and by a small peak in magnetic susceptibility.

Lithostratigraphic Unit IV

The TGR curve in Unit IV is relatively flat with a smooth bow shape with depressed areas at the base and top that are slightly enriched in K, reflecting the presence of glauconite, as also evidence by increased magnetic susceptibility. Sonic velocity, conductivity, porosity, and density signals show relatively constant values, except, again, at the lower and upper boundaries of the unit.

The transition to Unit III is marked by a peak in magnetic susceptibility just below the boundary and a peak in density just above. It also fits with a peak in sonic velocity, correlating with a low in conductivity. This boundary is visible on the acoustic image by a change in wall texture and an increase in caliper diameter above the boundary.

Lithostratigraphic Unit III

The TGR curve of Unit III is relatively flat, except at the lower and upper limits. The lowermost meter is slightly enriched in glauconite, as evidenced by increased K content, high magnetic susceptibility, and a low Th/K ratio. This interval is also marked by high sonic, high density, high magnetic sucsceptibility and low conductivity compared to the formation above, suggesting possible slight induration. Sonic, conductivity, porosity, and density signals also show relatively constant values, except at the boundaries. The remainder of Unit III is relatively homogeneous up to 644 mbsf, where a strong peak in P-wave velocity fits with a density peak, a conductivity low, and a diminution of the hole diameter, just above an unnamed sequence boundary (between surfaces m5.3 and m5.6).

At the transition to Unit II, we observe an increase in K content and magnetic susceptibility across the boundary. A small peak in sonic is also evidenced, just at the base of a small increase in density. Borehole diameter is slightly reduced at the boundary.

Lithostratigraphic Unit II

The TGR curve in Unit II shows a bow to serrated shape with intermediate values in the sand/silt intervals observed from the base of the unit to 602 mbsf and a flatish, serrated shape with intermediate to low values in the overall clayey silt above 602 mbsf to the top of the unit. Some intermediate sandy units (500–450 and 360–340 mbsf) are weakly expressed by irregular trends in the TGR curve. Below 602 mbsf, Unit II is rich in K. The K curve correlates with the magnetic susceptibility and sonic signals, and the low Th/K ratio attests to the presence of glauconite (except around 622 mbsf). The conductivity curve correlates with the porosity trend and mirrors density and sonic variations.

Above 602 mbsf, conductivity curve trends better correlate to the chlorinity curve, which is anticorrelated to density compared to the rest of the hole below. Sediments are almost glauconite free, as evidenced by high Th/K ratios, except between 500–495 and 480–469 mbsf in sand intervals with high density and sonic and low conductivity.

Above 469 mbsf, the TGR curve trend is much more regular compared to the formation below. The density curve decreases from 481 to 407 mbsf, reaching a minimum and increasing again toward 343 mbsf, where a peak is observed. This peak marks a clear petrophysical and geophysical boundary, as it correlates with a peak in conductivity and in P-wave velocity, a porosity low, and a clear low on the TGR curve and spectral elements. This interval fits with a cemented horizon observed in cores. A subhorizontal quartz vein was observed in Core 313-M0029A-71R (345.60 mbsf) in a cemented horizon, possibly related to overpressure. A sharp decrease in chlorinity at 346 mbsf suggests the 342–346 mbsf interval may act as a diffusion barrier to upgoing pore water (Fig. F36).

A number of petrophysical boundaries were recognized in Unit II. Most of the surfaces that correlate to sequence seismic boundaries show high densities and velocities. This explains the high impedance contrast at the origin of these high-amplitude seismic reflectors. Surface m5.2 does not fit this scheme and should possibly be placed a little bit lower in the section (612 mbsf) at a sharp impedance contrast.

The boundary with Unit I is characterized by a small TGR low and an increase in density. A porosity low is observed just above the transition.

Lithostratigraphic Unit I

The TGR curve of Unit I shows, from base to top, two bow-shaped serrated trends in intermediate values, a thick boxcar-shaped trend in low values, and a set of irregular to bell-shaped trends in intermediate values ending up in another boxcar-shaped trend in low values. These trends with overall low values correspond to sandy material to sand and clay alternations deposited in a range of settings from shelf (gradational bell shape) to shoreface, foreshore (boxcar shape), and coastal plain (irregular to bell shape).

Chlorinity curve shape is much more irregular in Unit I compared to the diffusive-advective profile it displays in the sedimentary units below (see "Geochemistry"). Comparisons between the TGR and chlorinity curves suggest that the higher chlorinities have been measured in low-TGR intervals, whereas the lowest chlorinities are encountered in higher TGR intervals (Fig. F36). This suggests that, as for Holes M0027A and M0028A, saltier water is stored in coarse-grained levels, whereas fresher water is observed in finer grained intervals.

Several petrophysical boundaries were picked in Unit I, but poor core recovery prevented any good correlation with seismic and sedimentary surfaces.

Stratigraphic surfaces and correlation with petrophysical intervals

Table T11 lists key petrophysical surfaces and intervals recognized in Hole M0029A. Many of these petrophysical horizons relate to stratigraphic surfaces (see penultimate column in Table T11), although small differences in depth are attributed to differences in picks of the surfaces (i.e., at the start of an increase in a parameter value or at the peak of the change). Petrophysical surfaces and intervals are defined using the full suite of logs and petrophysical parameters, chosen where more than one property displays a significant change. Surface m4 is located just below a TGR peak, although it is not identified as a petrophysical surface. Surface m4.3 is the only surface that does not match logging and petrophysical data. All parameters in this interval show minimal variation. Surface m5.2 may be better located lower (see "Unit III"). Surface m5.3 is an example of a surface that is particularly clear, with a kick in most petrophysical parameters. Some petrophysical surfaces/intervals do not clearly correlate with stratigraphic surfaces or lithostratigraphic units/subunits between 490 and 580 mbsf within Subunit IIC. This unit is characterized by a high degree of variability. Some surfaces are related to cemented horizons (see notes in last column of Table T11), whereas others illustrate minor sedimentological changes within lithostratigraphic units.

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