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Downhole measurements

The focus in this section is to provide a description of the wireline data sets collected in Hole M0027A and their downhole characteristics and variations. A few detailed examples are given, interpreting interesting aspects of the log trends. Trends in MSCL core measurements and equivalent borehole measurements are also discussed in this section. Some measurements display variation primarily with depositional sedimentary composition, whereas others are controlled more by postdepositional factors such as the degree of cementation and the type of interstitial formation water. Integration of core and logging data sets allows calibration of core data with in situ borehole properties and provides an assessment of the precise depth from which cores were collected. These correlations are described briefly in "Physical properties." At the end of the section, a synthesis of petrophysical data and downhole logs and derived quantities is presented for each stratigraphic unit.

Downhole measurements in Hole M0027A

In Hole M0027A, 2297 m of wireline logs was acquired, along with VSP measurements. Spectral gamma ray logs were acquired through steel drill pipe down to 603 m wireline log depth below seafloor (WSF), thus providing coverage in all except the bottommost section of the hole (see Table T7 in the "Methods" chapter). An open-hole run of the same probe from 410 to 621 m WSF provided an additional 18 m of coverage at the base of the hole.

Two sections of open hole were logged with other tools in Hole M0027A, from 192 to 342 m WSF (middle section) and from 418 to 623 m WSF (lower section). The base of the middle interval became shallower with each successive run because of the formation of a bridge and infilling from 342 m WSF. This prevented the section between 342 and 418 m WSF from being logged. In the lower section, the acoustic log was completed at 575 m WSF because of the timing of further wireline operations planned for this site.

Open-hole measurements were acquired for magnetic susceptibility, spectral gamma ray, acoustic imaging, conductivity, and sonic velocity. VSP measurements were acquired in the middle section in open hole and through pipe in the upper section (see Table T8 in the "Methods" chapter). Figure F56 provides an overview of all slimline downhole measurements taken in Hole M0027A.

In all figures, downhole geophysical data and images are plotted in meters below present-day seafloor based on the wireline scale (WSF) (see "Downhole measurements" in the "Methods" chapter). Where appropriate, boundaries between lithologic units (Units I–VII) are indicated to facilitate comparison with other data analyses in this hole and with analyses in other Expedition 313 holes.

Spectral gamma ray logs

Spectral gamma ray logs allow continuous characterization of the entire borehole. Comparison of through-pipe data and the open-hole logs indicates slight attenuation of the through-pipe signal but excellent matching of trends (Fig. F56). The TGR signal from the downhole probe and NGR measurements made on whole cores correlate very well, in some cases allowing accurate repositioning of core depth below seafloor (see "Physical properties").

In general, trends in TGR are dominated by the K component of the signal (Fig. F56). High amounts of K appear to be related to two main factors: silt/clay with high mica content and the occurrence of glauconitic sands.

Total counts in the upper 168 m (Unit I) are generally low. Through-pipe TGR values of ~20 counts per second (cps) and K amounts of <100 Bq/kg correspond to sandy intervals (3–8, 30–71, and 119–168 m WSF). A few excursions to higher values (>100 cps) correspond to clays, which were the intervals generally recovered. In Units II–IV, values are relatively constant (~150 cps) with small lows. In Unit V, with the exception of a small peak near the top related to high U (probably linked to organic matter described in cores) and high K, gamma counts are very low (<20 cps) to 400 m WSF, where they begin to rise. This interval of low counts is characterized by low core recovery and nonconsolidated sands. Cave formation around the pipe at these depths may have contributed to an underestimation of NGR emissions. Total counts in the lower part of Unit VI are relatively constant at slightly increased values compared to those observed in Units III and IV. In Unit VII, counts increase significantly to very high values (>500 cps) between 500 and 510 m WSF in an interval rich in U (top of Subunit IIB). Total counts, still high, then decrease progressively to 600 m WSF before gradually rising to the end of the log at 620 m WSF.

Along the hole, Th/K and U/K ratios are quite variable (Fig. F57). Around 355 and 430 m WSF, U/K is slightly lower at a cemented level. Transitions between low gamma ray in sandy intervals and high gamma ray in clay intervals can either be sharp (e.g., 77 and 96 m WSF) or progressive (e.g., 12–15 and 405–415 m WSF). In the latter case, Th/K and U/K ratios remain quite stable (Fig. F57), suggesting a progressive change in sand/clay ratio with no change in mineralogy.

Below 480 m WSF, high K values are not related to mica-rich clay and silt but to the occurrence of K-rich glauconite in the sediment. The presence of glauconite is also indicated by low Th/K values and correlates with high magnetic susceptibility (Figs. F56, F57). The low Th/K value in the middle part of the hole also corresponds with the occurrence of glauconite (between 337 and 355 m WSF). Between 50 and 75 m WSF, the Th/K ratio is also low but the cores appear to be glauconite free; however, core recovery is low.

The highest TGR values are located at 500 m WSF and are related to the combination of high U and high K (Fig. F56). The U spike is associated with interesting features just above: from 480 to 490 m WSF, K content increases with glauconitic content, whereas U and Th are depleted. Minimum U and Th contents occur between 490 and 495 m WSF in cemented glauconitic sandstones. Below these cemented rocks, Th values increase to an average of 25 Bq/kg, similar to values found in other parts of the hole, whereas U increases significantly to a maximum of 180 Bq/kg at 504.5 m WSF and then decreases gradually to an average of 50 Bq/kg below 560 m WSF.

The association of low U and Th also occurs when glauconite appears in the sediment from 340 to 355 m WSF. A spike of U at 361 m WSF underlies a cemented sandstone at 355 m WSF, which is accompanied by a progressive decrease in U to 380 m WSF. The pattern at 340–380 m WSF is located between a freshwater aquifer above and a saltier aquifer below (see "Geochemistry"). Observed patterns of K and U are interpreted as diffusion processes during diagenesis at the boundary between two aquifers with distinctly different chemical properties (e.g., especially Eh controlling the solubility of U).

Magnetic susceptibility logs

Magnetic susceptibility is often a clear indicator of lithological variations because of its high sensitivity to iron-bearing minerals. Susceptibility is generally anticipated to be higher in clays than in sands and higher where terrestrial input to the sediment is greater. In Unit I and the upper part of Unit II, magnetic susceptibility is high in clay intervals. In all holes, magnetic susceptibility signals from the EM51 downhole probe and MSCL correlate very well, allowing, in some cases, very accurate repositioning of core depth below seafloor.

In Hole M0027A, the highest magnetic susceptibility values are found around 390 m WSF in a clay interval in Unit II, as seen in the MSCL-measured magnetic susceptibility (Figs. F56, F58). Although glauconite, which is a mineral with high magnetic susceptibility, was not observed in these clays by the sedimentologists, XRD analyses show a ?glauconite? spike in these upper clay intervals. This suggests that fine-grained glauconite may be the influence on magnetic susceptibility. XRD analyses indicate that a peak in magnetite accompanies the glauconite peaks. This magnetite may be forming around the glauconite grains (see "Paleomagnetism;" see Fig. F46 in the "Site M0028" chapter). For the remainder of the hole below the base of the clay at ~209 m WSF, glauconite is the dominant control on magnetic susceptibility, which correlates with sedimentological observations of this mineral. In some cores, a clear correlation is observed between magnetic susceptibility peaks (both from the MSCL and the EM51) and darker layers containing sulfides, closely related to the formation of magnetic minerals such as greigite (Figs. F31, F58). Below this level, the magnetic susceptibility signal is much lower to the top of Unit VII. High values are then observed between 490 and 495 m WSF, followed by a progressive decrease in magnetic susceptibility to 600 m WSF. In Unit VII, the magnetic susceptibility signal correlates with K, and both closely reflect the glauconite content estimated by the sedimentologists (Fig. F59). A discussion of the cycles observed within this interval (540–585 mbsf) and their potential relationship to variations of glauconite transport on the off-apron toe-of-slope setting (see "Lithostratigraphy") can be found in "Paleomagnetism" (Fig. F55). Susceptibility variations commonly correlate with density variations because both parameters are high where glauconite is present.

Acoustic image logs

The ABI40 acoustic image log was run at a resolution of 72 parts per thousand in Hole M0027A. Acoustic amplitude and traveltime images provide an estimate of the borehole diameter and the induration and rugosity of the borehole walls. Softer intervals are characterized by shorter traveltimes and a lower amplitude signal, which can be used to pick out nonconsolidated intervals often related to sandier levels in the cores. The hardest zones are generally characterized by higher traveltimes and higher amplitudes (Figs. F51, F59). Centralization of the tool during acquisition affects the accuracy of the acoustic caliper calculated from the traveltime image. A generally large and variable diameter borehole is indicated in softer intervals and a more consistent smaller diameter in more indurated zones (Fig. F51). Thus, lithological changes as well as potential impedance contrasts can be identified. In Hole M0027A, sharp changes in the ABI40 signal between 310 and 326 m WSF mark a bridge formation at this depth during logging operations, leading to a sharp decrease in hole diameter and lower quality images because of the presence of particles in the hole.

Sonic velocity logs

On average, sonic velocity in Hole M0027A increases with depth, as would be expected (Fig. F56). It changes across most major lithology changes, varying from slight increases or decreases to more significant jumps (Fig. F56). Velocity is also affected by the induration of the sediment, and increases are observed in levels where sediment becomes harder or is cemented. As such, these changes tend to correlate with variations in conductivity (Fig. F56). Around 205 m there is an increase in velocity that corresponds with a change in lithology from sands to clays. A slight decrease is observed at the interface between Units II and III, when passing from sands (above) to clays (below). The velocity peak just above marks a cemented level. Two distinctive transition increases are apparent, one just above the interface between Units III and IV (295 m WSF) and the other between Units VI and VII (488 m WSF), with a step increase at the latter boundary. Velocity increases over a short interval at the first transition because of the presence of a 40 cm thick cemented horizon passing downhole to clay. This transition is also observed in the conductivity log. The second transition is similarly picked out by a magnetic susceptibility increase and a conductivity decrease. Within Unit VI, at 470 m WSF, a gradual increase in velocity corresponds with an increase in density identified in whole-core logging as the lithology grades from silt to fine sand. Velocities remain high with variation throughout Unit VII.

Conductivity logs

Conductivity in the borehole is influenced by a variety of aspects, including lithology, pore water content, and salinity. In Hole M0027A, conductivity clearly correlates with changes in water chlorinity (Fig. F60; see "Geochemistry"), emphasizing that conductivity measurements are at a first order driven here by pore water salinity. Note that higher conductivities (and associated chlorinities) mainly correlate with coarse-grained intervals. Conductivity in the middle logged section (195–330 mbsf) indicates, on average, a decrease through the section with few notable peaks to higher values, often associated with a decrease in Th content. An example of this is a peak in conductivity at 224 mbsf (Table T13), which likely correlates with the m5.2 surface (see "Stratigraphic correlation"). In the lower section, there is an overall increase in conductivity downhole. The lowest measured values are observed at the top of Unit V and within Unit VI, bounding both above and below the sands with high-chlorinity pore water observed between 350 and 405 m WSF (Fig. F60; see "Geochemistry"). A change in conductivity is also commonly observed at the boundaries of sedimentary units or just above or below a stratigraphic boundary, often associated with cemented intervals and located where conductivity anticorrelates with changes in sonic velocity.

Vertical seismic profiling

VSP data were acquired in open hole from 329 to 204 m WSF and through pipe from 204 m WSF to the seafloor (Fig. F61) (see "Downhole measurements" in the "Methods" chapter for more detail). 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. The time-depth relationship from the VSP survey can be compared with the relationship estimated by processing the seismic survey data (see Fig. F18A in the "Methods" chapter). All depths converted from traveltime in seismic profiles at this site were estimated using stacking velocities (see "Downhole measurements" and "Stratigraphic correlation," both in the "Methods" chapter).

Example of multilog interpretation

Petrophysical and downhole measurements can provide evidence for sequence boundaries. Figure F51 is a compilation of several parameters around the m5.3 boundary as defined in "Stratigraphic correlation." The boundary is located at the interface between lithologic Units II and III, at an abrupt transition at 236.16 mbsf from clay below to medium, moderately glauconitic sand above. In Figure F51, the core and data measured on the whole core have been shifted 0.50 m downhole to match the wireline signal. The m5.3 boundary is therefore located at 236.66 m WSF. The boundary is marked on the petrophysical data by a slight decrease in P-wave velocity and a slight increase in density when passing from clay to sands. These sands are characterized by a decrease in TGR counts and an increase in magnetic susceptibility (both on the EM51 log and the MSCL data), reflecting the presence of glauconite in the sand. On the ABI40 amplitude image, the m5.3 boundary is also characterized by a change in color and texture from light uniform colors in clay below to patchy dark red colors in the sand above. The caliper extracted from the acoustic image shows the hole diameter is larger in the sandy interval. Above this boundary, a cemented interval (235.22–235.63 m WSF) is clearly indicated by an increase in density and P-wave velocity. This interval is also visible on the ABI40 images.

Most lithological units and stratigraphic surfaces are often picked out by utilizing several of the wireline logs or petrophysical core data. This is clear on Figures F56 and F62 and in the last column of Table T13, which lists these surfaces against the petrophysical surfaces, where appropriate.

Downhole log and physical properties integration

This section combines results of logging and physical property measurements with the main characteristics of the lithostratigraphic units. It ends with a brief summary of the links between petrophysical intervals (Table T13) and stratigraphic surfaces (Fig. F62). All depths in Figure F62 are given in meters below seafloor and ignore small differences that may exist between core (mbsf) and log (m WSF) depths. Various points shown in Figure F62 can generally be seen in greater detail in various figures in "Physical properties" or "Downhole measurements." Numerical data are accessible online; see "Publisher's notes" for links to the database.

Lithostratigraphic Unit VIII

The upper part of this thin unit was logged with only some of the tools. Results are of poor quality.

Lithostratigraphic Unit VII

Lithostratigraphic Unit VII shows high K content and high magnetic susceptibility, density, and sonic velocity compared to the units above, reflecting glauconite (Fig. F62B). However, there is a high degree of variability in these parameters. Gamma ray from the base of the unit increases to an overall high rich in U at the Subunit VIIA–VIIB transition at 499.5 mbsf. U and Th contents decrease above to the boundary with Unit VI. The density log has a blocky appearance in the lower half of the unit (below 570 mbsf) but more stable density values at the top of the unit. A low at 499.50 mbsf is close to the U rich/poor transition. In this coarse-grained unit, chlorinity is high, reflecting salty pore water (Fig. F60; see also "Geochemistry") and conductivity, which slightly decreases uphole, clearly following the chlorinity trend. Lows in conductivity usually tie with sonic peaks and porosity lows, reflecting cemented or more indurated intervals rather than pore water chlorinity changes.

The transition to Unit VI is clearly marked by a decrease in K content, magnetic susceptibility, sonic velocity, electrical conductivity, and impedance. Density has a small peak just below the unit boundary, mirrored by porosity. The Th/K ratio clearly increases when entering into Unit VI.

Lithostratigraphic Unit VI

The data sets in Unit VI clearly evidence the two subunits identified in "Lithostratigraphy," which show very distinct lithologies (Fig. F62B, F62C). TGR in Subunit VIB is characterized by a relatively high signal indicative of clayey sediment and becomes more variable at the bottom of the unit below 465 mbsf (silt/sand interbedding with clay layers). Subunit VIB is also characterized by low conductivity compared to the sands above in Subunit VIA. From ~488 mbsf, there is a stepped density decrease (porosity increase) to 465 mbsf, where an MFS has been interpreted. This is also matched by the sonic curve. At the MFS, there is a small peak in the Th/K ratio. From the low at the MFS, density increases stepwise, paralleling small "coarsening"-upward motifs, to just above the Subunit VIA/VIB boundary. Entering the sands of Subunit VIA, TGR decreases gradually to very low values. Toward the top of the unit, a peak in Th a few meters below the Unit V/VI boundary probably characterizes the unconformity correlated with seismic sequence boundary m5.7. Density does not change significantly upsection in Subunit VIA except at the top. In the lower, fine-grained part of Unit VI, chlorinity is lower than in Unit VII but decreases to 418 mbsf. The electrical conductivity curve parallels the chlorinity trend as well as the porosity and density curves, reflecting either changes in water content volume or mineralogy, such as the presence of clay. In the upper coarse-grained sandy part of Unit VI, chlorinity increases drastically to salty pore water in parallel with MSCL conductivity between 412 and 343 mbsf (see further description in "Physical properties").

At the boundary with Unit V, gamma ray increases from very weakly serrated low values in two bell-shaped trends with sharp tops associated with seismic sequence boundaries m5.7 and m5.6. The latter peak in gamma ray is associated with a sharp increase in K and density, mirroring the density that characterizes the Unit V lower boundary.

Lithostratigraphic Unit V

Unit V is glauconite rich, as evidenced by a low Th/K ratio and high magnetic susceptibility. Density in this unit is high, paralleling the K curve and dominated by glauconite, although those correlations do not always fit with the glauconite estimates made by sedimentologists. The TGR curve displays the superposition of bell- and funnel-shaped curves. Highs in TGR correspond to high magnetic susceptibility, K, and density and are interpreted as an increase in glauconite content rather than clays. A density high at the base of the unit correlates with a carbonate cemented bed (see "Lithostratigraphy"). At ~350 mbsf, there is a low in density followed by a gradual increase upsection, mirrored in porosity. Conductivity is high in the unit, roughly paralleling the gamma ray trend, which is in adequation with the overall coarse-grained lithology, relatively low porosity, and high chlorinity measured on the cores. The decrease in conductivity at ~340 mbsf matches the decrease in chlorinity and the increase in porosity.

TGR changes at the boundary with Unit IV from a decreasing funnel shape below to a higher but constant and serrated curve above. This change correlates to a sharp increase in conductivity and decreases in density and porosity. In detail, the rate of density decrease is greater just at the boundary with a subsequent brief local high at seismic sequence boundary m5.45.

Lithostratigraphic Unit IV

Unit IV is characterized by a flat, serrated TGR curve typical of largely clayey material with high Th content at the bottom. Approximately the lower half of the unit is marked by a decrease in density mirroring a porosity increase, suggesting a weak fining-upward succession in an overall clayey sediment. Two sharp-topped bell-shaped sonic increases (324–309 and 309–296 mbsf) are anticorrelated but parallel to the conductivity curve. Increases in density with no decrease in porosity possibly indicate increases in heavy mineral content despite the lack of evidence along the magnetic susceptibility curve. In this fine-grained unit, chlorinity is low, slightly increasing uphole, according to the global trend of the conductivity logs.

At the boundary with Unit III, TGR changes from weak serrated intermediate values to boxcar-shaped lower values just above a sharp confined peak in TGR. Sonic velocity increases significantly at the small bed immediately above the boundary with a small low in conductivity and a small peak in U. Density decreases sharply above this bed, and porosity increases because of sharp grain size diminution.

Lithostratigraphic Unit III

Unit III shows TGR values serrated intermediate with boxcar to weak bell shapes characteristic of clays and silts, with a distinct low at 272 mbsf. This latter sand bed correlates to seismic sequence boundary m5.4, characterized by a decrease in density and a peak in the Th/K ratio, sonic velocity, and magnetic susceptibility. Density rapidly decreases and then slowly increases uphole to 272 mbsf (m5.4), mirrored by the porosity curve. Density rapidly decreases to a low at 265 mbsf and then stepwise increases to the top of the unit. At 240 mbsf, clay volume peaks and density increases, possibly related to a high content of heavy minerals (pyrite). Chlorinity is low in Unit III, increasing uphole and correlated with the conductivity curve.

The boundary with Unit II is characterized by a clear increase in sonic velocity, an increase and then decrease in conductivity, an abrupt change from high serrated gamma values to a weak funnel trend, a sharp increase in density, and decreases in porosity.

Lithostratigraphic Unit II

Unit II shows a composite TGR curve with a succession of boxcar trends in the lower part overlain by a tight serrated shape in the upper part. This shape corresponds to the superposition of an alternation of sand and clay motifs at the base with homogeneous clayey sediment at the top. These TGR trends are paralleled by Th, K, and density curves but are anticorrelated with porosity, which confirms the fining-upward clayey trends.

Several peaks in magnetic susceptibility correlate with peaks in conductivity and density, which are possibly correlated with glauconite occurrences at the base of the unit. At the m4.5 unconformity, density decreases sharply and sonic velocity peaks. The sharp change in the overall shape of the TGR trend at 205 mbsf is clearly evidenced on the acoustic images and conductivity curves. The trend shows a drastic increase in magnetic susceptibility due to a high content of ferromagnesian minerals. Chlorinity increases progressively from 188 mbsf uphole to salty water (see Fig. F60), in parallel with conductivity (see "Geochemistry"). In the units below, high chlorinities were usually correlated to coarse-grained intervals. In this unit, high chlorinity appears in much clayey sediment. However, peaks in conductivity between 235 and 232 mbsf fit with TGR gamma lows and may reflect the presence of saltier pore water in coarse sediments that have not been sampled for geochemical measurements.

The boundary with Unit I is characterized by a rapid funnel-shaped TGR trend at the transition between weakly serrated high values below and smooth boxcar low values above, indicating a rapid grain size increase from silty muds below to clean sands above. The boundary is underlined by an increase in Th/K ratio.

Lithostratigraphic Unit I

The TGR signature of Unit I is composed of two 50 m thick low-value boxcar-shaped trends interbedded with 20–40 m thick stacks of 10 m thick bell- and funnel-shaped trends with intermediate values. These evolutions are typical of alternation of clean shoreface sands with coastal plain sands, silts, and clays. Trends in TGR are paralleled by Th, K, and U curves. Porosity and density trends are highly variable but very well anticorrelated. The Subunit IC/ID boundary (seismic sequence boundary m1) occurs at the top of a stepped boxcar-shaped interval with increasing gamma values. The Pleistocene surfaces corresponding to marine isotope Chrons (MICs) 4, MIC3c, and MIC3a are characterized by abrupt decreases in TGR values at the top of a thick boxcar trend and at the base of bell and funnel trends, respectively.

Stratigraphic surfaces and correlation with petrophysical intervals

Table T13 summarizes the key petrophysical surfaces and intervals. Most of them correspond to stratigraphic surfaces recognized on the core and on seismic profiles (see penultimate column in Table T13). Small differences in depth are due to the lack of precision in the location of an event (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 by combining the full suite of logs and petrophysical parameters at places where more than one property displays a significant change. The m4 surface is imprecisely located because of a lack of data in this interval (see "Stratigraphic correlation"). The m3 surface is picked at the largest observed increase in gamma ray, but it falls a few meters above the seismic pick because of low core recovery. Flooding surfaces and transgressive surfaces are, in places, characterized by series of fluctuations in petrophysical parameters like density, magnetic susceptibility, and gamma ray (e.g., between 253 and 256 mbsf). Seven additional surfaces/intervals were picked from petrophysical data that are not obviously related to any seismic pick or major lithological unit/subunit transition. Some of these correspond to cemented horizons (see notes in last column of Table T13), whereas others reflect minor sedimentological changes.