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doi:10.2204/iodp.proc.310.108.2007

Downhole logging

Hole M0009B

Geophysical wireline operations were completed in Hole M0009B (100.31 mbsl) from 20.39 mbsf, with data coverage by nearly all slimhole tools to ~5.96 mbsf (position of bottom of the casing). All logs were acquired in an open hole. From the seafloor to 20.39 mbsf, total gamma radiation (TGR) is very low (~32 cps) despite logging speeds up to a maximum of 1.1 m/min. No clear differentiations between contributions of different elements (K, U, and Th) can be made. Borehole conditions were very hostile in the lower part of the borehole (16–20.39 mbsf) but were quite stable in the upper part (Fig. F71), with the exception of a cavity at 13.75 mbsf. Optical images were slightly affected by murky borehole waters, but generally the quality was very good. Acoustic images are equal in quality, but the ABI40 tool could not pass the aforementioned cavity at 13.75 mbsf (Fig. F72).

Two intervals can be identified in the logged section of Hole M0009B:

• Interval 1 (13.89–18.12 mbsf; Cores 310-M0009B-12R through 13R) is characterized by branching and encrusting coral growth forms. The density of the framework increases upsection, resulting in higher formation electrical resistivity values (from 1.1 Ωm at the bottom of the hole to 1.88 Ωm). Sonic P-wave velocities (V_P) range from ~1662 to ~4530 m/s. Sonic Stoneley waves could not be recorded in this interval. The temperature of the borehole fluid at the time of logging was ~25.87°C, pH values were ~8.16, and electrical conductivity was ~55.32 mS/cm (0.180 Ωm).
• The quality and resolution of the optical image allows for the identification and characterization of a second interval. Encrustations with microbialites are less pronounced in the basal part of Interval 2 (5.96–13.89 mbsf; Cores 310-M0009B-6R through 11R). Upsection, thickness and framework density increase, and the uppermost part of this interval (from 8.5 mbsf) is characterized by the occurrence of branching coral assemblages and a marked increase in microbialite encrustation. Formation electrical resistivity increased from 1.36 to 3.65 Ωm; V_P ranged from ~2145 to ~3479 m/s. Sonic Stoneley wave velocities ranged from ~932 to ~1463 m/s (Fig. F71). Because of the extremely porous (mesoscale) character of the formation, sonic data are not continuous and are of poor quality. Natural gamma ray values are slightly higher in Interval 2 than in Interval 1.

Hole M0009D

Geophysical wireline operations were completed in Hole M0009D (103.18 mbsl) from 31.80 to 4.58 mbsf. Open-hole logging was performed in two different stages because of borehole instability:

1. By positioning an open shoe casing at 19.55 mbsf, most of the older Pleistocene sequence could be logged (19.55–31.80 mbsf; Cores 310-M0009D-13R through 19R). Borehole conditions were very hostile for high-quality logging in the lower part, as can be observed from the caliper log (Fig. F73) and the borehole images. Cavities up to 0.5 m in height can be observed. Optical images were slightly affected by murky borehole fluids where cavities are present, but overall the quality is very good. Acoustic images were not affected by murky water conditions, and they complement the optical images by imaging the acoustic hardness of the borehole wall (Fig. F74). A spectral gamma ray log was made from 31.80 to 13.70 mbsf, where the upper 5.85 m were logged through the steel casing. Total gamma radiation (TGR) is very low (ranging from ~13 to ~32 cps), despite logging speeds up to a maximum of 1.1 m/min. No clear differentiation between contributions of different elements (K, U, and Th) can be made. Resistivity values ranged from 1.39 to 23.5 m (Fig. F73), and a marked interval between 18.0 and 20.5 mbsf with above-average (compared to the other data obtained during the expedition) resistivity values was observed in the uppermost part of this section. The temperature of the borehole fluid was ~25.55°C, pH values ranged from ~7.82 to 8.09, and electrical conductivity was ~55.0 mS/cm (0.182 Ωm). Because of the limited length of open hole below the casing and the hostile borehole conditions evidenced by low recovery and borehole images, it was decided not to run the mechanical caliper-sonic tandem in this interval.
2. By positioning an open shoe casing at 4.58 mbsf, most of the last deglacial sequence could be logged (4.58–19.55 mbsf; Cores 310-M0009D-5R through 13R). For safety reasons and time constraints, it was decided to make only two runs in this interval: (1) the Induction Resistivity Probe (DIL45) to probe for depth and at the same time measure an important lithological parameter and (2) an optical borehole televiewer (OBI40). Whereas the DIL45 tool could make the overlap with the previous section (below 19.55 mbsf), the OBI40 reached a maximum depth of 18.04 mbsf, just above the Unit I/II boundary. However, borehole conditions were extremely hostile in this interval, and major problems were encountered while lowering the tools in the borehole and logging up. The latter resulted in a substantial degradation of image data quality in the lower part of this section. The last deglacial sequence can be divided into three intervals.
• In Interval 1 (9.92–18.82 mbsf; Cores 310-M0009D-8R through 12R) from ~16.75 mbsf (poor-quality optical image), encrusting coral growth forms can be identified and grade to more branching coral assemblages from 14.91 mbsf onward. Formation electrical resistivity increased from 0.97 to 2.04 Ωm in a cavity at 15.1 mbsf. A gradation of relatively thick to thinner branches upsection can be observed from the optical image (Fig. F74). The top of this interval is marked by a heavily encrusted surface and shows a formation electrical resistivity maximum (2.48 Ωm).
• Interval 2 (7.05–9.92 mbsf; Core 310-M0009D-7R) is characterized at the base by an extremely open framework of branching coral growth forms. The overlying section consists of massive coral growth forms (Porites) (~8.50 mbsf). Formation electrical resistivity values decrease within this interval from 1.93 to 1.06 Ωm. The top-bounding surface consists of an extensively bored massive Porites.
• Interval 3 (4.58–7.05 mbsf; Cores 310-M0009D-5R through 6R) consists of a coralgal framework with branching and encrusting coral growth forms. The top part (from 5.56 mbsf) is dominated by thin encrusting corals. Formation electrical resistivity increases upsection.

Hole M0009E

Hole M0009E (94.94 mbsl) was logged for geophysical parameters. Geophysical wireline operations were completed from 14.92 mbsf with data coverage by all slimhole tools without repositioning the open shoe casing (fixed at 2.55 mbsf).

Borehole conditions were hostile, and the borehole was highly unstable, especially below 11 mbsf. The calipers show a large increase in borehole diameter below this depth, and the logging tools could not pass an obstruction at ~15 mbsf even after repetitive cleaning and hammering efforts. The optical image was affected by murky borehole waters. Acoustic images were not affected by this and show a high-quality virtual (acoustic) representation (at millimeter scale) of the lithologies cored (Fig. F75).

The logged section can be divided into three intervals:

• Interval 1 (11.03–14.92 mbsf; Cores 310-M0009E-7R through 9R) has branching coral growth forms in the middle part where framework density and pronounced encrusting increases upward, with massive Porites at the top. Natural gamma ray values are lowest in dense sections and show elevated values in the lower part of the interval. Resistivity values increase slightly from 1.4 Ωm at the base to 1.72 Ωm at the top, indicating less pronounced encrusting and infilling of pore space by microbialite crusts. Acoustic velocities could not be measured over this interval.
• Interval 2 (7.33–11.03 mbsf; Cores 310-M0009E-4R through 6R) is dominated by microbialite boundstone lithofacies that include encrusting coral growth forms. Resistivity values increase slightly from 1.78 Ωm at the base to 2.09 Ωm at the top. Compressional acoustic velocity (V_P) is highly variable (small-scale changes) and ranges from a minimum of 1823 m/s to a maximum of 2898 m/s. Stoneley wave velocity ranges from 930 to 1842 m/s. Gamma ray values (TGR) are fairly constant over the entire interval (Fig. F76).
• Interval 3 (2.55–7.33 mbsf; Cores 310-M0009E-2R through 4R) has a rubbly character at the base with encrusting coral growth forms that dominate the remainder of the interval. The abundance of encrusting microbialite varies within this interval, which is observed on the acoustic image and in the variation in formation resistivity (highest in intensely encrusted intervals). Natural gamma ray values are fairly constant but decrease toward the top (2.55 mbsf). Within the casing, a further decrease can be observed. V_P is highly variable, but a general increase in velocities can be observed from base to top (1823–3715 m/s). Stoneley wave (tube wave) velocities range from 1185 to 1825 m/s, but proper data coverage, especially in the rubbly and most open framework intervals, is poor. The temperature of the borehole fluid a few hours after drilling decreased from 26.2° to 25.0°C from base to top in Hole M0009E, pH values were ~8.14, and borehole fluid electrical conductivity decreased upsection from 55.79 to 55.1 mS/cm (0.179–0.181 Ωm).

Hole M0021B

Hole M0021B (81.70 mbsl) was logged for geophysical parameters (Fig. F77). Drilling depth was 32.81 mbsf, and geophysical wireline operations were completed in Hole M0023A from 17.07 mbsf with data coverage by all slimhole tools without repositioning the open shoe casing (fixed at 2.88 mbsf) under open (and very hostile) borehole conditions. Borehole conditions were harsh, and the borehole was highly unstable especially below 15 mbsf. The calipers show a large increase in borehole diameter below this depth, and the logging tools could not pass an obstruction at ~17 mbsf even after repetitive cleaning and hammering efforts. Optical images were seriously affected by murky borehole waters. Toward the top of the hole, the quality was better. Acoustic images were not affected and are a high-quality visual representation (millimeter scale) of cored lithologies (Fig. F78). In Hole M0021B, the last deglacial sequence can be divided into three intervals:

• Interval 1 (13.15–17.09 mbsf; Cores 310-M0021B-8R through 10R) consists of a rubbly basal section and a framework composed of branching coral colonies that can be identified from the acoustic image. Formation resistivity increased from 0.72 to 1.88 Ωm, natural radioactivity values decreased, the temperature of the borehole fluid was ~27.56°C, pH values were ~7.96, and borehole fluid electrical conductivity was ~56.48 mS/cm (0.177 Ωm).
• Interval 2 (6.5–13.15 mbsf; Cores 310-M0021B-4R through 7R) displays branching and encrusting coral growth forms in which the volume of microbialite crusts increases upsection; a more open framework with less microbialite dominates the top 1 m (7.5–6.5 mbsf). Formation resistivity increases from 1.82 to a maximum value of 3.85 Ωm at 7.59 mbsf, after which it decreases to a minimum of 1.8 Ωm at 6.51 mbsf. Natural radioactivity values were constantly low but slightly decreased upsection, the temperature of the borehole fluid was ~27.49°C, pH values were ~7.96, and borehole fluid electrical conductivity was ~56.4 mS/cm (0.177 Ωm). Acoustic formation velocities (V_P) are highly variable but show a general decrease from 3059 m/s at 7.8 mbsf to a minimum of 1797 m/s.
• Interval 3 (2.88–6.5 mbsf; Cores 310-M0021B-2R through 3R) has a cavity at its base with frameworks formed successively by branching coral colonies and encrusting growth forms at the top. Formation resistivity increased from 1.49 to 3.41 Ωm; natural radioactivity values increased in the rubble section but remained constant throughout the rest of the interval. Borehole fluid properties remained constant, and V_P was highly variable but showed a slight increase from base to top, where the minimum and maximum V_P values were 1797 and 3601 m/s, respectively.

Synthesis: geophysical downhole logging at Tiarei inner and outer ridges

Wireline logging operations at Tiarei sites produced nearly complete downhole coverage of the last deglacial sequence from 72 to 122 mbsl. Tools have different data coverage in various holes (see the “Downhole logging” sections in the individual site chapters). It was not possible to image the boundary between Units I and II, except in Holes M0021B and M0023B.

In Figure F79, borehole images and natural radioactivity logs (total counts) are plotted in meters below modern sea level. In each logged borehole, the boundary between the last deglacial and older Pleistocene sequences is indicated. The basal part of Unit I directly overlying this boundary is often semiconsolidated and poorly recovered and shows elevated natural gamma radioactivity values in proximal locations (e.g., Hole M0023B). Distal locations do not show this higher gamma ray signature, with the exception of Hole M0021B.

In Figure F80, borehole images and formation electrical resistivity (resistivity) are plotted in meters below modern sea level. The general resistivity pattern and absolute values in the last deglacial sequence along this transect are comparable across the holes. In each hole, the basal interval has the lowest resistivity values and values increase gradually uphole to a maximum value, after which a sharp negative excursion to lower values can be observed. However, the interval of increasing resistivity values is interrupted once by a subtle decrease in the middle part of the last deglacial sequence. The absolute values of this decrease are higher at distal sites than at the proximal ones. Caliper, borehole fluid characterization, and acoustic tools all contain particular information on parameters for specific intervals and lithologies.

Similar to the Maraa transects, the depth of the top of the older Pleistocene sequence below modern day sea level is highly variable and indicates a very uneven paleotopography before the last deglaciation.

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