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doi:10.2204/iodp.proc.310.105.2007

Downhole logging

Hole M0005D

Geophysical wireline operations were completed in Hole M0005D (59.63 mbsl) from 98.01 mbsf upward, with data coverage by all slimhole tools over the lowermost older Pleistocene sequence. A spectral gamma ray log was made through the steel drill pipes to obtain a continuous log over the entire interval comprising lithologic Units I and II (i.e., last deglacial and older Pleistocene sequences). From the seafloor to 32.36 mbsf, total gamma radiation (TGR) was very low (~15 cps), despite logging speeds up to a maximum of 1.1 m/min. A marked increase in TGR was observed within the older Pleistocene sequence in the interval between 32.36 and 52.0 mbsf, where U contributes most to TGR in the upper part (32.36–46.71 mbsf) and Th and minor K contribute most to TGR in the lower part. In the remainder of the older Pleistocene sequence, U contributes most to TGR values (Fig. F74).

Open borehole logging was performed in three different stages because of borehole instability:

1. By positioning an open shoe casing at 75.49 mbsf, the bottom part of Hole M0005D could be logged. Borehole conditions were exceptionally good for the lower part of the older Pleistocene sequence. From 77.66 mbsf to the base of the casing (75.49 mbsf; Core 310-M0005D-27R), the caliper showed a large increase in borehole diameter (from ~9.7 to 14.6 cm). Optical images were slightly affected by murky borehole waters at the top, but quality increased toward the base. Acoustic images were not affected by this and are high-quality virtual representations of the lithologies cored. Repetitive spectral gamma ray logs over this interval show that logging through the steel casing reduces the number of counts per second (in TGR) for each sample but that trends in natural radiation values with depth are the same (Fig. F74). Resistivity values range from 2.8 to 11 m (Fig. F74). Resistivity values drop significantly above 77.66 mbsf, corresponding to an increase in borehole diameter and soft formation reflectivity values in the acoustic image. Sonic P-wave velocities (V_P) range from 2300 to 4400 m/s. The top 2 m show a marked decrease in V_P to values as low as 1738 m/s. Sonic Stoneley-wave velocities range from 1085 to 1477 m/s, dropping to values of 333 m/s in the uppermost 2 m (Fig. F74). The temperature of the borehole fluid is ~22.8°C, pH values are ~8.07, and electrical conductivity is ~51.7 mS/cm (0.193 m).
2. By positioning the open shoe casing at 57.45 mbsf, the middle part (57.45–75.49 mbsf; Cores 310-M0005D-20R through 27R) of Hole M0005D in Unit II could be logged. Borehole conditions were very poor in this interval, as observed from the caliper logs. Borehole diameter was highly variable (from 10 to 15.7 cm). Optical images could not be recorded at this level because of unstable borehole conditions, which caused difficulties entering the borehole and pulling the tool up. Acoustic images were recorded from a maximum depth of 71.07 mbsf and show highly variable lithologic stacking based on acoustic reflectivity values. Overall, this interval is characterized by very low acoustic amplitudes; high impedance features appear only sporadically within the interval (Fig. F75). Between 60.40 and 62.75 mbsf, the acoustic borehole televiewer ABI40 image shows high impedance dense coral framework. Resistivity values are very low in the bottom part of this interval (~1.1 m) and increase from 67.36 mbsf to a maximum of 5.26 m at 61.11 mbsf (Fig. F74). The temperature of the borehole fluid increases from 22.56° to 23.84°C, pH values are ~7.78, and electrical conductivity is ~51.45 mS/cm (0.194 m).
3. By positioning an open shoe casing at 17.45 mbsf, the upper part of the older Pleistocene sequence and the lower 10 m of the last deglacial sequence (17.45–57.45 mbsf; Cores 310-M0005D-10R through 20R) of Hole M0005D could be logged. Borehole conditions were very poor in this interval, and only resistivity and spectral gamma tools could be run safely. Both tools, however, could not reach below 32.36 mbsf. Resistivity values range from 1.1 m at the base to 4.54 m in the middle of this interval (Fig. F74). No decoupling between deep and medium resistivity values was observed until reaching maximum overall resistivity values in the middle part.

Hole M0007A

At Site M0007, borehole conditions were extremely hostile and great difficulties were encountered while deploying the logging tools.

Geophysical wireline operations were completed in Hole M0007A (44.45 mbsl) from 41.63 mbsf upward, with data coverage by only three tools (Spectral Natural Gamma Probe [ASGR], Induction Resistivity Probe [DIL45], and Hydrogeologique Probe [IDRONAUT]) in the last deglacial carbonate sequence. Despite many efforts, tools with centralizers could not pass an obstruction directly below the shoe casing; however, a spectral gamma ray log through the steel drill pipes provided a continuous log of the drilled interval in Hole M0007A (Fig. F76).

Hole M0007B

In Hole M0007B (41.65 mbsl), geophysical wireline operations were completed from 45.08 mbsf upward, with data coverage by five tools (no Full Waveform Sonic Probe and Caliper Probe) in the short interval between 24.88 and 31.15 mbsf (Cores 310-M0007B-20R through 25R). A spectral gamma ray log through the steel drill pipes provided a continuous log of the drilled interval in Hole M0007B (Fig. F77). An open shoe casing was placed at 24.88 mbsf, and open borehole logging was only possible below this depth. The following geophysical characterization of Unit I at Site M0007 was made by integrating both boreholes. TGR was very low (~9 cps) from the seafloor to the base (0–45.08 mbsf; Cores 310-M0007B-1R through 34R), and a clear indication of what specific element contributed most to these counts cannot be made. Resistivity values range from a maximum of 3.38 m at 27.19 mbsf to 1.09 m at 16.31 mbsf. The temperature of the borehole fluid ranges from 26.5° to 27.2°C, pH values are ~8.1, and average electrical conductivity is ~56.3 mS/cm, whereas in Hole M0007B a sharp decrease to values below 49 mS/cm was observed from 29.31 mbsf. This decrease can be attributed to a large cavity and very open framework, which was observed in the acoustic image (ABI40 amplitude) of the borehole wall (ABI40 amplitude; Fig. F78).

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