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doi:10.2204/iodp.proc.340.107.2013

Downhole logging

Acquisition of downhole logging measurements including gamma ray, acoustic velocity, magnetic susceptibility, and electrical resistivity and images were obtained on 23 March 2012 in Hole U1397B. Two tool strings (Fig. F14) were used to log a ~140 m section of the hole over a ~14 h period. The borehole was very enlarged, degrading in condition throughout the course of the logging program, including infilling of at least 16.5 m.

Operations

Following the last core on deck, preparations for logging commenced, including circulating with weighted mud and pulling the pipe to 81.4 mbsf. Unlike previous logged holes during Expedition 340, Hole U1397B was not displaced with heavy mud because of the necessity to circulate while pulling pipe (see “Operations”). At 0515 h on 23 March 2012, pick up of the triple combo-Magnetic Susceptibility Sonde (MSS) tool string (see Fig. F11 in the “Methods” chapter [Expedition 340 Scientists, 2013a]) commenced. Because of hole conditions as reported by the drillers, the Hostile Environment Litho-Density Sonde (HLDS) was run without its ¹³⁷Cs source. It was also decided, on the basis of hole conditions, to log only to ~220 mbsf, in order to keep the logging tools above the region with the most difficult drilling/coring conditions. The tool string tagged the bottom at 223 mbsf (~30 m shallower than total drilled depth), indicating that the hole had either filled in or bridged over at this depth. An initial uplog was then undertaken to ~114 mbsf before the tool string was run back in the hole for a second and final uplog through the seafloor. On tagging the bottom of the hole, it was noted that 1 m of infill had occurred since the initial tag.

The original plan had been to conduct a VSP experiment using the VSI tool string as the second tool deployment at this hole. However, because of a very enlarged hole diameter (in excess of 20 inches over most of the logged interval), as reported by the caliper measurement from the triple combo, it was decided to cancel this tool string run. The VSI relies on good coupling with the borehole walls to anchor the sonde, which would not have been possible in this oversized borehole.

The FMS-sonic was the second tool string to be deployed in Hole U1397B, with tools being picked up at 1130 h. The downlog indicated seafloor at 2481.5 mbsl, and the hole depth was found to be shallower than that indicated by the triple combo, at a depth of 212 mbsf. An initial uplog to 115 mbsf was undertaken before returning to the bottom of the hole (206.5 mbsf = 5.5 m of infill over a ~20 min period) for a final uplog through the seafloor. Tools were at the surface by 1800 h, and the tools were laid out and the rig floor handed over to the drill crew by 1910 h.

Data processing and quality assessment

Figures F15, F16, and F17 show a summary of the main logging data recorded in Hole U1397B. Following acquisition, the downhole measurements data were processed. The total gamma ray log from the main pass of the triple combo was used as the reference log and was shifted to seafloor; it is the log to which all other logs (from this tool string and the FMS-sonic tool string) were depth-matched.

The oversized hole diameter has implications for all downhole logging measurements recorded in Hole U1397B; therefore, logging data should be interpreted with caution. The calipers run during these logging operations (HLDS and FMS) were generally opened to their full extent, and, as such, actual hole size through much of the open hole interval is not known. The HLDS caliper maximum reach is 20 inches, so the hole must have been generally greater than this in diameter (see hole size track in Fig. F15). As shown in Figure F16, the FMS caliper arms were also open to their full extent (~15 inches) over much of the logged interval. Hole diameter decreased between 92 and 124 mbsf between the triple combo and FMS-sonic tool string runs, indicating that the hole was gradually collapsing over the course of the 14 h logging period.

The condition and size of the borehole are principal factors in evaluating data quality. Because the tools could not be reliably positioned (centralized or eccentralized) within this enlarged borehole, data quality is likely compromised. Measurements such as gamma ray are clearly affected by the large borehole size, whereas deep reading measurements such as electrical resistivity and sonic velocity are less affected. Gamma ray measurements show some significant variability between passes of the same tool string (Fig. F18; 160–198 mbsf) and between the two tool string runs (Fig. F16; 106–160 mbsf). This suggests that the tool was in a different position relative to the borehole wall between passes and/or that the borehole condition was rapidly changing between tool runs. The degree of separation between the different electrical resistivity curves (R3, R5, RT) can be used as an indicator of the quality of the data. In this case, where the curves are greatly separated (e.g., Fig. F15; 200–210 mbsf) the measurements are likely dominated by the fluid resistivity as a consequence of the enlarged borehole. There are also intervals where the resistivity curves are closer together (Fig. F15; 160–170 mbsf), which indicates better quality data. Magnetic susceptibility data are also affected by enlarged borehole conditions; however, downhole trends and relative changes should be reliable. The high coherence indicated by the red areas in the compressional velocity (V_P) track in Figure F16 suggests that, despite the enlarged hole, the tool was able to capture the compressional wave arrivals over a significant portion of the logged interval. Both V_P and shear wave velocity (V_S) will be reprocessed postcruise in order to improve the velocity data set. Perhaps most significantly affected by the enlarged borehole conditions are the FMS images, which largely reflect the electrical properties of the borehole fluid rather than the formation. FMS pad contact with the borehole wall was rare (Fig. F19).

Logging stratigraphy

The ~140 m open-hole section of Hole U1397B was divided into four logging units on the basis of characteristics identified across the different geophysical properties measured. The different properties of these units are summarized below.

Logging Unit 1

Logging Unit 1 (85–90 mbsf) is characterized by relatively consistent values of resistivity (average = 1.49 Ωm) and gamma ray and an average value for V_P of ~1650 m/s. This is coupled with a generally decreasing trend in magnetic susceptibility.

Logging Unit 2

In contrast to the overlying unit, logging Unit 2 (90–127 mbsf) is characterized by four intervals on the scale of 5–10 m that each exhibit increasing resistivity and V_P downhole with sharp bases (Figs. F15, F16). The boundaries to these intervals correspond with distinct changes in the magnetic susceptibility log. Although the general trend in magnetic susceptibility is decreasing downward, local highs do correspond with the elevated resistivity and V_P values at the base of the intervals. Overall values of resistivity and V_P are elevated with respect to logging Unit 1 (mean resistivity = 1.99 Ωm; mean V_P = 1740 m/s).

Logging Unit 3

Logging Unit 3 (127–185 mbsf) shows a return to less variable resistivity and V_P values. The unit has been divided into two subunits on the basis of a subtle change in the character of the logs, most distinctive in the magnetic susceptibility. The amplitude of variations in the magnetic susceptibility data are much greater in logging Subunit 3A (127–155 mbsf) than in logging Subunit 3B (155–185 mbsf), although the frequency of changes is very similar.

Logging Subunit 3A

Resistivity is relatively constant in logging Subunit 3A, with an average value of 1.48 Ωm. Despite some moderately large excursions from the base value of magnetic susceptibility, there is no net change downhole. V_P shows low amplitude variability in Subunit 3A with somewhat higher values in the lower half of the subunit (Fig. F16). Overall V_P has an average value of ~1710 m/s.

Logging Subunit 3B

In combination with the drop in magnetic susceptibility amplitude, there is an increase in amplitude of the V_P log in logging Subunit 3B (mean = ~1760 m/s). The V_P peaks do not relate to the changes in the other logs in a consistent way. Instead, V_P highs in the top half of the subunit correspond to lows in magnetic susceptibility and resistivity, whereas in the lower half of the subunit the highs correspond to peaks in resistivity and magnetic susceptibility (Figs. F15, F16). The division of the subunit also marks a reduced separation between the true (RT) and deepest (R5) resistivity curves. True resistivity is relatively consistent downhole with a mean value of 1.49 Ωm.

Logging Unit 4

Logging Unit 4, which extends from 185 mbsf to the base of the hole, is marked by elevated values in resistivity (mean = 2.52 Ωm) and magnetic susceptibility relative to the overlying unit. There appears to be an interval of high resistivity and V_P between 185 and 190 mbsf. However, this measurement could be an artifact of degrading conditions at the base of the logged interval. Below this unit, resistivity increases toward the base of the unit with the curves becoming increasingly separated. Magnetic susceptibility, the measurement that extends to the greatest depth in the borehole, has a relatively consistent profile throughout the unit. V_P in this logging unit is elevated (average = ~1900 m/s) with respect to the overlying units and completes a downhole trend of increasing V_P .

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