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doi:10.2204/iodp.proc.302.202.2008

Logging data and absolute depth uncertainty

When incomplete recovery plagues a drilling hole, a valuable constraint on the relative depths of cores can be obtained by integrating logging data with petrophysical measurements made on recovered sediments. This is most commonly achieved by migrating core depths onto an equivalent logging depth (eld) scale. During ACEX, logging was conducted in Hole M0004B, providing the opportunity to integrate downhole natural gamma radiation (NGR) and compressional wave (P-wave) velocity measurements with similar data sets collected on whole cores (see the “Sites M0001–M0004” chapter).

Moderate-quality logging data were acquired on 2 passes between 210 and 65 mbsf. Offsets between the P-wave velocity (140–200 eld) and NGR measurements (155-180 eld), combined with the lithologically homogeneous character of Unit 1, make detailed integration of the core and logging depth scales problematic. The logging data do cross one of the largest changes in sediment physical properties encountered during ACEX. This transition occurs across lithologic Subunit 1.5, where a gradual progression is seen from fossil-poor glaciomarine deposits into biosiliceous and organic carbon-rich sediments (Moran et al., 2006). Subunit 1.5 is a 5.19 m long sequence defined by dramatic centimeter- to decimeter-scale, gray to black crosscutting couplets. The subunit extends from intervals 302-M0002A-44X-1, 95 cm (192.94 mbsf), to 46X-1, 113 cm (198.13 mbsf). The base of Subunit 1.5 marks a 26 m.y. hiatus separating Paleogene and Neogene sediments (Backman et al., 2008). It occurs in interval 302-M0002A-46X-1, 114 cm, at 198.7 mcd (equivalent to 198.14 mbsf).

In the petrophysics data, a drop in P-wave velocity and a corresponding peak in NGR mark the hiatus. Although the logging data do not capture the peak in NGR, as data acquisition ended prior to this depth being reached, the start of the increase in NGR seen in Cores 302-M0002A-44X and 45X is recorded in the logging data. Furthermore, positioning the sonic tool lower on the logging string allowed acquisition of sonic data deeper in the hole and appears to have captured the base of the subunit. A prominent feature in the sonic data is a 120 m/s increase in compressional wave velocity at 200 eld. Below this increase, there are four recorded “down-stepping” features in the sonic log. Given that the associated rise in NGR was not captured in the logging data, the hiatus likely corresponds to the drop in P-wave velocity at 205 eld (Fig. F3). Within this framework, the increase in compressional wave velocity at 200 eld is associated with the gradual increase seen in Core 302-M0002A-44X that begins at the top of lithologic Subunit 1.5 and culminates with a peak P-wave velocity of 1691 m/s at 193.85 mbsf (Section 302-M0002A-44X-2, 35 cm). However, this assumes that the depth separating Cores 302-M0002A-44X and 45X in the mbsf scale is accurate. If a larger gap exists between these cores, then the base of Subunit 1.5 could correspond to the drop in compressional wave velocity at 207 eld, or perhaps even lower, thereby extending the length of the transitional facies in Subunit 1.5.

The inferred ties between the core and logging data, bracketing the top and base of Subunit 1.5, can give a rough estimate of the accuracy of the mbsf depth scale. Errors in the mbsf depth scale related to inaccuracies in the rig floor measurement of pipe deployment are generally cumulative. Over the length of a hole they should not exceed the length of a drill pipe stand. Here the difference between the inferred logging depth to the base of lithologic Subunit 1.5 (205 eld) and its equivalent core depth (198.14 mbsf) is 6.86 m. This difference approximates the cumulative downhole error in the mbsf depths. Considering that the logged site (M0004B) was ~15 km from the cored site (M0002A), this error is not unreasonable.

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