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doi:10.2204/iodp.proc.303306.112.2006

Downhole measurements

Logging operations

Downhole logging operations were carried out after coring Hole U1313B to a depth of 302 mbsf (3727 mbrf) and displacing with sepiolite mud (see “Operations”). The drill pipe was raised to 65.3 mbsf (3489.6 mbrf) prior to logging. During logging operations, the sea state was fairly calm with a typical heave of 2 m or less. The LDEO-Borehole Research Group wireline heave compensator (WHC) was used throughout the logging operations in the open hole.

The initial plan was to use two tool string configurations, the triple combo with an additional MGT and the FMS-sonic tool string (see “Downhole measurements” in the “Site U1312–U1315 methods” chapter). However, shortly after deploying the triple combo-MGT tool string, power problems forced us to bring the tool string back on deck for examination. It was determined that the MGT tool was leaking and had caused damage to the telemetry cartridge below. The MGT was removed from the tool string and a new telemetry cartridge was installed on the tool string. Following the repairs, the triple combo was deployed successfully to the bottom of the borehole at 300 mbsf (3725.3 mbrf). Details of the intervals logged, together with the position of the drill bit, are shown in Figure F37.

Based on the very low amplitude resistivity results from the triple combo and the delays caused by earlier tool problems, it was decided not to deploy the FMS-sonic tool string. The second pass of the triple combo tool string was used to fully test new software for the new Schlumberger WHC system. These tests provided important performance information and showed promising results.

Data quality

Initial examination of the logging data from the triple combo tool string showed good quality data for several logs including natural gamma radiation, density, and porosity. This is supported by the caliper data, which show that the diameter of the borehole (drill bit size = 11.4 inches) ranged over just a few inches from ~10.6 to 15.5 inches over the entire interval (Fig. F38). The density and porosity tools require good borehole contact and are held against the borehole wall by an eccentralizer that is only effective in the open borehole below the drill pipe. Density and porosity data are also less reliable when the caliper has been closed before the tool string enters the base of the drill pipe (i.e., above ~75 mbsf during the main pass) (Fig. F38). Resistivity data from the formation are quite consistent between shallow spherically focused resistivity, intermediate medium induction phasor-processed resistivity, and deep induction phasor-processed resistivity, despite their low amplitude, indicating that the tool is getting reliable results. The gamma ray results are also very consistent and can be evaluated even through the drill pipe, despite some attenuation of the signal.

Results

The downhole logging data suggest that the formation is made up of two main sections that can be delineated at a depth of ~150 mbsf in almost every downhole log. This boundary shows up especially well in the gamma and resistivity data and is driven by increasing clay content in the sediments beginning at ~150 mbsf. This boundary is also seen clearly in core physical properties like magnetic susceptibility, lightness (L* values), and NRM paleomagnetic intensities. Visual core descriptions show that this boundary is gradational between 120 and 150 mbsf.

As is usual, the density and porosity logs are generally inversely related and show downhole trends of increasing density (1.6 g/cm³ at 80 mbsf to 1.9 g/cm³ at 300 mbsf) and decreasing porosity (~75% at 80 mbsf to ~50% at 300 mbsf) due to compaction. Resistivity values are low (~0.8–1.1 Ωm) reflecting the generally moderate- to high-porosity sediments. However, there are two significant changes in resistivity patterns downcore. The first one is at 150 mbsf and corresponds to the major change in lithologies noted above. The second is at 240 mbsf and may correspond to a significant change in grain size. Photoelectric effect factor values increase from 3.0 b/e^– (clay rich) in the upper 150 m to as high as 4.5 b/e^– (calcite rich) in the lower 150 m, consistent with the lithologies (see “Lithostratigraphy”).

Gamma ray values from the total spectral gamma ray (HSGR) log vary between 7 and 25 gAPI throughout the upper 150 mbsf, as they vary consistently with glacial–interglacial changes in clay content (Fig. F39). The values measured through the drill pipe in the upper 65 m (2–5 gAPI) are lower but are attenuated by a factor of ~4–5. Below 150 mbsf, gamma radiation decreases to values between 5 and 8 gAPI as a result of decreasing clay content. The cycles seen in the upper part continue in the lower section but with reduced amplitude.

The gamma ray value is driven by the Th concentrations (1–3 ppm) derived from clay content. The relatively low U (<0.5 ppm) and K content (<0.05 wt%) of the formation results in very similar HSGR and computed gamma ray headspace (HCGR; summation of Th and K gamma rays only) values. The U data suggest that total organic carbon values in the logged interval are consistently very low, as shown by discrete samples (see “Geochemistry”). Th, U, and K display very similar trends downhole that are consistent with major lithology changes. One observation seen downhole is the consistent pattern of U and Th in the upper 150 m and out-of-phase behavior below 150 mbsf (Fig. F39).

Core-logging comparisons

The downhole data display consistent cyclic decimeter- to meter-scale variations that are the result of changes in lithology. A comparison of logging- and core-derived density, natural gamma radiation, and porosity (not shown) records show very good agreement in downhole trends and patterns (Fig. F40). Measured density values range from 1.6 to 1.85 g/cm³ in both core and logging data. Although scaled differently (cores in total counts per second and logs in gAPI), the gamma ray data suggests that 1 m or even smaller-scale patterns can be recognized in both the core and logging records (Fig. F40).

In Figure F41, two comparisons of the logging gamma ray, core gamma ray, and L* values in the upper 70 mbsf of Hole U1313B are shown to demonstrate the remarkable degree of correlation possible between core and logging records. Even as amplitude of cycles decreases downcore, correlating these records with logging as a depth reference will be possible at meter (at least) scale to more precisely determine the amount of core expansion in the spliced core record.

Based on preliminary results of biostratigraphy and paleomagnetism, the estimated sedimentation rate determined from the age model for the last 5.4 m.y. is remarkably constant at ~4 cm/k.y. To show how logging data can be used at this site for constraining age-depth models in detail, a simple linear plot of the gamma ray logging data over the interval from 0 to 225 mbsf and the benthic oxygen isotope stack (Lisiecki and Raymo, 2005) was constructed for the last 5.4 m.y. (Fig. F42). The overall patterns of variability in both curves track each other quite well in a variety of frequencies over the whole record. This correlation has tie points at only the top and bottom (0 and 5.4 Ma) with no stretching or squeezing in between. This correlation is expanded over four time intervals (Fig. F43A, F43B, F43C, F43D) to highlight how faithfully the gamma ray profile tracks the oxygen isotope curve in rather amazing fashion despite large changes in signal amplitude. The predicted ages versus logging depth (mbsf) match the biostratigraphic and paleomagnetism datums very well over the last 5.4 m.y., providing additional support for the extraordinarily constant sedimentation rates determined for this site.

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