Downhole logging was performed in Hole 1241B after it had been drilled to a depth of 395 mbsf with an 11.438-in APC/XCB drill bit (see "Operations") and displaced with sepiolite mud and the pipe was set at 82 mbsf. Two tool string configurations were run, the triple combo-MGT and the FMS-sonic (see "Downhole Measurements" in the "Explanatory Notes" chapter). No problems were encountered while logging, and all passes reached the base of the hole. Details of the intervals logged with each configuration, together with the position of the drill bit, are shown in Figure F32. The Dipole Sonic Imager (DSI) on the FMS-sonic was run in P&S (middle frequency), lower dipole (low frequency), and first motion detection (FMD) modes. Weather was excellent, and the sea state was calm with peak heave <2 m. The wireline heave compensator was used throughout the logging operation.
The caliper log (Fig. F33) shows that the borehole was relatively smooth and that the diameter varied between 10.5 and 14.3 in (mean = 12.0 in; standard deviation = 1.1 in), resulting in excellent data from the density, porosity, and FMS tools that require good borehole contact. Hole deviation increased slightly with depth, reaching a maximum of 2.6°. Downhole log-derived densities mirror the downhole porosities and closely match with core measurements (Figs. F34, F35). NGR measurements are highly reproducible between tools and passes and are similar to the core-derived natural gamma record from Hole 1241A (Figs. F33, F36). Sonic velocities were low but generally reliable from 180 mbsf to the bottom of the hole (Fig. F34).
Overall, the physical properties at the site are homogeneous, suggesting a fairly uniform lithology throughout the sequence. No clear compaction-related trends are indicated with depth. Instead, sonic velocity, density, and resistivity show broad maxima from 260 to 280 mbsf associated with a minimum in porosity. One short and two long intervals marked by particularly low density, resistivity, and sonic velocity are present at 165-168, 192-258, and 305-365 mbsf, respectively, and correspond to intervals with relatively high diatom abundances in the cores (see "Biostratigraphy"). Superimposed upon these broad-scale changes in physical properties, meter-scale covariations in density and resistivity, similar to those observed at Sites 1238 and 1239, occur throughout the sequence (Figs. F34, F37). As at the earlier sites, these meter-scale density and resistivity fluctuations are most likely related to the silica to carbonate oscillations observed in the cores (see "Lithostratigraphy"). Banding on the FMS images occurs on the same scale as the density and resistivity changes associated with the nannofossil to diatom oscillations (Fig. F38).
The NGR activity in Hole 1241A shows significant meter- and dekameter-scale variability superimposed upon a general increase with depth (Figs. F33, F36). The spectral gamma results from the Hostile Environment Gamma Ray Sonde (HNGS) tool (Fig. F36) show very low Th activity throughout the sequence and K activity at or below the tool detection limit. In contrast, the U activity and variability is much greater, reaching its highest values below 310 mbsf. The high overall U activity dominates the total gamma ray activity of the sediments. As at Sites 1238 and 1239, the dominance of the U activity over Th and K activity indicates that changes in sedimentary redox conditions related to organic matter, rather than terrigenous input, control sediment gamma ray activity at Site 1241. However both the U and total counts are significantly lower at Site 1241 than at Sites 1238 and 1239, suggesting both organic and terrigenous components are relatively lower at Site 1241.
Log-derived density records show close agreement with core measurements down to the meter scale (Fig. F35). Using the downhole log density and natural gamma records as a depth reference, core measurements were mapped to an equivalent log depth (eld) using the software program Sagan in order to identify more precisely the size and position of core breaks within the XCB section (see "Composite Section").
Despite the close relationship between core and log densities, the lower resolution of the downhole log prevents core-log comparisons to the decimeter and centimeter scale. However, the relationship between resistivity and density in the downhole logs is strongly linear (r = 0.96) throughout the logged sequence, suggesting a common lithologic control on both properties (Fig. F37). This circumstance provides an opportunity for much higher resolution core-log comparison based on the FMS log, which uses 64 microelectrodes to generate an electrical conductivity image of the borehole with resolution of ~1 cm. A comparison between the spherically focused resistivity log, with a resolution similar to that of the downhole density log, and the conductivity curve derived by averaging the 64 channels of FMS data, suggests that a significant amount of higher-frequency variability is missed by the conventional resistivity log at this site (Fig. F39). This higher-frequency microconductivity variability is comparable with the GRA density records to the centimeter scale (Fig. F38), suggesting that the FMS data may allow centimeter-scale core-log comparisons for much of the sequence.