After coring had reached maximum depth at 367 mbsf, Hole 1103A was reamed, the RCB bit was released, and the pipe was pulled up to 84 mbsf. We ran the TC (natural gamma, porosity, density, and resistivity), GHMT (natural gamma, magnetic susceptibility, and total magnetic field), and FMS-sonic tool strings (see Fig. F26 and "Downhole Measurements" in the "Explanatory Notes" chapter). The last core arrived on deck at 0710 hr on 25 March, and logging operations finished at 0700 hr on 26 March (Table T4). The wireline heave compensator was used for all passes.
During the TC run, we encountered a hole blockage at 242 mbsf that the tool string could not pass. The hole was then logged up to pipe, with no repeat section. Apart from the initial blockage, hole conditions were good, and no further constrictions were met. After logging the open hole, the TC was initially unable to pass into the base of the pipe and came free only after 2 hr (see "Operations,"). Once the tool had returned to the ship, the accelerator porosity sonde (APS) bow-spring (a 1.5-m-long strip of metal) was found to be missing. The GHMT logged both on the way down to 241 mbsf and up to the pipe and seafloor. The FMS-sonic tool string was run without its normal centralizer unit (comprising three 1-m-long bow-springs) to prevent it from becoming entangled with the APS bow-spring, left downhole after the first run. The FMS tool failed at 148 mbsf on the first upward pass and at 180 mbsf on the second.
Hole 1103A was a good hole for logging, apart from the blockage at 242 mbsf. The borehole width varied between 30 and 41 cm, and <1 m of fill accumulated between the first and last logging runs. Most logs give reliable values (Figs. F27, F28), although we could not confirm this from comparison with core data because core recovery was only significant below 247 mbsf. Anomalous values in the total magnetic field and susceptibility logs near 117 mbsf may be caused by the APS bow-spring, which had been lost in the hole.
The sonic first-arrival traveltimes (autopicked during logging) are of poor quality. Those from the lower of the two digital sonic tool (SDT) transmitters (LTT3 and LTT4) are completely unusable, and those from the upper transmitter (LTT1 and LTT2) are often underestimates because the first arrival was sometimes incorrectly picked in the noise preceding the true first arrival. These traveltimes were converted to velocities by dividing the transmitter-receiver separation by the traveltimes, and then adding 10% to approximate the extra path length and slower water velocities traversed by the sonic wave between the tool and borehole wall. Normally, this correction is not necessary, as velocities are calculated using one traveltime subtracted from another (e.g., the 3.0 m minus the 2.4 m separation traveltime) so that the wave path in the borehole fluid is common to both and is canceled out. We cannot use this method because there are so few depth intervals in which both LTT1 and LTT2 are good. Even using our approximate method, the resulting velocity log shows variations consistent with expectations from the porosity and density logs, and velocity values (~2.1 km/s) comparable with those derived from seismic-survey stacking velocities. Figure F27 shows the velocities derived from the LTT1 and LTT2 traveltimes from the two FMS-sonic passes. Additionally, coherency analysis, performed on the Schlumberger Maxis computer, promises to provide intervals of velocity log with more accurate absolute values. The sonic velocities are discussed further in "Seismic Stratigraphy".
Up until the points where the FMS failed, good images were obtained. About half the buttons on the fourth pad failed to function.
Unit 1 is characterized by low (25%-35%) porosity and high resistivity, density, and velocity. Magnetic susceptibilities are variable. This unit seems to have a distinct lithologic character and may contain more chlorite than the underlying units (Fig. F29).
Unit 2 generally has 35%-50% porosity but contains a subunit (130-145 mbsf) of lower porosity (25%-40%). Susceptibilities are low. The subunit is bounded at the top and bottom by thin (2 m) beds of distinctly higher resistivity and lower susceptibility, in contrast to the similarity of resistivity and magnetic susceptibility in other parts of the logs.
Unit 3 has high susceptibilities, and the FMS images show that clasts are quite common.
Unit 4 contains fewer clasts than Units 3 and 5, and there is a reduction in magnetic susceptibility and resistivity.
Unit 5 is similar to Unit 3 but with lower porosities and higher resistivities. A 30-cm-thick layer (or flat boulder?) of very low (0%-10%) porosity occurs at 212 mbsf, and a 1-m-thick layer of ~20% porosity lies at 210 mbsf, the only level in which layering is evident in the FMS images.
Unit 6 is marked by a distinct jump to higher porosity (50%) and lower density, resistivity, and velocity. Porosity reaches 60% in a zone of highly variable log behavior (233-242 mbsf). The logs end at 242 mbsf, but Unit 6 cannot extend much deeper, as the first core with significant recovery (178-1103A-27R), characterized by a porosity of 20% and velocities of ~3 km/s, begins at 247 mbsf. The transition between the base of Unit 6 and the top of Core 178-1103A-27R thus represents the largest physical properties contrast observed in the hole, probably caused by induration.
FMS images from the two passes are good, which can be attributed in part to favorable borehole conditions. Some buttons of one pad failed, which led to a high-resistivity trace that will be corrected by further processing postcruise. In places, the images are affected by tool stick and slip. The images are repeatable between the two runs. In Units 3 and 5, the FMS images are characterized by the presence of resistive spots (light colored in the image), which are caused by pebbles, because they have a much lower porosity than the surrounding sand, silt, and clay matrix. One of the biggest pebbles, observed in the traces of two of the FMS pads, can be identified at 157 mbsf (Fig. F30). Some conductive (dark) spots are also apparent and could be caused by bad pad contact caused by the seawater-filled depression in the wall left by pebbles plucked during drilling.
Layering is observed only rarely in these images (Fig. F30). The clearest layer is highly resistive and is located at 211.5 mbsf. Its shape, and presence of layered sediment nearby, indicates that it is probably a planar layer; however, the almost zero porosities suggest that the "layer" might be a boulder perforated by coring. This feature corresponds to high resistivity, high gamma ray, and high susceptibility in the logs.
The total magnetic field (MAGB) log (Fig. F31) contains many positive spikes that are grouped in distinct intervals. They are most probably caused by large clasts located close to the borehole wall and are most abundant in Units 1, 3, and 5.
As the first stage in the procedure to recover a polarity stratigraphy (see "Downhole Measurements" in the "Explanatory Notes" chapter), the clast-affected intervals were manually removed from the log, along with the modeled pipe effect. The magnetic anomaly in the borehole caused by the sediment's induced magnetization was calculated from the magnetic susceptibility log. The similarity between the total anomaly and the anomaly resulting from the induced magnetization shows that in the sediment, the induced magnetization dominates the remanent magnetization. This is what would be expected from coarse-grained sediment: neither the large size of the magnetite particles nor the environment of deposition are ideal for recording the geomagnetic field with any fidelity. Hence a magnetic polarity stratigraphy could not be derived from the GHMT logs aboard ship but may be possible for the clast-free intervals after postcruise processing.
The Lamont-Doherty temperature-logging tool recorded the temperature of the fluid in Hole 1103A during the TC run. The curve has a temperature gradient of ~16ºC/km (Fig. F32). The downhole and uphole curves show a constant offset of ~0.6ºC because the borehole continued to re-equilibrate during acquisition. These temperatures do not represent in situ formation temperatures.