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doi:10.2204/iodp.proc.307.104.2006

Downhole measurements

Logging operation

Following completion of RCB coring operations (bit size = 9⅞ inch) in Hole U1317D, the hole was conditioned with a wiper trip and was displaced with 84 bbl of 8.9 ppg sepiolite mud. The drill pipe was withdrawn to 77 mbsf in preparation for logging. All three scheduled tool strings were run: (1) the triple combo tool string, (2) the FMS-sonic tool string, and (3) the WST for a check shot survey (see “Downhole measurements” in the “Methods” chapter). A summary of the logging operation, including tools used, is provided in Figure F31 and Table T9. The heave conditions were good, typically <2 m throughout the logging operation. All passes encountered an obstruction 19 m above the bottom of the hole at 251 mbsf (Fig. F31). The obstruction was probably material that fell from higher in the hole. Tool string rig-up was begun on 6 May 2005 at 1200 h and the full operation completed by 7 May at 0230 h.

The triple combo tool string was run first and was successfully lowered into the hole to 25 mbsf. In order to run the WST in accordance with the Marine Mammal Policy (i.e., during daytime and in the absence of marine mammals in the vicinity of the ship prior to and during the check shot survey), it was run before the FMS-sonic tool string. A total of 13 check shot stations were acquired at ~15 m spacing, avoiding the washed-out section (Table T10). A first pass up to pipe depth (77 mbsf) was followed by a second pass up to seafloor after the pipe was raised by 7 m. The Dipole Sonic Imager was run in P-wave and S-wave monopole and dipole shear modes and in first-motion detection mode. The P-wave velocity logs are of excellent quality. The FMS-sonic tool string was retrieved to the rig floor at 0230 h on 7 May.

Data quality

The triple combo tool string caliper indicated that the hole conditions were good except in a short section of the borehole (128–132 mbsf). Below 132 mbsf and to 142 mbsf, the triple combo caliper indicated minor localized washouts confirmed by the calipers of the FMS-sonic tool string (Fig. F32A, F32C). The accelerometer data from the Temperature/Acceleration/Pressure (TAP) tool indicate that stick-slip of the triple combo tool string remained at low levels through the entire open interval (Fig. F32B). Similar to the triple combo tool string, vertical acceleration (A_z) of the FMS-sonic tool string remained at levels characteristic of normal stick-slip motion (Fig. F32D). The orientation of the reference pad of the FMS (Fig. F32G, F32H) shows an absence of rotation due to the elliptical shape of the hole (major axis oriented along 30°). As a result, the two passes followed exactly the same path (Fig. F32I) and consequently do not improve the FMS coverage of the borehole wall. The FMS images are generally of good quality except (1) within the cave initially detected by the caliper of the triple combo tool string, (2) in the lower part of the logged section where spurious features were interpreted as drilling marks, and (3) images from two of the FMS pads that were worn and produced images that were less clear than those produced by the other two pads.

The level of NGR in the penetrated formation is low and the signal is attenuated through the pipe. The step in gamma radiation at the sediment/water interface was observed at 816 mbrf. The seafloor depth differs by <2.0 m from the seafloor depth determined by the driller. Using this reference scale, the pipe depth was placed at 77 mbsf. Data from the triple combo tool string served as reference by which the features in the equivalent logs of subsequent FMS-sonic tool string runs were matched (Fig. F33B, F33C).

Logging stratigraphy

Two main logging units are characterized based on (1) the impulsive characters of the resistivity (Fig. F34C), bulk density (Fig. F34D), and acoustic velocity (V_P) (Fig. F34F) logs in the bottom part of the open hole and (2) major changes in the cyclicity of the porosity log (Fig. F34D).

The first logging unit has been further divided into two subunits based on a clear shift in gamma radiation (Fig. F34A).

Logging Subunit 1a: Base of pipe (77 mbsf)–106 mbsf

Logging Subunit 1a is characterized by a progressive enrichment in Th (from 2 to 6 ppm) and an increase in density (from 1.55 to 2.0 g/cm³) with depth. The acoustic velocity is constant with an average value of ~1700 m/s. Resistivity values are low (~1 Ω·m) (Fig. F34C). Logging Subunit 1a correlates with lithostratigraphic Unit 1, the Pleistocene coral mound succession (see “Lithostratigraphy”).

Logging Subunit 1b: 106–141 mbsf

Overall, this subunit is characterized by cyclic fluctuations of intermediate frequency in the gamma radiation, K, porosity, bulk density, and acoustic velocity logs. The lower boundary of logging Subunit 1b is defined by the minimal value in the gamma ray and porosity logs and a step in the resistivity, density, and sonic velocity (V_P) logs. The acoustic velocity increases slightly over the unit up to 2000 m/s.

Logging Unit 2: 141–bottom of the logged section (251 mbsf)

Logging Unit 2 is characterized by localized spikes in the resistivity and acoustic (V_P) logs. These spikes parallel the photoelectric effect factor log and suggest a cementation/​lithification of carbonate-rich layers. This interpretation is consistent with the corresponding decrease of gamma radiation attributed to a decrease in clay content. Logging Unit 2 correlates with the bottom part of lithostratigraphic Unit 2 Miocene drift sediments (see “Lithostratigraphy”).

Discussion

Core-log integration

Total gamma ray counts and densities have the potential to be correlated to downhole logging data and finally presented in the equivalent logging depth scale (Fig. F35).

Check shot survey and core-log-seismic integration

The one-way sonic traveltimes from the ship to the WST downhole were deduced from the first arrival in the recorded waveform. Interval velocities were calculated from the one-way traveltimes (Fig. F34F). These measurements provide an independent way to validate the correlation of the borehole to the seismic sections in addition to the synthetic seismogram. Using the excellent-quality density and acoustic logs (Fig. F34E, F34F), a synthetic seismogram will be constructed postcruise and will provide a means of matching the reflections expected from the formation (based on measured physical properties from logging and core sources) with those in the seismic data.

Thermal measurements

Three independent sets of thermal measurements were collected at this site (Fig. F36). The first data set contains three in situ measurements of formation temperature measured by the APCT during drilling operations in Hole U1317A. During the logging operation, the temperature of the borehole fluid was measured by the Schlumberger Environmental Measurement Sonde and Lamont-Doherty Earth Observatory Borehole Research Group TAP tool. Despite the constant shift of 2°C between these two temperature logs, the temperature measurements show a linear increase with depth. No significant fluid flow that could be implied by changes in the gradient of the temperature was detected. Thermal conductivity measured on cores has values that are between 0.8 and 1.8 W/m·K and are relatively constant in the 65–125 mbsf interval. An overall decrease of thermal conductivity with depth is observed.

Microresistivity FMS images

Of all our downhole logs, only the microresistivity (FMS; sampling rate = 0.1 inch) images can resolve high-frequency cycles and the thin lithified carbonate-rich layers in detail. Figure F37 allows comparisons between the texture of the two distinct logging units. Manual picking of sinusoidal features on the FMS images allows us to identify and characterize the strike and dip of the bedding or factures. Although the FMS images in the bottom portion of the hole may have been impaired by drilling disturbance, if bedding orientation is confirmed by postcruise analyses, these images may be of great value to establish the structural context of the base of the mound.

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