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doi:10.2204/iodp.proc.307.105.2006

Downhole measurements

Logging operations

The triple combo and FMS-sonic tool strings (Fig. F8 in the “Methods” chapter) were planned for Hole U1318B. After completion of the APC and XCB drilling operation (bit size = 11 inches) at 242 mbsf, the hole was displaced with 102 bbl of 8.9 ppg sepiolite mud and the pipe set at 70 mbsf in preparation for logging. A summary of the logging operation in Hole U1318B is provided in Figure F25 and a breakdown of the chronology of logging operation is given in Table T11, including some details of the tools used. The tool rig-up was begun on 10 May at 1030 h and rig-down completed by 10 May 2005 at 2100 h. The triple combo tool string was run to the bottom of the hole (242 mbsf) and logging began at 1030 h on May 10. The caliper readings from the triple combo tool string suggested that the hole was in very good condition for logging. Difficulties in starting the minitron (neutron source), necessary to evaluate the porosity (APLC) of the formation, suggested a malfunction, but the other tools collected good-quality data. The FMS-sonic tool string was rigged up and run to the bottom of the hole, and two passes were successfully acquired. Logging operations were completed by 2100 h on 10 May. The heave conditions were excellent, typically <1 m throughout the logging operation. Consequently, the wireline heave compensator experienced no problems during this operation.

Data quality

The triple combo tool string caliper indicated that the hole conditions were very good in the entire open interval (70–242 mbsf) (Fig. F26A) with values smaller than 12 inches. Vertical acceleration (A_z) of the FMS-sonic tool string, which integrates the effects of heave, sidewall contact, and wireline stretch on the tool string, indicated that hole conditions and stick-slip of the tool remained at low levels until the cable head entered the BHA (Fig. F26B). The orientation of the reference pad of the two passes of the FMS images (Fig. F26E, F26F) shows a very slow and progressive rotation. The two passes followed exactly the same path (Fig. F26G) in the entire open section of the hole (70–242 mbsf) and did not improve the coverage of the borehole. The FMS images are of good quality except for slight degradation of the images from two worn pads. A very high frequency alternation between high and low resistivity is observed in most of the open hole, but mainly in the lower part of the borehole. Some portion of this alternation, especially in the lower section (below 190 mbsf), may be interpreted as drilling marks, but generally its origin is unknown as high-frequency changes in resistivity are not expected from the homogeneous sediments of this part of the hole. It might be an artifact. The Accelerator Porosity Sonde experienced some trouble downhole, and the porosity log must be interpreted with caution. The quality of the sonic log is discussed in detail in “Discussion.”

The original logs were depth-shifted to the seafloor, based on the step in gamma radiation in the triple combo logs (Fig. F27A). The seafloor depth differs by <2 m from the seafloor depth determined from the mudline. 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. F27B, F27C).

Logging stratigraphy

The logged section and logging units are characterized by (1) clear shifts in total spectral gamma radiation (HSGR) (Fig. F28A) and bulk density (RHOM) (Fig. F28D) and (2) the impulsive character of the resistivity (Fig. F28C) and acoustic velocity (V_P) (Fig. F28F) logs. Four logging units were defined (Fig. F28G).

Logging Unit 1: base of pipe (70 mbsf)–81 mbsf

Logging Unit 1 is characterized by (1) a high level of gamma radiation (~60 gAPI), (2) a downhole decrease in porosity, and (3) the progressive increase of the photoelectric effect factor (PEF) (from 1.25 to 3 b/e^–). The base of logging Unit 1 is, thus, marked by low porosity and corresponds to lithostratigraphic Unit 1 (see “Lithostratigraphy”).

Logging Unit 2: 81–87 mbsf

Logging Unit 2 is characterized by a major peak in gamma radiation (>120 gAPI), principally associated with an enrichment in uranium (to 16 ppm). This logging unit is also marked by a subtle change in resistivity and slight increase in density. The average velocity of this interval is ~1650 m/s. Logging Unit 2 corresponds to a lithified sandstone including a bivalve bed (see “Lithostratigraphy”).

Logging Unit 3: 87–188 mbsf

Logging Unit 3 is characterized by a regular cyclicity of gamma radiation with a slight increase with depth mainly associated to the potassium and thorium contribution of natural radioactivity. Electrical resistivity is low and shows only small-amplitude variations in this unit. Density and PEF logs anticorrelate with gamma radiation, suggesting some variation in carbonate and clay content with depth; low gamma radiation and high PEF can be interpreted to be more carbonate rich and high gamma radiation and low PEF to be more clay rich. The background velocity is ~1600 m/s. The unit is characterized by distinct peaks in acoustic velocity (up to 2000 m/s) and corresponds to siltstone of lithostratigraphic Unit 3 (see “Lithostratigraphy”).

Logging Unit 4: 188 mbsf–bottom of the logged section (242 mbsf)

Logging Unit 4 is characterized by the lowest porosity recorded in the open hole. This unit is also marked by a decrease in gamma radiation (from 60 to 50 gAPI). This unit presents localized peaks in electrical resistivity concomitant with an increase in density and thus a decrease in porosity. The average acoustic velocity of this logging unit is >1850 m/s with peaks of ~2200 m/s. Logging Unit 4 corresponds to the lower part of lithostratigraphic Unit 3 (see “Lithostratigraphy”).

Discussion

When in situ velocity of the formation is close to the velocity of the drilling mud (~1600 m/s), the P-wave labeling algorithm in the Schlumberger software has initial difficulty in identifying the P-wave of the formation measured by the Dipole Sonic Imager (Figs. F28F, F29). The two passes of the sonic logs have been improved by reprocessing (Fig. F28F), but some errors in the semiautomatic picking of the P-wave persist (Fig. F29).

Comparison between continuous and in situ logs with the whole-core MST records (gamma radiation, density, and velocity) and discrete sample MAD data provides the basis for depth-matching the core-derived mcd scale to a logging equivalent depth and correction of physical properties measured on expanded cores (Fig. F30).

The microresitivity (FMS) (vertical resolution = <1 cm) images can finely resolve the high-frequency cycles seen, for example, in natural gamma radiation data of logging Units 3 and 4 (87–242 mbsf).

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