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doi:10.2204/iodp.proc.307.103.2006

Downhole measurements

Logging operations

The triple combo and FMS-sonic tool strings (see Figure F8 in the “Methods” chapter) were planned for use in Hole U1316A. After completion of the APC and XCB drilling operation (bit size = 11 inches) at 134 mbsf, the hole was displaced with 45 bbl of 8.9 ppg sepiolite mud in preparation for logging. In order to get a maximum coverage of the short open hole interval, we decided to set up the pipe at 30 mbsf and break down the classic triple combo tool string into two shorter tool strings. The first included the Hostile Environment Gamma Ray Sonde, Accelerator Porosity Sonde, and Hostile Environment Litho-Density Sonde. The second included the digital Dual Induction Tool and the Dipole Sonic Imager from the standard FMS-sonic tool string. Unfortunately, an obstruction located 15 m below the pipe prevented logging. After extensive pumping and moving of the pipe without success, the operation in Hole U1316A was aborted (Table T11). Based on this experience, we took a more conservative approach (deeper pipe depth and heavier tool string) for the second attempt at logging Site U1316 in Hole U1316C. After completion of the RCB drilling operation (bit size = 9⅞ inches) at 143 mbsf, Hole U1316C was displaced with 44 bbl of 8.9 ppg sepiolite mud and the pipe set at 57 mbsf in preparation for logging. A summary of the logging operation at Hole U1316C is provided in Figure F33, and breakdowns of the chronology of the logging operations in Holes U1316A and U1316C are provided in Tables T11 and T12, respectively. These tables include some details of the tools used. For Hole U1316C, tool rig-up was begun on 8 May 2005 at 0300 h and logging was completed by 8 May at 1330 h. The triple combo tool string was run to the bottom of the hole (1097 mbrf, 143 mbsf) and logging began at 0635 h on May 8. The caliper readings from the triple combo tool string suggested that the hole was in perfect condition for logging. The standard FMS-sonic tool string was rigged up and run to the bottom of the hole. Two passes were successfully acquired. The FMS-sonic tool string was retrieved to the rig floor, and logging operations were completed by 1330 h on 8 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 hole conditions were very good in the entire open interval (57–143 mbsf) (Fig. F34A). Vertical acceleration 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 moderate levels until the cable head entered the BHA (Fig. F34B). The orientation of the reference pad of the FMS (Fig. F34E, F34F) shows a very slow and progressive rotation. The two passes followed exactly the same path (Fig. F34G) in the bottom part of the hole (96–143 mbsf). In the upper part of the open hole, the two passes are complementary and improve the coverage of the borehole. The FMS images are of good quality. Electrical resistivity progressively increases downhole with highly resistive layers identified in the standard resistivity logs and the FMS images.

The original logs were depth-shifted to the seafloor. The seafloor depth was based on the step in gamma radiation in the triple combo logs (Fig. F35A). The seafloor depth differs by 1 m from the seafloor depth determined from the mudline. Using this reference scale, the pipe depth is placed at 57 mbsf. Gamma ray data from the triple combo tool string served as a reference by which the features in the equivalent logs of subsequent FMS-sonic runs were matched (Fig. F35B, F35C). The depth adjustments required to bring the match log in line with the base log have been applied to all other logs from the same tool string.

Logging stratigraphy

The logged section and logging units are characterized by the following:

1. The impulsive characters of resistivity (SFLU, IMPH, and IDPH) (Fig. F36C), porosity (APLC) (Fig. F36D), bulk density (RHOM) (Fig. F36D), and acoustic velocity (V_P) (Fig. F36F) logs in the upper, central, and lower part of the open hole.
2. Clear shifts in total spectral gamma radiation (HSGR) (Fig. F36A).
3. Major changes in the cyclicity of thorium (HTHO) (Fig. F36B) and porosity (APLC) (Fig. F36D) logs.

Five logging units have been defined (Fig. F36G). All of these units correspond to lithostratigraphic Unit 3 (see “Lithostratigraphy”).

Logging Unit 1: Base of pipe (57–67 mbsf)

Logging Unit 1 is characterized by a constant level of gamma radiation (~35 gAPI) and the impulsive nature of the electric resistivity logs. The base of this unit is characterized by a pronounced low in gamma radiation and porosity, and major peaks in resistivity and density. The mean acoustic velocity in logging Unit 1 is ~1750 m/s.

Logging Unit 2: 67–77 mbsf

Logging Unit 2 is characterized by a progressive enrichment in gamma radiation mainly due to its Th (2–7 ppm) and K (0.7–1.2 wt%) contributions. Porosity, density, P-wave velocity, and electrical resistivity are particularly constant over this interval. The base of this unit is characterized by the highest gamma radiation recorded in the open hole. Acoustic velocity in logging Unit 2 is 1950 m/s.

Logging Unit 3: 77– 99 mbsf

Logging Unit 3, like logging Unit 1, is characterized by the impulsive characteristics of the electrical resistivity, porosity, and density logs. The increase of density associated both with an increase in the photoelectric factor and a decrease in gamma radiation suggests an enrichment in carbonate content. These thin layers are associated with a significant increase in acoustic velocity (up to 2400 m/s).

Logging Unit 4: 99–118 mbsf

Like logging Unit 2, this unit is characterized by low and constant electrical resistivity values. The density log is anticorrelated with both the porosity and gamma radiation logs, suggesting subtle variations in carbonate/clay content. The base of this unit is characterized by high porosity (low density) and a major drop in acoustic velocity.

Logging Unit 5: 118–143 mbsf (bottom of the logged section)

Depending on the location of the probe in the tool string, logging Unit 5 has been partly logged by these probes. Available records show high-frequency variations of resistivity and density similar to logging Units 1 and 3. The difference between logging Units 1, 3, and 5 is the progressive increase in the magnitude of variations with depth. Electrical resistivity increases up to 5 Ω·m, and acoustic velocity is >2700 m/s.

Discussion

Comparison between continuous and in situ logs with the whole-core MST records (natural 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. This is a necessary step to correct for the expansion of the cores and obtain correct sedimentation and mass accumulation rate (see “Stratigraphic correlation”). The total gamma radiation counts, porosity, density, and velocity have the potential to be correlated to downhole logging data (Fig. F37).

Microresistivity (FMS; vertical resolution = <1 cm, sampling rate = 0.1 inch [2.5 mm]) images can resolve the thickness and dip of the high-density layers (Fig. F38), which can be related to the sigmoidal reflectors identified in the seismic lines.

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