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doi:10.2204/iodp.proc.314315316.118.2009

Data and log quality

Hole C0006A

Available data

Hole C0006A was drilled with MWD-APWD tools. All data were sent to the surface through the drilling fluid telemetry system (see Fig. F3 in the “Expedition 314 methods” chapter). At the end of the drilling operation, time and depth information were merged and data was processed following the data flow presented in Figure F3 in the “Expedition 314 methods” chapter. Retrieved data include

1. Surface drilling control parameters: ROP, hook load (HKLD), surface weight on bit (SWOB), SPPA;
2. Downhole drilling parameters: drill bit (collar) rotation (CRPM_RT), PowerPulse turbine rotation speed (TRPM_RT);
3. Annular pressure data: average annular pressure (APRS_MWD) and temperature (ATMP_MWD) and equivalent circulating density (ECD_MWD); and
4. Gamma ray log (GR_RAB) for further depth correlation over interval 0–879 m LSF (3903.5–4782.5 m DRF).

Depth shift

For this hole, the mudline (seafloor) was identified from the first break in the gamma ray log (GRM1) found at 3903.5 m DRF (Fig. F5) while tagging the seafloor without ROV monitoring. GRM1 is particularly noisy at the seafloor interface because the fast ROP (jet-in) in the unconsolidated formation is incompatible with a reliable statistical count of the radioactive elements of the formation and possible flow of mud around the bit. The depth-shifted version of the surface and downhole drilling data and downhole ECD, APRS, and GRM1 is given in Figure F6. To help connect the time and depth version of the data, the time-depth relationship for Hole C0006A is given in Figure F7.

Logging data quality

Except for the MWD tool GRM1 log, which is directly related to the formation properties (lithology), all other logs are direct surface drilling and downhole measurements. APRS, ATMP, and ECD derived from APRS show an expected increase with depth. As GRM1 has a high depth of investigation, it is considered reliable despite the lack of hole shape (caliper) data. No repeat data were available in this hole; however, this GRM1 log is well correlated with the gamma ray log (GR_RAB) of the geoVISION resistivity tool (GVR) from Hole C0006B. Minor depth discrepancies can be attributed to lateral variations/​heterogeneities between these two holes (Fig. F8).

Hole C0006B

Available data

Hole C0006B was drilled with LWD-MWD-APWD tools. As in Hole C0004A, the adnVISION tool was deployed to obtain ultrasonic caliper data. Despite the loss of real-time communication with the LWD tools (1500 h on 13 November 2007; 345.2 m LSF), drilling operations were conducted to TD (885.5 m LSF) and memory data were successfully downloaded.

Depth shift

For Hole C0006B, the mudline (seafloor) was identified from the first break in the gamma ray log (GR) and resistivity logs (RES_RING, RES_BIT, RES_BD, RES_BM, RES_BS) at 3899.5 m DRF, showing a discrepancy with drillers depth by 0.5 m (3900.0 m DRF) (Fig. F9). Uncertainty in picking the mudline is clearly within ±1 m because of the washing out of the top few meters of the unconsolidated formation by drilling fluid and the resultant mixing (formation suspension) at the mudline interface, blurring gamma ray and resistivity readings.

For Hole C0006B, the depth-shifted version of the main drilling data and geophysical logs are given in Figures F8 and F10, respectively. Figure F11 presents the time-depth relationship linking the time (Fig. F2) and depth (Figs. F8, F10) version of the data in Hole C0006B.

Logging data quality

Figures F8 and F10 show the quality control logs for Hole C0006B LWD data. The target ROP of 30 m/h (±5 m/h) was generally achieved to TD (“Hole C0006B” in “Operations”). This ROP was sufficient to record 1 sample every 4 cm over the majority of the hole. SPPA increased with depth from 10 to 18 MPa, and no noticeable change in APRS and ECD was observed until the loss of the real-time communication with the LWD-APWD tools. Hole deviation quickly reached 5° (250 m LSF), but memory data show that hole deviation stabilized with depth, remaining close to 5° at 885.5 m LSF.

Hole conditions are highly variable with depth. Sonic caliper values from the adnVISION tool that should be 8.5 inches (21.6 cm) for a perfect in-gauge hole instead show values >10 inches (25.4 cm) for the upper depth interval (0–200 m LSF), the lower depth interval (710–855 m LSF [last caliper reading]), and a few localized (approximately meter scale) washouts. All these depth intervals are characterized by low gamma ray counts suggesting caving in sand-rich layers.

Comparison between deep button (RES_BD) and shallow button (RES_BS) resistivity values shows that drilling fluid invasion is concomitant with low gamma ray depth intervals in spite of the short time after bit measurements. Combined with hole conditions and caliper information, these layers can possibly be interpreted as permeable sand-rich layers.

Because of the limited time available before the end of the cruise, sonicVISION data for Hole C0006B were processed postcruise by the Schlumberger Data Consulting Specialist. The depth interval of usable processed data was limited by the failure of real-time communication and powering of the sonic tool (1500 h on 13 November 2007; 345.2 m LSF) and the possible damage of the transmitter. At the time real-time communication failed, the sonic tool switched from turbine mode to battery mode. Because of the low downhole temperature (~3°C), the batteries did not provide enough voltage to the transmitter, therefore limiting the available energy to excite the formation. Possible damage of the transmitter resulting from improper stabilization of the tool and/or severe drilling conditions (stick-slip or shocks) may have also impaired data quality. As a result, only the uppermost 160 m has been processed by combining the results of wide and leaky-P processing, attempting to select wide results when available. Quality control analysis of sonic data is based on examination of plots showing sonic waveforms and slowness coherence images for common receiver data and common source data. From 36 to 160 m LSF, sonic data quality is moderate; discontinuous transit times have been picked using mostly wide processed data. Above 36 m LSF, formation arrival can not be distinguished from mud arrival (Table T4).

Overall quality of the resistivity images used in structural interpretation is very good (Fig. F10; Table T5). The following descriptions of logging units include areas of apparent artifacts probably reflecting hole or tool conditions and not real geology. This assessment was based on the shallow level of investigation of the GVR tool, displayed as a static, not a dynamically renormalized, image.

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