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doi:10.2204/iodp.proc.314315316.116.2009

Data and log quality

Hole C0004B

Available data

Hole C0004B was drilled with LWD-MWD-APWD tools. MWD-APWD data was transmitted in real time with a limited set of LWD data (see Table T2 in the “Expedition 314 methods” chapter). Because the only radioactive source on board was cemented with the BHA at the previous site (see “Operations” in the “Expedition 314 Site C0003” chapter), density and porosity measurements could not be made. However, the adnVISION tool was deployed without the radioactive source to obtain ultrasonic caliper data. When LWD tools were recovered on the rig floor, memory data were successfully downloaded and processed according to the data flow described in “Onboard data flow and quality check” and Figure F8 in the “Expedition 314 methods” chapter. A list of available LWD data is given in Table T3 in the “Expedition 314 methods” chapter.

Depth shift

The mudline (seafloor) was identified from the first break in the gamma ray and resistivity logs (Fig. F5). In Hole C0004B, a mudline was picked at 2666 m DRF, showing a discrepancy of 0.5 m with drillers depth (2666.5 m DRF; 2637 m mud depth below sea level [MSL]). The depth-shifted versions of the main drilling data and geophysical logs are given in Figures F6 and F7, respectively. Figure F8 presents the time-depth relationship linking the time (Fig. F4) and the depth version (Figs. F6, F7) of the data in Hole C0004B.

Logging data quality

Figure F6 shows the drilling control logs. After jet-in to 60.5 m LSF, the initial target ROP of 40 m/h was generally achieved to ~100 m LSF where it was then reduced to 30 m/h to TD of 400 m LSF. SWOB was set to a minimal value (<5 kkgf for most of the drilled interval). SPPA was maintained at constant value (~15–18 MPa) for the entire drilling period, and a normal (hydrostatic) increasing trend in APRS and ECD was observed. The four ultrasonic azimuthal calipers (C15, C26, C37, and C48) showed bad borehole condition with washouts exceeding 2 inches (5.08 cm) in most places, except between 60 and 80 m and below 290 m LSF (last caliper reading at 370 m LSF). Because no density and porosity data were available, these washouts are of minor concern regarding the quality of the available logs.

The geophysical logs are shown in Figure F7. Time after bit (TAB) measurements for ultrasonic caliper logs are always <45 min. TAB measurements were ~5–10 min for the natural gamma ray log, except in a depth interval corresponding to pipe connections and the wiper trip (99–234 m LSF; 0330 to 0430 h on 3 November) where they exceeded 2 h. TAB measurements for resistivity were between 5 and 10 min. Comparison between deep button (RES_BD) and shallow button (RES_BS) resistivity values showed a larger separation, suggesting possible drilling fluid invasion.

The sonicVISION data for Hole C0004B were processed by the Schlumberger Data Consulting Specialist onboard the Chikyu. As a result of the processing, two products were delivered. The first product relies on a broad band-pass filter (10–16 kHz) on the data acquired during drilling, referred to as “wide.” The second product relies on a very narrow band-pass filter (6–7 kHz) designed to pass only the “leaky-P” arrival applied to the data acquired during drilling. The composite sonic velocity curve prepared for this hole included data from both processed logs (Table T3). In the upper part of the hole (0–94.5 m LSF), the results of the leaky-P processed data were used. The leaky-P processed data were also used for intervals where the formation arrival could not be distinguished from the mud arrival. The wide data were the most reliable in the rest of the hole; therefore, these data were used to assemble the composite log from 94.5 to 389.5 m LSF (last sonicVISION value).

The quality control analysis of the sonic data is based on examination of the plots showing the sonic waveforms and the slowness coherence images for the common receiver data and the common source data. The full versions of these quality control plots are available as picture description standard format files in the raw data for the expedition. Examples of data of quality Types 0 and 1 are shown in “Data and log quality” section of the “Expedition 314 Site C0001” chapter.

As stated above, the uppermost 60.5 m LSF was jetted-in without tool rotation resulting in the lack of resistivity images for this interval. To ~100 m LSF, the ROP was significantly higher than in previous holes (40 versus 30 m/h). The images initially showed some pixel effect that was removed by postprocessing (3 × 3 pixels up to 9 × 9 pixels smoothing filtering). The smoothing of the image did not blur the geological features. Unlike previous image data, no depth mismatches have been observed at this hole.

Resistivity images are of very good quality. The resistivity image log from Hole C0004A extends from 129 to 1398 m LSF (Table T4). Overall, the quality of the image data is excellent. The log is marked by three 1 m intervals of poor quality where the image is smeared, perhaps resulting from lack of rotation. A short interval of variable resistivity around the hole circumference occurs from 129 to 197 m, probably caused by hole eccentricity. Two features, absolutely horizontal with respect to the borehole, are suspected artifacts. Sharp changes in resistivity along knife-edge planar horizontal surfaces typically bound decimeter or thicker domains. Centimeter-scale horizontal variations in resistivity are also suspected to be artifacts because of their thinness, regularity, and horizontal orientation. Interpretation of resistivity image data is further discussed in “Structural geology and geomechanics” in the “Expedition 314 Site C0001” chapter.

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