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doi:10.2204/iodp.proc.314315316.114.2009

Data and log quality

Hole C0002A

Available data

Hole C0002A was drilled with LWD-MWD-APWD tools. MWD-APWD data were transmitted in real time with a limited set of LWD data (see Table T2 in the “Expedition 314 methods” chapter). 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 (GR) and resistivity logs (Fig. F4). In Hole C0002A, the mudline was picked at 1964.5 m DRF, showing no discrepancy with drillers depth (water depth = 1964.5 m DRF; 1936 meters below sea level [mbsl]). The depth-shifted versions of the main drilling data and geophysical logs are given in Figures F5 and F6, respectively. Figure F7 presents the time-depth relationship linking the time (Fig. F3) and depth version (Figs. F5, F6) of the data from Hole C0002A.

Logging data quality

Figure F5 shows the drilling control logs. The target ROP of 30 m/h (±5 m/h) was generally achieved until TD (1401.5 m LSF). This ROP was sufficient to record one sample per 4 cm over the majority of the hole. WOB was set to a minimal value (<5 kkgf for most of the drilled interval). Surface pump pressure (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 azimuthal calipers (C15, C26, C37, and C48) showed poor borehole condition with washouts exceeding 2 inches (5.08 cm) in most places except between 830 and 930 m LSF, where the hole was almost in gauge (stand-off < 1 inch [2.54 cm]). Stick-slip and friction increased linearly with the length of the penetrated interval but never exceeded the critical limit of 250 rpm that could impair geoVISION resistivity tool (GVR) image quality. In the washed out lower section of the hole (950–1400 m LSF), the GVR experienced numerous shocks with peak intensity sometimes >25 g without any failure.

Time after bit (TAB) measurements were ~5–10 min for the gamma ray log, except in a short depth interval corresponding to pipe connections where they sometimes exceeded 1 h. The gamma ray log is highly correlated to borehole shape, suggesting lithology-controlled washouts of sandy layers. TAB measurements for density and neutron porosity logs are ~45 min. In spite of the major washouts, the image-derived density (four bin average maximum azimuthal density [IDRO]) and image stand-off correction (IDDR) provide valuable logs. TAB measurements for resistivity were between 5 and 10 min. Where hole conditions were good, comparison between deep button (RES_BD) and shallow button (RES_BS) resistivity values showed that drilling fluid invasion was not significant, consistent with the short TAB readings. Elsewhere, the discrepancy between deep and shallow readings reflects possible invasion in permeable (sandy) layers or poor hole conditions (washouts, also associated with sandy layers).

The sonicVISION data for Hole C0002A were processed by the Schlumberger Data Consulting Services (DCS) specialist on board the Chikyu. Three products were produced. The first analysis relies on a broad band-pass filter on the data acquired during drilling, referred to here as “wide.” The second relies on a very narrow band-pass filter, designed to pass only the leaky P-wave arrival on the data acquired during drilling, referred to as “leaky-P.” The third relies on the wide filtering on the data acquired at high speed while pulling out of the hole, referred to as “widefast.” As a result, the composite sonic velocity curve for this site includes data from all three processed logs (Table T3). In the upper half of the hole (0–634 m LSF), the results of all three processed logs were used. The leaky-P data were only used for intervals where neither of the other logs allowed reliable picks. The wide data were the most reliable in the bottom half of the hole, so these data were used to assemble the composite log from 634 to 1293 m LSF.

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 (Fig. F8). The full versions of these three plots are available as picture description standard (Schlumberger) format files in the raw data for the expedition (available at sio7.jamstec.go.jp). Figure F8 illustrates a typical example of an interval in which few if any picks are possible (Quality Type 3 in Table T3). Examples of Types 0–2, are shown in “Data and log quality” in the “Expedition 314 Site C0001” chapter.

Density, gamma ray, and resistivity images are of very good quality. However, depth shifts of several tens of centimeters are observed in these images. These depth shifts are neither systematic nor constant with depth. They even occur within the images from the same tool (e.g., geoVISION deep, medium, and shallow resistivity images). All images and scalar logs were processed with the same time-depth conversion files; however, the origin of these local depth shifts is not clear and can not be realistically attributed to preprocessing of the data. At first, these depth shifts seemed localized in zones of high contrast (resistivity, density, and gamma ray values) suggesting a possible combination of (1) tool deviation in respect to layering and (2) shoulder effect. Selected examples were sent to a Schlumberger DCS specialist on shore for further investigation. He reported similar observations on past drillship expeditions, suggesting that incomplete heave compensation and rapid vertical movements between the sampling of all data (sampling rate = 5 s) are the origins of the depth shifts. Most of the analyses conducted on board were done on the shallow resistivity image. Density (density correction and photoelectric factor [PEF]) and gamma ray images have not been used for log characterization, physical properties, structural analyses, or log-seismic integration.

The resistivity image log from Hole C0002A extends from 129 to 1398 m LSF (Table T4). Overall, the quality of the image data is excellent (except isolated spurious depth shifts discussed above). The log is marked by three 1 m intervals of poor quality where the image is smeared, perhaps because of lack of rotation. A short section of variable resistivity around the hole circumference occurs from 129 to 197 m LSF, probably because of hole eccentricity. Sharp changes in resistivity along knife-edge planar horizontal surfaces, typically bounding decimeter or thicker domains, are suspected to be drilling artifacts. Centimeter-scale horizontal variations in resistivity are also suspected to be artifacts because of their thinness, regularity, and horizontal orientation. Overall, the general structural patterns and sedimentary features are apparent. Interpretation of resistivity image data is further discussed in “Structural geology and geomechanics” in the “Expedition 314 Site C0001” chapter.

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