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doi:10.2204/iodp.proc.319.103.2010

Logging and data quality

Logging results

Logging data collected during MWD (gamma ray logs) and wireline logging Runs 1 and 2 are presented in Figures F6 and F7 (see also C0009_F1.AI in LOGGING in "Supplementary material"). Detailed discussion and interpretation of individual logs are incorporated into the subsequent disciplinary sections (i.e., lithology, structural geology, and physical properties). MDT data collected during wireline logging Run 3 and VSP data (zero-offset, circular, and walkaway) from Run 5 are presented in "Downhole measurements." Logging Run 4 consisted of a junk basket run before the VSP, which was conducted inside the casing (Fig. F8).

Depth correction of wireline log data

In order to correlate observations from wireline logs with core and cuttings, depth-shifting of the data to a common reference datum was required (see "Introduction" in the "Methods" chapter for general information on depth reference terminology). We chose to register the depths for the logs to the bottom of the 20 inch casing (703.9 m DSF or 2786 m DRF), because it is clearly identifiable in all three wireline logging runs (Fig. F9). The 20 inch casing, bottom of the 26 inch hole, and bottom of the 17 inch hole are all clearly detected by caliper and resistivity data in wireline logging Run 1 and by caliper data in Run 2 (Figs. F9, F10). The natural gamma ray (NGR) data collected by wireline logging Runs 1 and 2 show a clear increase corresponding to the bottoms of the 26 and 17 inch holes, although this response may also be the result of hole shape. The NGR data collected during VSP operations also show an increase corresponding to the bottom of the 17 inch hole.

Depth references for each logging run are listed in Table T6 and shown graphically in Figure F11. The height of the rig floor (rotary table) is 28.3 m above sea level (presuming minimal variation in this parameter during drilling operations), and water depth is reported as 2054.0 m. Wireline log depths are all tied to the drillers depth at the 20 inch casing shoe, located at 2786.2 m DRF (703.9 m DSF, 2785.0 m wireline depth below rig floor [WRF]). Therefore 2081.1 m (2785.0–703.9) is subtracted from the logging depth (WRF) to give the depth below seafloor of the wireline logging data, corrected relative to the drillers depth. This value is given in meters WMSF (referring to the matching of the individual wireline logging runs to each other). After registering the depths in this way, WMSF depth as reported here is equivalent in depth to DSF.

FMI images have been corrected for tool acceleration during the logging run (see "Logging" in the "Methods" chapter) as well as relative to the other wireline logs. Depths of these corrected data are also reported in meters WMSF (relative to the seafloor).

Operations

MWD measurements were taken during three different drilling phases, and five wireline logging runs were conducted in Hole C0009A (Fig. F12). MWD measurements were taken during drilling, the first three wireline logging runs were conducted after riser drilling and coring, and the last two runs were conducted after the hole was cased and cemented (Fig. F8).

On completion of drilling with a 12¼ inch bit to 3686 m DRF and a wiper trip, the first wireline logging tools were rigged at 2230 h on 7 July 2009. The passive heave compensator (see Fig. F4B in the "Methods" chapter) was set up at 0100 h on 8 July. This was the first time the passive heave compensator was used instead of the active heave compensator, which is typically used for riserless logging operations. Setup and use of this system throughout wireline logging operations was smooth.

Logging Run 1 began at 0145 h with the EMS-HRLA-PEX-gamma ray tool string (23.6 m long) (Fig. F12). Density, porosity, gamma radiation, caliper, laterolog resistivities, and mud properties were measured during this run. The tool string could not pass beyond 1585.9 m WMSF after several attempts, and hence the hole was logged from that depth to the 20 inch casing shoe (703.9 m WMSF), and the lowermost 8 m of the cored zone at the bottom of hole was not logged. A repeat section of 92 m was logged from the bottom to 1493.9 m WMSF. The main log covers an interval between 703.9 and 1585.9 m WMSF.

Logging Run 2 consisted of FMI, Sonic Scanner, and HNGC (spectral gamma) (31.77 m long tool string). The tool string could not be lowered below 1581.1 m WMSF. Run 2 was logged from that depth to the 20 inch casing shoe and included a repeat section for the interval from 763.9 to 849.5 m WMSF.

Logging Run 3 consisted of a 23.44 m long tool string with the MDT to conduct measurements of stress magnitude, pore pressure, and permeability. Data collected from the first two logging runs were used to select the MDT test locations, and gamma ray was included on the MDT tool to refine targeting of the measurement intervals. Because this tool was being deployed for the first time in scientific ocean drilling, there were multiple discussions among the science party, Center for Deep Earth Exploration (CDEX) science support and operations, the Schlumberger engineer, and specialists to develop a final deployment plan.

Logging Run 4 consisted of a junk basket run with an 8¼ inch gauge ring run inside the casing. Logging Run 5 consisted of the walkaway, circular, and zero-offset VSP experiments and used 16 VSI shuttles.

Log data quality

Wireline log data quality is assessed primarily by cross-correlating available logs and hole shape. Density, neutron porosity, and resistivity images are affected the most by borehole conditions. Resistivity and gamma ray data may be degraded where borehole diameter greatly increases or is washed out. Deep investigation measurements such as resistivity and sonic velocity are the least sensitive to borehole conditions. Environmental corrections are applied to the original data immediately after data acquisition at the well site by the field engineer. If necessary, additional correction and data processing are conducted at the well site and/or consulting centers by specialists to reduce these effects. Data quality indicators are divided into three log types: sonic velocity, resistivity images, and other logs (except neutron porosity); they are shown together with composite logs and the lithologic units (Fig. F7).

Available data

Hole C0009A was drilled with MWD APWD for the 26 inch hole, MWD Power-V for the 12¼ inch hole, and a conventional drilling assembly for hole opening to 17 inches. Three separate wireline logging runs were conducted after opening the cored interval to 12¼ inch diameter. Wireline data were processed and depth-shifted on board the Chikyu by the Logging Staff Scientist and logging specialists (see details in the "Methods" chapter). Lists of available data from both logging while drilling (LWD)/MWD and wireline logging are provided in Tables T2 and T4 in the "Methods" chapter.

Repeatability

To ensure data quality, repeat runs were conducted over 92 m for logging Run 1 and 85.6 m for logging Run 2, and comparisons were made between several logs to confirm reproducibility during and between the runs. The comparison between the main and repeat sections of Runs 1 and 2 indicates good correlation between the two except for the neutron porosity. The large scatter and large absolute values of neutron porosity raised some doubt about the data quality. Neutron porosity is affected by standoff and is generally not reliable in zones of washout, breakouts, and significant borehole rugosity. By restricting the data to a zone of small standoff for High-Resolution Density Device (HRDD) (standard resolution density standoff <0.02 inch), we may recover a better correlation between the main log and the repeated section (see "Physical properties").

Data quality

Three different calipers were run for hole diameter measurement. Among them, the six arm caliper from EMS provides the best indication of hole shape. Because of hole shape effects, resistivity data below 1283 m WMSF are of poor/medium quality. Resistivity images from FMI are also degraded by borehole shape and fracturing at depths below 1283 m WMSF. Because of their high sensitivity to hole shape, density and porosity were also affected significantly below 3365 m DRF (1282.7 m WMSF). Anomalous density values were recorded at several depths, and porosity data exhibit wide variation throughout the measured range. Gamma ray data exhibit a steady trend and do not exhibit any indication of poor data quality. In Figure F7, the caliper logs indicate that the borehole is in-gauge from the top to 1284.9 m WMSF; however, the borehole below 1284.9 m WMSF is highly rugose and possibly washed out.

Sonic velocity data processing and quality

Immediately after data acquisition from the Sonic Scanner, the data were sent to Schlumberger's Tokyo Data and Consulting Service for further processing available only with in-house software. The processing procedure included extracting compressional, shear, and Stoneley velocities using BestDT, a sonic processing module on Schlumberger's GeoFrame software. The waveforms are of very high quality, except for a few intervals with very large borehole size. The monopole waveforms consist of very good formation arrivals, and the compressional velocity results are also clear. The dipole waveforms contain decent formation arrivals. After detailed analysis on dispersion plots, the best processing results were obtained using multishot shear parametric inversion (MSPI) processing rather than slowness time coherence. MSPI processing is based on dispersion analysis by a curve-fitting technique to compute the correct shear velocity. In some intervals, the shear slowness processed from cross-dipoles indicates the presence of anisotropy (intrinsic or stress induced) as well as borehole alteration. Stoneley wave results are more sensitive to borehole conditions, and hence more affected by washout than other velocity data, especially for the interval below 1284.9 m WMSF.