Wireline Standard Data Processing
ODP logging contractor: LDEO-BRG
Location: East Tasman Plateau (Tasman Sea)
Latitude: 43° 57.5545' S
Longitude: 149° 55.7169' E
Logging date: 2-3 May 2000
Bottom felt: 2633 mbrf (used for depth shift to sea floor)
Total penetration: 766.5 mbsf
Total core recovered: 237.09 m (79.6 %)
Logging string 1: DIT/APS/HLDS/HNGS
Logging string 2: GHMT/DSI/NGT (main and repeat passes)
Hole 1172D was logged under conditions of high ship heave, which meant that the wireline heave compensator (WHC) stopped repeatedly during the first run, and was not used for most of the second. Despite this, the data are of good quality. Heave was too high to run the FMS tool string, as planned.
The following bottom-hole assembly depths are as they appear on the logs after differential depth shift (see "Depth shift" section) and depth shift to the sea floor. As such, there might be a discrepancy with the original depths given by the drillers onboard. Possible reasons for depth discrepancies are ship heave, malfunction of the wireline heave compensator, and drill string and/or wireline stretch.
DIT/APS/HLDS/HNGS: Bottom-hole assembly at 150 mbsf
GHMT/NGT/DSI: Bottom-hole assembly at 164 mbsf (main pass)
Depth shift: The original logs were depth matched to the NGT from the GHMT/NGT/DSI run and were then shifted to the sea floor (-2633 m). The sea floor depth is determined by the step in gamma ray values at the sediment-water interface. In this case it is the same as the "bottom felt" depth given by the drillers (see above).
Depth matching is typically done in the following way. One log is chosen as reference (base) log (usually the total gamma ray log from the run with the greatest vertical extent), and then the features in the equivalent logs from the other runs are matched to it in turn. This matching is performed automatically, and the result checked and adjusted as necessary. The depth adjustments that were required to bring the match log in line with the base log are then applied to all the other logs from the same tool string.
For Hole 1172D, the SGR log from the GHMT/DSI/NGT main run was used as the reference log for two reasons: first, because the head tension (DF) log seemed less erratic than the DF log from the DIT/APS/HLDS/HNGS run. Second, because when the Wireline Heave Compensator stops, as it did often during the DIT/APS/HLDS/HNGS run, it stops at either end of its track, and has to be reset to the middle of the track to start up again. The resulting depth errors can be on the order of 3 m
Due to the WHC repeatedly stopping during the DIT/APS/HLDS/HNGS run, there are offsets between some logs, for example between the acoustic data and the resistivity logs at 350 mbsf, a depth where the gamma ray logs match well. The Wireline Heave Compensator stopped between 331-350, 355-439, 624-645, and 662-706 mbsf on the DIT/APS/HLDS/HNGS run.
Gamma-ray processing: NGT data have been processed to correct for borehole size and type of drilling fluid. The HNGS data were corrected for hole size during the recording.
Acoustic data processing: The DSI waveform data were processed during logging to give DTCO (compressional wave slowness) and DTSM (shear wave slowness). The DTCO from the main pass looks reasonable and matches well to the 100 m of the repeat pass. The DTSM appears to have some unreasonable values in the 490-493, 450-465, and 560-615 mbsf intervals. No editing was performed, and the compressional and shear slownesses were then converted to velocities.
High-resolution data: Bulk density and neutron porosity data were recorded at a sampling rate of 2.54 and 5.08 cm respectively. The enhanced bulk density curve is the result of Schlumberger enhanced processing technique performed on the MAXIS system onboard. While in normal processing short-spacing data is smoothed to match the long-spacing one, in enhanced processing this is reversed. In a situation where there is good contact between the HLDS pad and the borehole wall (low density correction) the results are improved, because the short-spacing has better vertical resolution.
Geological Magnetic Tool: The Geological Magnetic Tool collected data at two different sampling rates, the standard 0.1524 m rate and 0.0508 m. A handful of anomalous spikes are present in the magnetic field (MAGB) log, but the data appear reliable. Both data sets have been depth shifted to the reference run and to the sea floor.
null value=-999.25. This may replace recorded log values or results, which are considered invalid.
During the processing, quality control of the data is mainly performed by cross-correlation of all logging data. Large (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization (APS, HLDS) and a good contact with the borehole wall. From 168-412 mbsf, Hole 1172D was between 11 and 13 inches wide with a few washouts, from 412-505 mbsf the hole had a widely varying diameter, to 19 inches, and below 505 mbsf, the hole was a little rugose, but mostly between 10 and 13 inches wide.
Data recorded through bottom-hole assembly, such as the gamma ray data, should be used qualitatively only because of the attenuation on the incoming signal.
Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL).
Details of standard shore-based processing procedures are found in the "Explanatory Notes" chapter, ODP IR Volume 189. For further information about the logs, please contact:
E-mail: Trevor Williams
E-mail: Cristina Broglia