Wireline Standard Data Processing
ODP logging contractor: LDEO-BRG
Location: West Tasmania Slope (Tasman Sea)
Latitude: 42° 36.58' S
Longitude: 144° 24.76' E
Logging date: March, 2000
Bottom felt: 2474 mbrf
Total penetration: 883.5 mbsf
Total core recovered: 837.1 m (94.7%)
Logging string 1: DIT/APS/HLDS/HNGS
Logging string 2: GHMT/DSI/NGT (main and repeat)
This hole contained a number of bridges that made getting the tool strings to the bottom of the hole difficult, and also caused irregular tool speed while logging. The DIT/APS/HLDS/HNGS tool string reached total depth whereas the GHMT/DSI/NGT stopped 153 m above the bottom of the hole, even after a pipe trip made to clear the bridges. A planned FMS run was not run due to the poor borehole conditions and time constraints.
The wireline heave compensator was used to counter ship heave (typically 3 m).
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 and drill string and/or wireline stretch.
DIT/APS/HLDS/HNGS: Bottom-hole assembly at ~99 mbsf
GHMT/DSI/NGT: Bottom-hole assembly at ~99 mbsf (main pass)
Depth shift: Since the depth match between the total gamma of the two logging runs is somewhat ambiguous, the conductivity from the GHMT (MAGC) was matched to the DIT from the DIT/APS/HLDS/HNGS main pass. This led to an improved depth match. All the logs were then shifted to the sea floor (-2475 m). The sea floor depth is determined by the step in gamma ray values at the sediment-water interface. It differs 1 m from 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.
The depth match between the main passes of the two tool strings is fairly robust for most of the hole, but a lack of correlateable features in the intervals 305-350 and 425-555 mbsf makes the matches there more questionable.
Depth matching was also made difficult by the variation in tool speed caused by the tool strings dragging on the bridges while logging up the hole. While the cable speed (CS) at the surface remains fairly constant, the actual tool speed is slowed, as indicated by increased tension at the head of the tool (DF, down hole force). For example, density and porosity measurements are offset from each other in intervals of high DF, for example from 410-440 and 720-755 mbsf.
Gamma-ray processing: The gamma ray logs from the NGT have been re-processed to account for borehole size and drilling fluid. The HNGS data were corrected for hole size during the recording.
Acoustic data processing: The DSI waveform data were re-processed onboard to give DTCO (compressional wave slowness) and DTSM (shear wave slowness). The DTCO from the main pass looks reasonable and matches fairly well the 100 m of the repeat pass. The DTSM appears to have unreliable values from 268-273 mbsf. From 300-538 mbsf there is high amplitude DTSM variability that has no corresponding variability in the DTCO, thus the shear velocity data in this interval should be regarded with caution. 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 value may replace invalid log values or results.
Large (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization (APS, HLDS) and a good contact with the borehole wall. In general, Hole 1168A was very rough, with caliper ranges from 4 to 19 inches. Above 150 m, the hole is washed out beyond the maximum extent of the cailper arm (19 inches). The caliper was closed from 420-431 mbsf and 719-741 mbsf to help prevent tool sticking in bridged intervals. APS and HLDS data quality is poor in the rough and/or wide intervals.
The SFLU resistivity log contains anomalous spikes from 478-512 mbsf and 587-594 mbsf.
The MAGB total magnetic field curve contains isolated anomalous spikes, but in general the MAGB log is good. There is about a 10 nT offset in MAGB between the main and the repeat passes. There is also an offset between the MAGS and RMGS magnetic susceptibility logs, of unknown cause. There is an anomalous spike in the MAGS main pass at 306-309 mbsf.
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.
Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 189. For further questions about the logs, please contact:
E-mail: Cristina Broglia