Wireline Standard Data Processing
ODP logging contractor: LDEO-BRG
Location: Great Australian Bight (SW Pacific Ocean)
Latitude: 33° 18.9624' S
Longitude: 127° 36.1236' E
Logging date: November, 1998
Bottom felt: 229 mbrf (used for depth shift to sea floor)
Total penetration: 833.4 mbsf
Total core recovered: 22.4 m (6.12 %)
Logging string 1: DIT/APS/HLDS/HNGS (upper
and lower sections)
Logging string 2: FMS/GPIT/SDT/NGT (2 passes; no FMS on repeat pass)
Wireline heave compensator was used to counter ship heave during the DIT/APS/HLDS/HNGS uplog and the main pass of the FMS/GPIT/SDT/NGT. Seas were calm during the entire logging operations.
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, use of wireline heave compensator, and drill string and/or wireline stretch.
DIT/APS/HLDS/HNGS: Bottom-hole assembly at ~105 mbsf
DIT/APS/HLDS/HNGS: Drill pipe at ~25 mbsf
FMS/GPIT/SDT/NGT: Bottom-hole assembly at ~105 mbsf.
Depth shift: Original logs have been interactively depth shifted
with reference to NGT from DIT/APS/HLDS/HNGS run and to the sea floor (-229 m).
The program used is an interactive, graphical depth-match program, which allows
to visually correlate logs and to define appropriate shifts. The reference and
match channels are displayed on the screen, with vectors connecting old
(reference curve) and new (match curve) shift depths. The total gamma ray curve
(SGR) from the NGT tool run on each logging string is used to correlate the
logging runs most often. In general, the reference curve is chosen on the basis
of constant, low cable tension and high cable speed (tools run at faster speeds
are less likely to stick and are less susceptible to data degradation caused by
ship heave). Other factors, however, such as the length of the logged interval,
the presence of drill pipe, and the statistical quality of the collected data
(better statistics is obtained at lower logging speeds) are also considered in
the selection. A list of the amount of differential depth shifts applied at
this hole is available upon request.
Gamma-ray processing: NGT data from FMS/GPIT/SDT/NGT runs have not been environmentally corrected due to the lack of a reliable hole diameter measurement (see below). The HNGS data from DIT/HLDS/APS/HNGS was corrected during the recording.
Acoustic data processing: The array sonic tool was operated in two modes: linear array mode, with the 8-receivers providing full waveform analysis (compressional and shear) and standard depth-derived borehole compensated mode, including long-spacing (8-10-10-12') and short-spacing (3-5-5-7') logs. The data is of very good quality and does not need any processing; compressional velocity has been computed from both modes.
High-resolution data: Neutron porosity data were recorded at a sampling rate of 5.08 cm.
null value=-999.25. This value generally replaces log values or results, which are considered, invalid (ex. processed sonic data).
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. Prior to logging, the hole was conditioned with barite, which has a high density and photoelectric effect; usually, when barite is used, this is clearly observed on the density log, which shows an increasing trend toward the bottom of the hole, where the heavy barite tends to accumulate. This is not observed on either the density or photoelectric log in this hole. An increase of the photoelectric values, coupled with slightly negative density correction would indicate presence of barite in the middle part of the hole (~350-430 mbsf); this seems to be confirmed by the much lower caliper readings, which possibly indicate the presence of barite mud cake.
Data recorded through bottom-hole assembly, such as the gamma ray data above 105 mbsf, 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) and
on the FMS string (C1 and C2), main pass. The former shows a rather smooth
hole, ranging in diameter from about 11 to 16 inches; this does not seem very
consistent with the indication of swelling clays in the upper part of the hole
and the few obstructions encountered during the recording. Based on the fact
that it took several attempts to close the caliper in order to fit it through
the 4-in drill pipe, it is very likely that the caliper was encrusted with the
heavy mud (barite) used to condition the hole and did not function properly. On
the other hand, the caliper measurements recorded by the FMS tool string in the
upper part show a very irregular hole, with some questionable low readings,
possibly indicating a collapsing hole. In conclusion, neither reading seems to
give a completely reliable indication of the shape and diameter of the hole.
Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR Volume 182. For further questions about the logs, please contact:
E-mail: Cristina Broglia