Wireline Standard Data Processing
ODP logging contractor: LDEO-BRG
Location: Walvis Ridge (tropical SE Atlantic)
Latitude: 19° 37.1818' S
Longitude: 11° 19.1598' E
Logging date: September, 1997
Bottom felt: 805.5 mbrf
Total penetration: 452.7 m
Total core recovered: 393.5 m (91.4 %)
Logging string 1: DIT/SDT/NGT
Logging string 2: HLDS/APS/HNGS
Logging string 3: FMS/GPIT/NGT (2 passes)
Logging string 4: GHMT/NGT (2 passes)
Wireline heave compensator was used on all runs to counter ship heave.
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/SDT/NGT: Bottom-hole assembly at ~74 mbsf
HLDS/APS/HNGS: Bottom-hole assembly at ~70 mbsf
FMS/GPIT/NGT: Did not reach bottom hole assembly (pass 1)
FMS/GPIT/NGT: Bottom-hole assembly at ~71 mbsf (pass 2)
GHMT/NGT: Bottom-hole assembly at ~71.5 mbsf (main pass)
GHMT/NGT: Did not reach bottom hole assembly (repeat pass).
Depth shift: Original logs have been interactively depth shifted with reference to NGT from DIT/SDT/NGT run and to the sea floor (- 804 m). This amount corresponds to the sea floor depth observed on the logs and differs 1.5 m from the drillers' "bottom felt" depth.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 or HSGR) from the NGT or HNGS tools 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 AND HNGS data have been processed to correct for borehole size and type of drilling fluid.
Acoustic data processing: The array sonic tool was operated in standard depth-derived borehole compensated mode, including long-spacing (8-10-10-12') sonic logs. The logs have been processed to eliminate some of the noise and cycle skipping experienced during the recording. Using two sets of the four transit time measurements and proper depth justification, four independent measurements over a -2ft interval centered on the depth of interest are determined, each based on the difference between a pair of transmitters and receivers. The program discards any transit time that is negative or falls outside a range of meaningful values selected by the processor.
High-resolution data: Neutron porosity data (APS) were recorded at a sampling rate of 5.08 cm (2 in) in addition to the standard sampling rate of 15.24 cm (0.5 ft).
Geological Magnetic Tool: The Geological Magnetic Tool collected data at two different sampling rates, 0.1524 and 0.0508 m. Both data sets have been depth shifted to the reference run and to the sea floor.
null value=-999.25. This value may replace recorded 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. Hole deviation can also affect the data negatively; the FMS, for example, is not designed to be run in holes deviated more than 10 degrees, as the tool weight might cause the caliper to close.
Data recorded through bottom-hole assembly, such as the NGT and HNGS data above 74 mbsf, should be used qualitatively only because of the attenuation on the incoming signal. Invalid gamma ray readings were observed at 44.5-51 and 64-68 mbsf (DIT/SDT/NGT run).
Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 175. For further questions about the logs, please contact:
E-mail: Cristina Broglia