Wireline Standard Data Processing
Location: Great Bahama Bank (tropical NW Atlantic)
Latitude: 24° 32.763' N
Longitude: 79° 15.650' W
Logging date: March, 1996
Bottom felt: 494.5 mbrf
Total penetration: 677 mbsf
Total core recovered: none.
Logging string 1: DIT/SDT/GPIT/NGT
Logging string 2: FMS/GPIT/NGT
Logging string 3: APS/HLDS/HNGS
Logging string 4: ACT/GST/NGT
Logging string 5: WST
Wireline heave compensator was used 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/GPIT/NGT: Bottom-hole assembly at ~106.5 mbsf
FMS/GPIT/NGT: Bottom-hole assembly at ~106.5 mbsf
APS/HLDS/HNGS: Bottom-hole assembly at ~106 mbsf
ACT/GST/NGT: Did not reach bottom-hole assembly.
Depth shift: Original logs have been interactively depth shifted with reference to HNGS from APS/HLDS/HNGS run and to the sea floor (- 493.5 m). This amount differs 1.0 m from the "bottom felt" depth given by the drillers and is based on correlation between logs and lithologic markers seen on core. 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 from the NGT and/or HNGS 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 environmental corrections: Corrections for borehole size and type of drilling fluid were performed on the NGT data from the FMS/GPIT/NGT and DIT/SDT/GPIT/NGT tool strings. HNGS data from the APS/HLDS/HNGS tool string were corrected in real-time during the recording.
Acoustic data processing: The array sonic tool was operated in standard depth-derived, borehole compensated, long spacing (8'-10'-10'-12') mode. The sonic logs were of very good quality and did not require any reprocessing.
High-resolution data: Neutron porosity data were recorded at a sampling rate of 0.0508 m with the APS tool. Due to a software problem no high resolution HLDS data could be recorded by the MAXIS acquisition system.
null value=-999.25. This value generally appears in discrete core measurement files and also it 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 gamma ray data above 106 mbsf should be used qualitatively only because of the attenuation on the incoming signal. Invalid gamma ray data were recorded at 100 mbsf during the APS/HLDS/HNGS run and at 99-107 mbsf during the DIT/SDT/GPIT/NGT run.
Hole diameter was recorded by the caliper on the HLDS tool (LCAL) and the caliper on the FMS string (C1 and C2).
For further information about the logs, please contact:
E-mail: Cristina Broglia