Wireline Standard Data Processing
ODP logging contractor: LDEO-BRG
Location: Ceara Rise (tropical NW Atlantic)
Latitude: 5° 27.313' N
Longitude: 43° 44.890' W
Logging date: March, 1994
Bottom felt: 4023.5 mbrf
Total penetration: 531.9 mbsf
Total core recovered: 472.39 m (88.8 %)
Logging String 1: DIT/SDT/HLDT/NGT
Wireline heave compensator was used to counter the mild ship heave. The heave compensator was switched off at ~98 mbsf.
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/HLDT/SDT/NGT: Bottom-hole assembly at ~54 mbsf.
Depth shift: All original logs have been depth shifted to the sea floor (- 4021.1 m). The amount of depth shift differs from the "bottom felt" depth given by the drillers because it incorporates some additional depth shift applied by the logging scientist during correlation of the logs with the Multi-sensor Track (MST) data from core.
Gamma-ray processing: 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') and short-spacing (3-5-5-7') logs. The long spacing sonic 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. The processing yields decent results below 120 mbsf; above this depth the original transit times were of very poor quality and the results are possibly invalid.
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 HLDT pad and the borehole wall (low density correction) the results are improved, because the short-spacing have better vertical resolution.
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 (CNTG, HLDT) and a good contact with the borehole wall.
Data recorded through pipe, such as the spectral gamma ray above 54 mbsf, should be used qualitatively only because of the attenuation on the incoming signal.
Hole diameter was recorded by the hydraulic caliper on the HLDT tool (CALI). The caliper closed at 65 mbsf, therefore any density data recorded between this depth and the bottom of the pipe at 54 mbsf could not be corrected for hole size and is not presented. Also, the density data recorded between 65 and 107 mbsf should be used cautiously, as the reading are badly affected by the irregular borehole.
Details of standard shore-based processing procedures are found in the "Explanatory Notes" chapter, ODP IR Volume 154. For further information about the logs, please contact:
E-mail: Cristina Broglia