Wireline Standard Data Processing
ODP logging contractor: LDEO-BRG
Location: Amazon Fan (equatorial NW Atlantic)
Latitude: 5° 5.796' N
Longitude: 46° 48.738' W
Logging date: April, 1994
Bottom felt: 3376.8 mbrf
Total penetration: 254.2 mbsf
Total core recovered: 177.38 m (69.8 %)
Logging string 1: DIT/SDT/HLDT/CNTG/NGT
Logging string 2: FMS/GPIT/NGT (pass 1, pass 2 top and bottom)
Logging string 3: GHMT/NGT (2 passes)
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/HLDT/CNTG/NGT: bottom hole assembly at ~67 mbsf
FMS/GPIT/NGT: all passes recorded open hole
GHMT/NGT: both passes recorded open hole
Depth shift: The NGT (SGR curve) from the FMS passes correlated very poorly with the SGR from the reference run. Therefore, first FMS pass 1 was correlated to FMS pass 2 top using the caliper, then both depth shifted FMS pass 1 and FMS pass 2 top were depth shifted with reference to DIT/SDT/HLDT/CNTG/NGT, using SGR for correlation. Finally, FMS pass 2 bottom was correlated to the depth shifted FMS pass 1 using SGR and the DIT/SDT/HLDT/CNTG/NGT repeat run was correlated to DIT/SDT/HLDT/CNTG/NGT main pass using resistivity. The second pass of the GHMT logs has been first depth shifted to match the first pass, using the different susceptibility curves for correlation. Both passes have then been depth shifted with reference to the main DIT/SDT/HLDT/CNTG/NGT pass, using the NGT for correlation. The match, however, is generally poor, due to the different NGT signature. All logs have then been depth shifted to the sea floor (- 3376 m). This amount corresponds to the water depth as observed on the DIT/SDT/HLDT/CNTG/ NGT logs and differs 0.6 m from the "bottom-felt" depth given by the drillers.
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: Data have been processed to correct for borehole size and type of drilling fluid.
Acoustic data processing: The 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.
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.
Geological Magnetic Tool: The Geological Magnetic Tool collected data at two different sampling rates, the standard 0.1524 m rate 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 generally appears in discrete core measurement files and also it may replace invalid recorded log values or results.
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. 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 CNTG and NGT data above 67 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) and on the FMS string (C1 and C2). The HLDT caliper, however, did not work properly during the main pass in the lower 145 m of the hole. For this reason, both caliper and density data have been spliced with the data from the repeat run, during which the caliper worked properly, allowing for an appropriate correction of the density data. The high density data have also been spliced.
Details of standard shore-based processing procedures are found in the "Explanatory Notes" chapter, ODP IR Volume 155. For further information about the logs, please contact:
E-mail: Cristina Broglia