Wireline Temperature Data Processing
ODP logging contractor: LDEO-BRG
Location: tropical NW Pacific Ocean
Latitude: 18° 38.52' N
Longitude: 156° 21.582' E
Logging date: December, 1989
Bottom felt: 5685 mbrf (used for depth shift to sea floor)
Total penetration: 511.2 mbsf
Total core recovered: 59.2 m (18 %)
Logging string 1: DIT/SDT/HLDT/NGT
Logging string 2: FMS/GPIT/NGT (2 passes)
Logging string 3: ACT/GST/NGT
Wireline heave compensator was not operational during the recording with DIT/SDT/HLDT/NGT and ACT/GST/NGT ogging strings
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, lack of wireline heave compensator, and drill string and/or wireline stretch.
DIT/SDT/HLDT/NGT: Bottom-hole assembly at ~68 mbsf
FMS/GPIT/NGT: Bottom-hole assembly at ~65 mbsf
ACT/GST/NGT: Bottom-hole assembly at ~65 mbsf.
Depth shift: Original logs have been interactively depth shifted with reference to NGT and caliper from FMS/GPIT/NGT runs and to the sea floor (- 5685 m). The FMS/GPIT/NGT runs present the best data, as the Wireline Heave Compensator was fully operational. The NGT on the ACT/GST/NGT run shows a decent correlation with the reference run and could be used to define the tie points. The DIT run, instead, was badly affected by the lack of WHC during the recording, by frequent pulls during the recording, and by the fact that it was mounted below the HLDT, which carries a radioactive source; as a result, the NGT shows higher counts than the other runs and almost no correlation with the reference run. The CALI channel, instead, has been used for correlation with the caliper from the FMS runs; the results of this depth shifting, however, should be used with caution. 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 the ACT/GST/NGT and FMS/GPIT/NGT runs have been processed to correct for borehole size and type of drilling fluid. The NGT from the FMS runs is the best NGT recorded at this hole; the statistics of the ACT/GST/NGT run was partly affected by the lack of WHC. No processing has been performed on the NGT from the DIT/SDT/HLDT/NGT run as the data is of extremely poor quality.
Acoustic data processing: The array sonic tool was operated in standard depth-derived borehole compensated mode, including long-spacing (8-10-10-12') logs. Because of the low quality of the transit times which were frequently affected by cycle skipping, no standard processing could be performed. The LTT4 channel, however, has been edited through correlation with the resistivity curve and an uncompensated velocity has been calculated.
Geochemical data: Geochemical processing of Hole 801B was performed in 1990-1991. In 1996, revision of both geophysical and geochemical data of Hole 801B revealed that the data had not been properly depth matched. Due a technical problem with the processing program, the new depth shifts could not be applied to the final results. For this reason, though the data is available online, it should be used with caution when compared to the other logs.
null value=-999.25. This value generally appears in discrete core measurement files and also it may replace invalid log values or results (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 (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 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).
Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 129. For further questions about the logs, please contact:
E-mail: Cristina Broglia