Wireline Standard Data Processing
ODP logging
contractor: LDEO-BRG
Hole: 841C
Leg: 135
Location: Tonga Trench (tropical SW Pacific)
Latitude: 23° 20.72' S
Longitude: 175° 17.879' W
Logging date: February, 1991
Bottom felt: 4821 mbrf (used for depth shift to sea floor)
Total penetration: 775 mbsf
Total core recovered: no coring performed. Hole dedicated solely to logging.
Logging
Runs
Logging string 1: FMS/GPIT/NGT (2 passes)
Logging string 2: DIT/SDT/HLDT/CNTG/NGT
No information available from logging reports about use of wireline heave compensate counter ship heave.
Bottom-hole
Assembly
The following bottom-hole assembly/pipe are as they appear on the logs after differential depth shift (see ³Depth shift² 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 and drill string and/or wireline stretch.
FMS/GPIT/NGT: Recorded open-hole
DIT/SDT/HLDT/CNTG/NGT: Recorded open-hole.
Processing
Depth shift: Original logs have been interactively depth shifted with reference to NGT from FMS/GPIT/NGT pass 1 and to the sea floor (- 4821 m). 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 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') logs. 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.
Quality
Control
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 (HLDT, CNTG) 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.
Hole diameter was recorded by the hydraulic caliper on the HLDT tool (CALI) and on the FMS tool string (C1 and C2) and by the 3-arm mechanical caliper device (MCD) on the DIT/SDT/HLDT/CNTG/NGT tool string.
Additional information about the logs can be found in the Explanatory Note and Site Chapter, ODP IR volume 135. For further questions about the logs, please contact:
Cristina
Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia