Wireline Standard Data Processing
ODP logging contractor: LDEO-BRG
Well name: 761C
Location: Exmouth Plateau (SE Indian Ocean)
Latitude: 16° 44.23' S
Longitude: 115° 32.10' E
Logging date: July, 1988
Bottom felt: 2179.9 mbrf (used for depth shift to sea floor)
Total penetration: 436.7 mbsf
Total core recovered: 72.2 m (16 %)
Logging string 1: DIT/SDT/NGT (GPIT removed after computer crash onboard)
Logging string 2: ACT/GST/NGT
There are no information in the logging report from this hole about use of wireline heave compensator to counter ship heave.
The following bottom-hole assembly/pipe 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/NGT: Bottom-hole assembly at ~ 125 mbsf
ACT/GST/NGT: Recorded through bottom-hole assembly (290 mbsf to t.d. and drill pipe.
Depth shift: Original logs have been interactively depth shifted with reference to NGT from ACT/GST/NGT run and to the sea floor (- 2179.9 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 from the DIT/SDT/NGT run have been processed to correct for borehole size and type of drilling fluid. During conventional log processing, no correction has been performed on the NGT data (from geochemical run) recorded trough pipe; in fact the formation was irradiated during the geochemical tool calibration from ~ 290 to 345 mbsf. During the geochemical processing performed in 1990, however, some correction fro the presence of drill pipe and bottom hole assembly was attempted, with dubious results; the gamma ray data through the BHA (290 mbsf to t.d.) in particular are of very poor quality and they have not been included in the database. The rest of the data should be used qualitatively.
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 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.
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. No SFLU is included in the database because the data was consistently too low during the recording.
Data recorded through bottom-hole assembly and pipe should be used qualitatively only because of the attenuation on the incoming signal. No gamma ray data below 290 mbsf is included in the database (see above).
Hole diameter was recorded by the 3-arm mechanical caliper on the DIT tool string.
Further information about logging in Hole 761C can be found in the Site Chapter, ODP IR Volume 122. For further information please contact:
E-mail: Cristina Broglia