Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Well name: 750A

Leg: 120

Location: central S Indian Ocean

Latitude: 57¡ 35.54' S

Longitude: 81¡ 14.42' E

Logging date: April, 1988

Bottom felt: 2041 mbrf (used for depth shift to sea floor)

Total penetration: 460.5 mbsf

Total core recovered: 70.5 m (15 %)

Logging Runs

Logging string 1: DIT/SDT/AMS/NGT (3 sections and downlog)

Wireline heave compensator failed just after logging started; sea conditions deteriorated as logging was progressing.

Bottom-hole Assembly

The following bottom-hole assembly depth is as it appears on the logs after differential depth shift (see ÒDepth shiftÓ 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.

DIT/SDT/AMS/NGT: Bottom-hole assembly at ~52 mbsf (upper section)

DIT/SDT/AMS/NGT: Bottom-hole assembly at ~77 mbsf (downlog).

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from DIT/SDT/AMS/NGT downlog run and to the sea floor (- 2041 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') and short-spacing (3-5-5-7') logs. Because of the extremely poor quality of the sonic logs, none of the standard processing can be performed and no acoustic data are included in the database. Sonic waveform processing is suggested in order to get meaningful results.

Quality Control

null value=-999.25. This value generally replaces recorded log values or results which are considered invalid.

During the processing, quality control of the data is mainly performed by cross-correlation of all logging data: with the exception of acoustic data, logging data are generally of good quality.

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 3-arm mechanical caliper device (MCD).

Additional information about the logs can be found in the ÒExplanatory NotesÓ and Site Chapter, ODP IR volume 120. For further questions about the logs, please contact:

Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia