Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Well name: 758A

Leg: 121

Location: Broken Ridge (tropical SE Indian Ocean)

Latitude: 5° 23.049' N

Longitude: 90° 21.673' E

Logging date: June, 1988

Bottom felt: 2934.5 mbrf (used for depth shift to sea floor)

Total penetration: 676.8 mbsf

Total core recovered: 453.8 m (67 %)

Logging Runs

Logging string 1: DIT/BHC/GR

Logging string 2: ACT/GST/NGT

Logging string 3: BHTV

No information available about use of Wireline Heave Compensator.

Bottom-hole Assembly

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.

DITBHC/GR: Bottom-hole assembly at ~42 mbsf.

ACT/GST/NGT: Bottom-hole assembly at ~42 mbsf.

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from ACT/GST/NGT run, and to the sea floor (- 2934.5 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: Data form the ACT/GST/NGT run have been processed to correct for borehole size and type of drilling fluid. Two processings were performed on the NGT data; the former one, which uses an old processing routine, was performed in 1998, when the original geochemical processing was carried out. The latter one, which uses current processing techniques, was performed in January 1996. The former is presented along with the other geochemical logs, the latter with the conventional logs. They are labelled respectively ngt_old.dat and ngt_new.dat.

Acoustic data processing: The good quality of the acoustic data did not require any processing.

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).

Data recorded through bottom-hole assembly such as the NGT data above 42 mbsf should be used qualitatively only because of the attenuation on the incoming signal.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 121. For further questions about the logs, please contact:

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