Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Well name: 641C

Leg: 103

Location: Galicia Bank (NE Atlantic)

Latitude: 42° 9.3' N

Longitude: 12° 10.9' W

Logging date: June, 1985

Bottom felt: 4650 mbrf (used for depth shift to sea floor)

Total penetration: 305.2 mbsf

Total core recovered: 112.5 m (72.9 %)

Logging Runs

Logging string 1: LDT/CNTG/NGT (upper and lower sections).

First logging attempt with acoustic-gamma ray-caliper tool string failed due to a combination of electronic failure and presence of impassable bridges in the hole.

No wireline heave compensator was used to counter ship heave resulting from the mild sea conditions (1-3 ft).

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.

LDT/CNTG/NGT: Bottom-hole assembly at ~128.5 mbsf (upper section).

LDT/CNTG/NGT: Bottom-hole assembly at ~160 mbsf (lower section).

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from LDT/CNTG/NGT upper part and to the sea floor (- 4650 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.

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 (CNTG, HLDT) and a good contact with the borehole wall.

Data recorded through bottom-hole assembly should be used qualitatively only because of the attenuation on the incoming signal.

No hole diameter was measured.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 103. For further questions about the logs, please contact:

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