Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 859B

Leg: 141

Location: Chile Triple Junction (SE Pacific)

Latitude: 45° 53.72' S

Longitude: 75° 51.33' W

Logging date: December, 1991

Bottom felt: 2760 mbrf

Total penetration: 476.1 mbsf

Total core recovered: 169.67 m (46 %)

Logging Runs

Logging string 1: DIT/SDT/HLDT/NGT

Logging string 2: FMS/GPIT/NGT (2 passes)

Logging string 3: ACT/GST/NGT

Logging string 4: WST

Wireline heave compensator was used to counter ship heave.

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.

DIT/SDT/HLDT/NGT: Bottom-hole assembly at 48.5 mbsf

FMS/GPIT/NGT: Bottom-hole assembly at 48.5 mbsf (pass 1)

FMS/GPIT/NGT: Bottom-hole assembly at 27 mbsf (pass 2)

ACT/GST/NGT: Bottom-hole assembly at 38.5 mbsf.

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from DIT/SDT/HLDT/NGT run and to the sea floor (- 2754 m). This value corresponds to the depth of the mudline observed on the logs and as such differs from the "bottom felt" depth given by the drillers (2760 mbsf). 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 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) 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.

Because of the presence of barite in the mud, the density data must be used cautiously, particularly in the bottom of the hole. The Photoelectric Effect (PEF) curve in particular was badly affected by the presence of barite and it has not been included in the database.

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, the hydraulic caliper on the HLDT tool, and the caliper on the FMS string (C1 and C2). Because in the case of elliptical holes the mechanical caliper provides a measurement which is half-way between short and long axis, and the hydraulic caliper measures the major axis, the caliper recorded by the FMS string (C1 and C2) usually provides a more accurate measurement of the hole diameter.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 141. For further questions about the logs, please contact:

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