Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Well name: 646B

Leg: 105

Location: Eirik Ridge (Labrador Sea)

Latitude: 58° 12.559' N

Longitude: 48° 22.147' W

Logging date: October, 1985

Bottom felt: 3458.7 mbrf (used for depth shift to sea floor)

Total penetration: 766.7 mbsf

Total core recovered: 402.9 m (53 %)

Logging Runs

Logging string 1: DIT/LSS/GR

Logging string 2: GST/CNTG/NGT (upper and lower sections)

Wireline heave compensator was intermittently 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/LSS/GR: Bottom-hole assembly at ~205 mbsf

GST/CNTG/NGT: Bottom-hole assembly at ~205 mbsf (upper section).

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from GST/CNTG/NGT run (upper and lower sections) and to the sea floor (- 3458.7 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 gamma ray curve (GR or SGR) recorded 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: Because of the good quality of the acoustic logs, no processing has been necessary.

Geochemical data: The elemental yields recorded by the GST tool represent the relative contribution of only some of the rock-forming elements (iron, calcium, chlorine, silicon, sulfur, hydrogen, gadolinium, and titanium - the last two ones computed during geochemical processing) to the total spectrum. Because other rock-forming elements are present in the formation (such as aluminum, potassium, etc.), caution is recommended in using the yields to infer lithologic changes. Instead, ratios are more appropriate to determine changes in the macroscopic properties of the formation. The ACT (Aluminum Clay Tool) was introduced during Leg 109, when it became routinely part of the geochemical tool string. For this reason, until Leg 109 no geochemical processing was performed and therefore the elemental yields should be used cautiously to infer lithologic changes.

Quality Control

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. Large (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization (CNTG, LDT) 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. Invalid NGT data were recorded in the 205-305 mbsf interval, due to problems with the minitron source.

Invalid acoustic data were recorded in the 736-745 mbsf interval.

Hole diameter was recorded by the 3-arm mechanical caliper device (MCD).

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 105. For further questions about the logs, please contact:

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia