Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 856-H

Leg: 139

Location: Middle Valley, Juan de Fuca Ridge (NE Pacific Ocean)

Latitude: 48° 26.020' N

Longitude: 128° 40.859' W

Logging date: July, 1991

Bottom felt: 2434.5 mbrf (used for depth shift to sea floor)

Total penetration: 93.8 mbsf

Total core recovered: 18.9 m (20 %)

Logging Runs

Logging string 1: DIT/SDT/NGT

Logging string 2: GST/NGT

Logging string 3: FMS/GPIT/NGT

No wireline heave compensator used to counter ship heave resulting from the rough sea conditions.

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//NGT: Bottom-hole assembly at ~ 20 mbsf

ACT/GST/NGT: Bottom-hole assembly at ~ 20 mbsf

FMS/GPIT/NGT: Bottom-hole assembly at ~ 20 mbsf.

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from GST/NGT run; because none of the original logs shows the bottom of the pipe at 2454.5 mbrf (20 mbsf), the data has been subsequently applied a ~ 6 m shift downward to match the bottom of the pipe at this depth. Finally, they have been shifted to the sea floor (- 2434.5 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.

Acoustic data processing: The array sonic tool was operated in standard depth-derived borehole compensated mode, including long-spacing (8-10-10-12') logs. Because of the extremely noisy character of the sonic logs, no processing has been performed at this stage.

Geochemical processing: Because of malfunction of the ACT component of the geochemical tool string which prevented acquisition of aluminum concentration data, no processing has been performed. 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 usually 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.

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. 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.

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 and by the caliper on the FMS string (C1 and C2).

Additional information about the logs can be found in the ≥Explanatory Notes≤ and Site Chapter, ODP IR volume 139. For further questions about the logs, please contact:

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