Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 857D

Leg: 139

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

Latitude: 48° 26.517' N

Longitude: 128° 42.651' W

Logging date: August, 1991

Bottom felt: 2431.5 mbrf (used for depth shift to sea floor)

Total penetration: 936.2 mbsf

Total core recovered: 37.36 m ( %)

Logging Runs

Logging string 1: DIT/SDT/NGT

Logging string 2: HLDT/CNTG/NGT (2 passes)

Logging string 3: FMS/GPIT/NGT (3 passes; no good data acquired during pass 3)

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

Casing

The following casing 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: Casing at ~ 576 mbsf

HLDT/CNTG/NGT: Casing ~ 576 at mbsf

FMS/GPIT/NGT: Casing at ~576 mbsf.

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from DIT/SDT/NGT run and to the sea floor (- 2431.5 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: NGT data from the DIT/SDT/NGT and HLDT/CNTG/NGT (2 passes) runs have been processed to correct for borehole size and type of drilling fluid. Because of the very high logging speed used during the recording of the FMS runs, the NGT statistics cannot be considered very reliable and the data has not been processed.

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, resulting from the jerky movement of the tool in the hole, no processing has been performed at this stage.

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. 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.

The hydraulic caliper on the HLDT tool did not open, therefore no reliable correction for the borehole could be performed. Because of the slight deviation of the hole the tool was possibly leaning against the borehole wall. The density data from the two passes do not correlate very well and also do not show any correlation with other logs. Based on the Schlumberger's logging report the best density measurement was acquired during pass 2. 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). Because of the rough seas, sticking problems and pad failures, the FMS string data are often suspicious. The caliper measurements cannot be considered reliable, because of damage to one of the arms.

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