Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 909-C

Leg: 151

Location: Yermac Plateau (Arctic Ocean)

Latitude: 80° 15.894' N

Longitude: 6° 35.430' W

Logging date: September, 1993

Bottom felt: 2529 mbrf

Total penetration: 1061.8 mbsf

Total core recovered: 604.77 m (61%)

Logging Runs

Logging String 1: DIT/SDT/HLDT/NGT (upper and lower sections; HLDT failed to operate in upper section)

Logging String 2: FMS/GPIT/NGT (lower section, main and repeat). No FMS/GPIT/NGT run in the upper section due to time limitations.

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/HLDT/SDT/NGT (lower section): Bottom-hole assembly at ~589 mbsf.

FMS/GPIT/NGT (lower section): Bottom-hole assembly at ~590 mbsf.

DIT/HLDT/SDT/NGT (upper section): Bottom-hole assembly at ~89 mbsf.

Processing

Depth shift: The lower DIT/HLDT/SDT/NGT run has been depth shifted with reference to the lower FMS/GPIT/NGT run and then both lower DIT/HLDT/SDT/NGT and FMS/GPIT/NGT runs have been depth shifted with reference to upper DIT/SDT/HLDT/NGT. Lower FMS/GPIT/NGT has been applied a constant depth shift of - .53 m. All logs have been shifted to the sea floor (- 2526 m). This amount differs from the bottom-felt depth because the mud line on the logs is 3 m higher than the drillers' depth.

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 (CNTG, HLDT) and a good contact with the borehole wall. Invalid density readings are observed at 11.5, 31, and 77.5 mbsf.

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. Hole 909C has a quite severe deviation from the vertical: ~14 degrees at 600 mbsf up to a maximum of 25.6 degrees at 1010 mbsf. The effect of the deviation is to strongly eccentralize the tool string in the borehole; while this might not be a problem for tool strings such as the DIT/HLDT/SDT/NGT which are usually run eccentralized, it is a serious potential problem for the FMS string. The deviation, combined with the tool weight, might cause the caliper arms to close. Therefore, caution is suggested when interpreting the data recorded by the FMS string.

Hole diameter was recorded by the hydraulic caliper on the HLDT tool (CALI)and on the FMS string (C1 and C2).

Details of standard shore-based processing procedures are found in the "Explanatory Notes" chapter, ODP IR Volume 151. For further information about the logs, please contact:

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