Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 994-C

Leg: 164

Location: Blake Ridge (NW Atlantic)

Latitude: 31° 47.139' N

Longitude: 75° 32.753' E

Logging date: November, 1995

Bottom felt: 2810.1 mbrf

Total penetration: 703.5 mbsf

Total core recovered: 501.5 m (71.3 %)

Logging Runs

Logging string 1: DIT/SDT/HLDT/CNTG/NGT

Logging string 2: ACT/GST/NGT

The 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/CNTG/NGT: Bottom-hole assembly at ~77.5 mbsf.

ACT/GST/NGT: Bottom-hole assembly at ~74.5 mbsf.

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from DIT/SDT/HLDT/CNTG/NGT run and to the sea floor (- 2809.5 m). This amount corresponds to the depth of the sea floor as seen on the logs and differs 0.6 m from the "bottom felt" depth given by the drillers. 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 SDT was operated in standard depth-derived borehole compensated mode including long (8'-10'-10'-12') and short-spacing mode (3'-5-5-7'). The long-spacing 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.

High-resolution processing: No high resolution logging performed at this hole.

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. The quality of the density data was greatly impaired by the degraded hole conditions and by the presence of gas hydrates, which yielded bulk density values as small as 1.2 g/cm3 in the 215 mbsf-t.d. interval. Invalid density spikes were observed at 395-405, 410-413, 426-428, and 442-450 mbsf.

Data recorded through bottom-hole assembly, such as the NGT data above 77.5 mbsf, should be used qualitatively only because of the attenuation on the incoming signal. Invalid gamma ray data were recorded at 63-77.5 mbsf during the DIT/SDT/HLDT/CNTG/NGT run.

The quality of the acoustic data was greatly impaired by the degraded hole conditions, which yielded original transit time values in excess of 220 msec/ft. The processing produced acceptable results that should be regarded from a qualitative standpoint only; invalid data are still present at 85 and 205-214.5 mbsf.

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

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia