Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 953C

Leg: 157

Location: Gran Canaria (tropical NE Atlantic)

Latitude: 28° 39.014' N

Longitude: 15° 8.671' W

Logging date: September, 1994

Bottom felt: 3588.9 mbrf

Total penetration: 1158.7 mbsf

Total core recovered: 556.53 m (48 %)

Logging Runs

Logging string 1: DIT/SDT/HLDT/CNTG/NGT (downlog and uplog)

The wireline heave compensator was used to counter ship heave resulting from the mild sea conditions (0.2-1 m).

Bottom-hole Assembly/Drill Pipe

The following bottom-hole assembly and drill pipe 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 ~ 372 mbsf

DIT/SDT/HLDT/CNTG/NGT: Drill pipe at ~ 287.5 mbsf.

Processing

Depth shift: Original logs have been interactively depth shifted with reference to gamma ray and resistivity data from downlog. Due to hole obstructions, the data recorded uphole is locally stretched. The amount of stretch differs for each tool and therefore different amounts of depth shift have been applied in the attempt to account for the stretch. Two zones are particularly affected: from about 478 to 528 mbsf and from 908 mbsf to total depth. Therefore the data quality over these two intervals is very questionable. After interactive depth shift the logs have been shifted to the sea floor (3591.5 m). This amount corresponds to the mudline as seen on the gamma ray log (uplog); it differs 2.6 meters from the "bottom felt" depth (see above).

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-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.

High-resolution data: Bulk density and neutron porosity data were recorded at a sampling rate of 2.54 and 5.08 cm respectively. The enhanced bulk density curve is the result of Schlumberger enhanced processing technique performed on the MAXIS system onboard. While in normal processing short-spacing data is smoothed to match the long-spacing one, in enhanced processing this is reversed. In a situation where there is good contact between the HLDT pad and the borehole wall (low density correction) the results are improved, because the short-spacing have better vertical resolution.

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.

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 hydraulic caliper on the HLDT tool (CALI); the caliper tool was closed several times during the uplog in attempt to reduce the severe overpull cause by hole caving.

The following is a list of the reliable logs from both uplog and downlog (these are also the channels displayed in the log summaries figures of the ODP IR volume 157):

resistivity (both downlog and uplog)

sonic (uplog)

gamma ray (downlog; uplog can be used as well, except in the intervals where the data is stretched -see above, DEPTH SHIFT section-)

density and caliper (uplog)

neutron (uplog)

Details of standard shore-based processing procedures are found in the "Explanatory Notes" chapter, ODP IR Volume 157. For further information about the logs, please contact:

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