Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 956B

Leg: 157

Location: Gran Canaria (tropical NE Atlantic)

Latitude: 27° 36.895' N

Longitude: 16° 9.797' W

Logging date: September, 1994

Bottom felt: 3453.4 mbrf (used for depth shift to sea floor)

Total penetration: 703.5 mbsf

Total core recovered: 297.4 m (54 %)

Logging Runs

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

Logging String 2: ACT/GST/NGT

Logging String 3: FMS/GPIT/NGT (2 passes)

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 302.5 mbsf

ACT/GST/NGT: bottom hole assembly at 302.5 mbsf

FMS/GPIT/NGT: bottom hole assembly at 284 mbsf.

Processing

Depth shift: All original logs have been interactively depth shifted with reference to NGT from DIT/SDT/HLDT/CNTG/NGT run and to the sea floor (-3453.4 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: 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. 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. Invalid gamma ray spikes were recorded at 261-267 and 286.5-292.5 mbsf. The extremely high values recorded at 615-620 mbsf correspond instead to highly radioactive sediments rich in sphene and zircon which can be high in Thorium content.

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 157. For further information about the logs, please contact:

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia