Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 926B

Leg: 154

Location: Ceara Rise (Tropical NW Atlantic)

Latitude: 3° 43.148' N

Longitude: 42° 54.507' W

Logging date: February, 1994

Bottom felt: 3609.5 mbrf (used for depth shift to sea floor)

Total penetration: 605.8 mbsf

Total core recovered: 593.25 m (97.9 %)

Logging Runs

Logging string 1: DIT/SDT/HLDT/NGT

Logging string 2: GHMT (2 passes)

Logging string 3: ACT/GST/NGT

Wireline heave compensator was used to counter the mild ship heave. The WHC was turned off at 105 mbsf during the DIT/SDT/HLDT/NGT run, at 97 mbsf during the ACT/GST/NGT run, and at 100.5 mbsf during the GHMT run.

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: Bottom-hole assembly at ~77 mbsf for DIT tool, at 49 mbsf

for NGT tool

GHMT: Bottom Hole Assembly at ~78.5 mbsf (main pass)

GHMT: Recorded open-hole (repeat pass)

ACT/GST/NGT: Bottom-hole assembly at ~64 mbsf.

Processing

Depth shift: The DIT/SDT/HLDT/NGT run was first depth shifted +6.5 m for consistency with the core-log correlation performed onboard using Multi Sensor Track data. Then all original logs have been interactively depth shifted with reference to NGT from the DIT/SDT/HLDT/NGT run and to the sea floor (- 3609.5 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. The GHMT data could not be depth-matched to the reference run using the SGR channel, as no NGT tool was included in the GHMT tool string; however, a rough depth match was possible by correlating the magnetic susceptibility channel (MAGS) with the SGR reference channel. 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') and short-spacing (3-5-5-7') logs. The long spacing sonic logs have been processed to eliminate some of the noise and cycle skipping experienced during the recording. The data recorded in short spacing mode in the section of the hole below 400 mbsf are of very good quality; in the upper part, instead, processing has been performed on long spacing data, which appear to be of slightly better quality than short spacing one. The two portions of transit times have then been merged. 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.

Geological Magnetic Tool: The Geological Magnetic Tool collected data at two different sampling rates, the standard 0.1524 m rate and 0.0508 m. Both data sets have been depth shifted to the sea floor. Because the sensor for MAGB (Earth Magnetic Field) did not give any meaningful signal downhole, the data is not included in the database.

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 closed at about 80 mbsf and therefore no borehole size correction can be performed between this depth and the bottom of the pipe. For this reasong, this portion of the data is not included.

Details of standard shore-based processing procedures are found in the "Explanatory Notes" chapter, ODP IR Volume 154. For further information about the logs, please contact:

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia