Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Well name: 925C

Leg: 154

Location: Ceara Rise (tropical NW Atlantic)

Latitude: 4° 12.256' N

Longitude: 43° 29.349' W

Logging date: February, 1994

Bottom felt: 3051.5 mbrf

Total penetration: 360.1 mbsf

Total core recovered: 368.78 m (102.4 %)

Logging Runs

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

Logging string 2: GHMT (2 passes; susceptibility data only)

Logging string 3: FMS/GPIT/NGT (3 passes)

Wireline heave compensator was used to counter the moderate ship heave. The WHC

was switched off at 85 mbsf before entering the pipe.

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 ~60 mbsf

GHMT: Recorded open-hole

FMS/GPIT/NGT: Bottom-hole assembly at ~60 mbsf

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from FMS/GPIT/NGT main pass and to the sea floor (- 3049 m). This amount differs from the "bottom felt" depth given by the drillers because it incorporates some additional depth shift applied by the logging scientists during correlation of the logs with the Multi Sensor Track (MST) data from core. 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. 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 magnetometer component of the Geologic Magnetic Tool failed during the transit downhole, thus only susceptibility data were collected. Both main and repeat pass have been depth shifted to the reference run and to the sea floor (see Depth Shift section).

Quality Control

null value=-999.25. This value may replace recorded log values or results which are considered invalid.

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 such as the gamma ray logs which were partially recorded through bottom-hole assembly should be used qualitatively only because of the attenuation on the incoming signal. Invalid gamma ray data was recorded at 37 and 55 mbsf during the DIT/SDT/HLDT/CNTG/NGT run.

Hole diameter was recorded by the hydraulic caliper on the HLDT tool (CALI) and on the FMS string (C1 and C2). The HLDT caliper started closing at about 85 mbsf, therefore the density data recorded above that depth should be used cautiously as they were not corrected for the actual hole diameter.

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