Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 925A

Leg: 154

Location: Ceara Rise (Tropical NW Atlantic)

Latitude: 4° 12.256' N

Longitude: 43° 29.349' W

Logging date: February, 1994

Bottom felt: 3053 mbrf

Total penetration: 930.4 mbsf

Total core recovered: 490.05 m (52.7 %)

Logging Runs

Logging string 1: DIT/SDT/HLDT/CNTG/NGT (3 passes)

Logging string 2: FMS/GPIT/NGT

Wireline heave compensator was used to counter the moderate ship heave. The WHC was switched off at 240 mbsf during DIT/SDT/HLDT/CNTG/NGT pass 1, 301.5 mbsf during DIT/SDT/HLDT/CNTG/NGT pass 3 and 283.5 mbsf during FMS/GPIT/NGT.

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 (pass 1): Bottom-hole assembly at ~186 mbsf

DIT/SDT/HLDT/CNTG/NGT (pass 2): Bottom-hole assembly at ~ 452.5 mbsf

DIT/SDT/HLDT/CNTG/NGT (pass 3): Bottom-hole assembly at ~252 mbsf

FMS/GPIT/NGT: Bottom-hole assembly at ~254 mbsf.

Processing

Depth shift: All original logs (including high resolution logs) have been interactively depth shifted with reference to NGT from DIT/SDT/HLDT/CNTG/NGT pass 3 and to the sea floor (- 3046.6 m). The amount of depth shift 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. 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: NGT data from THE DIT/SDT/HLDT/CNTG/NGT pass 3 have been processed to correct for borehole size and type of drilling fluid. Data from passes 2 and 3 are invalid.

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 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.

The processing has been performed both on the data recorded in "long-spacing " (curves LTT1, LTT2 etc.) and "short spacing" (curves TT1, TT2 etc.) mode during runs 1 and 3. "Short-spacing" mode data from runs 1 and 3 have been merged prior to processing. "Long-spacing" mode data have been processed from run 3 and only above 790 m, as one of the curves (LTT3) was very noisy in the lower part of the hole. The reprocessed "long-spacing" mode data look better than the "short-spacing" mode data thorugh most of the hole. The curves resulting from the two different processings have been merged as follows:

187.5-265 mbsf: run 1 (short spacing mode)

265-750 mbsf: run 3 (long spacing mode)

750-890 mbsf: run 3 (short spacing mode)

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.

Runs 1 and 3 were merged as follows:

resistivity data: spliced at 350 mbsf.

density data (both standard and high resolution): spliced at 350 mbsf.

neutron data (both standard and high resolution): spliced at 350 mbsf.

acoustic data: see Acoustic data processing section.

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.

No valid density data were recorded during pass 2 of the DIT/SDT/HLDT/CNTG/NGT because caliper did not open. No valid gamma ray data were recorded during passes 1 and 2 of the DIT/SDT/HLDT/CNTG/NGT.

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), and on the FMS string (C1 and C2). The HLDT caliper started closing at about 227 and 291 mbsf during DIT/SDT/HLDT/CNTG/NGT runs 1 and 3 before getting into the pipe. The density data recorded above those depths is extremely noisy as it could not be corrected for the actual hole diameter, and therefore is not included.

Because of the frequent stops and pulls experienced during the recording, the data from the DIT/SDT/HLDT/CNTG/NGT run 3 should be used with caution in the following intervals: 410, 505, 540, 605, and 705 mbsf. In addition, the density data show invalid spikes at 445-450,493-497, and 587 mbsf.

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