Wireline Standard Data Processing
ODP logging contractor: LDEO-BRG
Well name: 395A
Location: Mid Atlantic Ridge (central tropical N Atlantic)
Latitude: 22° 45.35' N
Longitude: 46° 04.9' E
Logging date: June, 1986
Bottom felt: 4494 mbrf (used for depth shift to sea floor).
Total penetration: 664 mbsf (drilled during DSDP Leg 45)
Total core recovered: 106 m (18%)
Logging string 1: ACT/GST/NGT
Logging string 2: DIT/LSS/GR (second pass to record sonic waveforms))
Logging string 3: LDT/CNTG/GPIT/NGT (2 passes)
The wireline heave compensator was tested during this leg; it was used on both main passes of the LDT/CNT/GPIT/NGT logs. Ship heave ranged between 3 and 5 ft during the recording of the other logs.
The casing shoe was set at 112 mbsf during leg 45. Even after accurate depth correlation (see "Depth shift" section) between runs, the logs show the casing shoe at different depths; furthermore, these depths differ from the depth given by the drillers. Possible reasons for this discrepancy are ship heave, use of wireline heave compensator, and drill string and/or wireline stretch.
DIT/LSS/GR: Casing at 112.5 mbsf.
ACT/GST/NGT: Casing at 118 mbsf.
LDT/CNTG/GPIT: Casing at 115 mbsf.
Depth shift: Original logs have been interactively depth shifted with reference to NGT from LDT/CNTG/GPIT/NGT run and to the sea floor (- 4494 m). This value located the casing shoe at 112.5 mbsf on the DIT/LSS/GR logs and at 115 mbsf on the HLDT/CNTG/GPIT/NGT logs. 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 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.
Geochemical data: The geochemical data were of poor quality and are not included in the database.
null value=-999.25. This value generally appears in discrete core measurement files and also it may replace invalid log values or results (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, LDT) and a good contact with the borehole wall.
Data recorded through casing should be used qualitatively only because of the attenuation on the incoming signal.
No good measurement of hole diameter was recorded during Leg 109. A good caliper measurement was obtained during DSDP Leg 78.
Resistivity and acoustic data were spliced at the following depth:
resistivity: 205, 325, and 470 mbsf
acoustic: 200, 325, and 465 mbsf.
For further information about the logs, please contact:
E-mail: Cristina Broglia