Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 527
Leg: 74
Location: Walvis Ridge (tropical SE Atlantic)
Latitude: 28° 2.49' S
Longitude: 1° 45.8' E
Logging date: June 1980
Sea floor depth ("bottom felt"): 4437 mbrf
Total penetration: 384.5 mbsf
Total core recovered: 243.9 m (63 % of cored section)
Oldest sediment cored: Calcareous claystone ( Middle Maestrichtian)
Lithologies: Clays, carbonatic oozes, and chalks (sediments); basalt (basement)
The logging data were recorded by Gearhart-Owen in GO format. The data was digitized by Centerline Data and subsequently processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory in February 2005.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. CDL/GR/TEMP |
Downlog
|
0
|
369
|
112
|
TEMP and GR recorded. GR invalid
|
|
Main
|
96
|
355
|
|
Reference
|
|
|
Repeat
|
259
|
358.5
|
|
Main spliced
to repeat section
|
|
| 2. BHC/CL/GR |
|
113.1
|
346.8
|
|
BHC and GR malfunctioned
|
The CDL/GR/TEMP tool string was run first and a main and repeat run were performed; the temperature log was acquired during the lowering of the tool string to the bottom of the hole. The BHC/CL/GR was lowered next, but the BHC/GR tools malfunctioned and only CL was acquired. The hole caved in and the rest of the logging program (resistivity and final pass of the temperature tool) could not be completed.
The depths in the table are for the processed logs (after depth matching between passes and depth shift to the sea floor). Generally, discrepancies may exist between the sea floor depths determined from the downhole logs and those determined by the drillers from the pipe length. Typical reasons for depth discrepancies are ship heave, wireline and pipe stretch, tides, and the difficulty of getting an accurate sea floor from the "bottom felt" depth in soft sediment.
Note: the original logs show the repeat section spliced to the main section; the entire interval was digitized.
Depth match and depth shift to sea floor: The original logs were depth matched to the GR log from the main pass of the CDL/GR/TEMP tool string, and were then shifted to the sea floor (- 4435 m).
Depth matching is typically done in the following way. One log is chosen as reference (base) log (usually the total gamma ray log from the run with the greatest vertical extent and no sudden changes in cable speed), and then the features in the equivalent logs from the other runs are matched to it in turn. This matching is performed manually. The depth adjustments that were required to bring the match log in line with the base log are then applied to all the other logs from the same tool string.
None of the logs crossed the mudline. The sea floor depth given by the drillers ("bottom felt") was 4437 mbrf. To match the core sediments-basalt boundary set at 341.5 mbsf, however, the log data was shifted 4435 m.
A null value of -999.25 may replace invalid log values.
The gamma ray logs were not converted into standard API units: they are expressed as count per second (cps).
The quality of the data is assessed by checking against reasonable values for the logged lithologies, by repeatability between different passes of the same tool, and by correspondence between logs affected by the same formation property (e.g. the resistivity log should show similar features to the sonic velocity log). In Hole 527 there is only a very short overlap between main and repeat run; in this interval, the logs show good repeatability. The gamma ray recorded downlog, however, does not correlate with the gamma ray from the main and repeat runs and it is therefore considered invalid.
A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (CDL). Hole diameter was recorded by the 3-arm caliper device (CL) on the BHC/CL/GR tool string. According to the caliper log, the hole looks very smooth, around 10Ó, throughout the entire logged interval. Because logging operations had to be suspended due bad hole conditions, it is possible that this measurement does not reflect the true conditions of the hole.
Additional information about the drilling and logging operation can be found in the Operations section of the Site Chapter in DSDP Initial Reports volume 74 . For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia