Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 505B
Leg: 69
Location: Costa Rica Rift (tropical NE Pacific)
Latitude: 1° 55.2' N
Longitude: 83° 47.4' W
Logging date: October 1979
Sea floor depth (echo sounding): 3517 mbrf
Total penetration: 178 mbsf
Total core recovered: 6.85 m (16.3 % of cored section)
Oldest sediment cored: none
Lithologies: highly fractured basalt.
The logging data was recorded by Gearhart-Owen in LIS format. Data were processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory in April 2005.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. CDL/GR/TEMP |
Downlog
|
16
|
189
|
141
|
TEMP only
|
|
Pass
1
|
100
|
189.1
|
|
Reference
|
|
|
Pass
2
|
103.3
|
189.8
|
|
|
|
| 2. Magnetometer |
Main
|
107.1
|
183.3
|
|
Magnetic
susceptibility
|
|
Pass
1
|
84.3
|
188.1
|
|
Magnetic field
|
|
|
Pass
2
|
110.2
|
189
|
|
Magnetic field
|
|
|
Pass
3
|
101.3
|
146.4
|
|
Magnetic field
|
|
| 3. Borehole Televiewer |
|
|
|
|
Failed
|
| 4. LL3/NL/GR |
|
99.1
|
174.1
|
108
|
|
| 5. TEMP |
Downlog
|
|
|
|
No data available
|
Three suites of Gearhart-Owen logs were run successfully in Hole 505B. In order to ensure that each tool string reached the basement, the bottom hole assembly was lowered so that its end was just below the sediment-basement contact. Then, during the trip uphole the drill pipe was raised 28 m in order to log the lower part of the sediments as well. The Russian magnetometer and the borehole televiewer were run between the first and fourth logging pass. Logging operations ended with a final temperature measurement, not available on the original tape.
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.
Depth match and depth shift to sea floor: The original logs were depth-matched to the GR log from Pass 1 of the CDL/GR/TEMP tool string, and were then shifted to the sea floor (- 3517 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.
As none of the logs reached the mudline, the sea floor depth used to shift the logs to the sea floor corresponds to the depth determined by echo sounding (-3517 m).
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 density log). Excellent correlation is observed among the different logs in the basement section of the hole. An exception is represented by the GR log run with the LL3/NL tool string, which does not show the same features as the one run with the CDL/TEMP/GR tool string (2 passes).
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 hydraulic caliper on the CDL tool (CALI); the log shows a very irregular and enlarged hole, and very poor correlation between the two passes recorded.
A null value of -999.25 may replace invalid log values.
Additional information about the drilling and logging operation can be found in the Operations section of the Site Chapter in DSDP Initial Reports volume 69. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia