Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 467
Leg: 63
Location: San Miguel Gap (NE Pacific)
Latitude: 33° 50.97' N
Longitude: 120° 45.47' W
Logging date: October 1978
Sea floor depth (drillers'): 2146 mbrf
Total penetration: 1041.5mbsf
Total core recovered: 426.3 m (41 % of cored section)
Oldest sediment cored: claystone (Middle Miocene)
Lithologies: silty clay, diatomaceous nannofossil clay, calcareous claystone, lapilli tuff and nannofossil clay chalk and limestone.
The logging data was recorded by Gearhart-Owen in their own format and subsequently translated to LIS format. Because some of the data had been incorrectly translated, the logs were digitized by Centerline Data in 2004. They were processed in November 2004 at the Borehole Research Group of the Lamont-Doherty Earth Observatory.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. BHC/CL/GR |
Main
|
78
|
1041
|
88
|
Reference
|
|
Repeat
|
995
|
1041
|
|
|
|
| 2. Temperature |
Downlog
|
0
|
191
|
122
|
|
The main run with the BHC/CL/GR tool string was completed successfully; the repeat collected valid acoustic data only above 1014 mbsf. The temperature tool got stuck at 191 mbsf, 69 meter into open hole; the cable became knotted below the drill pipe, preventing retrieval. Drill pipe and cable were pulled up together, and eventually the logging tool was retrieved. No further logging was attempted.
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 drillers' depth in soft sediment.
Depth match and depth shift to sea floor: The BHC/CL/GR logs were depth-matched to the GR log from the main pass of the BHC/CL/GR tool string, and were then shifted to the sea floor (- 2146 m). The main pass was chosen as the reference run because it was the longest pass, though it did not reach the sea floor.
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.
The sea floor depth used for depth shifting was the drillers' water depth (see above), as none of the standard logs reached the mudline.
Sonic data: Velocity was calculated from the delay times. In the repeat pass there were no valid readings below 1014 mbsf, due a problem during data acquisition.
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).
Gamma ray logs recorded through bottom hole assembly (BHA) and drill pipe should be used only qualitatively, because of the attenuation on the incoming signal. The thick-walled BHA attenuates the signal more than the thinner-walled drill pipe. The BHC/CL/GR main run entered the bottom hole assembly at 88 mbsf, the temperature tool at 121 mbsf.
A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (FDC, CNL, not employed in this hole). Hole diameter was recorded by the mechanical caliper on the BHC/CL/GR tool string.
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 63. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia