Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 415A
Leg: 50
Location: Agadir Canyon (NE Atlantic)
Latitude: 31° 1.72' N
Longitude: 11° 39.11' W
Logging date: September 1976
Sea floor depth ("bottom felt"): 2817 mbrf
Sea floor depth (step in GR log): 2817 mbrf
Total penetration: 1079.5 mbsf
Total core recovered: 24.55 m (20 % of cored section)
Oldest sediment cored: Marlstone (Albian)
Lithologies: mudstones, limestones, marlstone
The logging data was recorded by Schlumberger in LIS format. Data were processed 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. GR |
Main
|
0
|
908
|
73 (csg) |
Reference
|
|
Repeat
|
657.1
|
748.4
|
|
|
Only a gamma ray log was recorded through pipe, because of the rapidly deteriorating hole conditions.
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 repeat run was depth matched to the main run; both were then shifted to the sea floor (- 2817 m). The main pass was chosen as the reference run because it was the only run to cross the sea floor. The sea floor depth was determined by the step in gamma ray values at 2817 mbrf. This coincides with the sea floor depth given by the drillers (see above).
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.
At Hole 415A the quality of the data was assessed by repeatability between the two passes of recorded. The correlation is excellent.
Gamma ray logs recorded through bottom hole assembly (BHA), drill pipe, drill pipe and casing 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. Above 809 mbsf the log was acquired through both drill pipe (7 inches thick) and casing (16 and 11 3/4 inches thick); below this depth it was recorded through bottom hole assembly only. As a result, the data can be greatly attenuated; the drill pipe collars result in anomalous low GR spikes every 9.5 m starting at 797 mbsf upward, while generally low GR readings are observed through the BHA below 797 mbsf..
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 50. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia