Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 471
Leg: 63
Location: Magdalena Island (NE Pacific)
Latitude: 23° 28.93' N
Longitude: 112° 29.78' W
Logging date: November 1978
Sea floor depth ("bottom felt"): 3115.5 mbrf
Sea floor depth (step in GR log): mbrf
Total penetration: 823 mbsf
Total core recovered: 356.4 m (43 % of cored section)
Oldest sediment cored: Metalliferous sediment (Middle Miocene)
Lithologies: Nannofossil silty clay, diatomaceous clay/ooze, porcellanite claystone, silty/calcareous sandstone, hemipelagic claystone, and metalliferous sediment (sediments); altered diabase (basement)
The logging data was recorded by Gearhart-Owen in GO format. The logs were digitized by Centerline Data in 2004. They were processed in February 2005 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
|
147
|
826.9
|
158
|
|
|
Repeat
|
766.6
|
826.9
|
|
|
|
| 2. CDL/GR |
Main
|
156
|
824
|
156.5
|
|
|
Repeat
|
766
|
823
|
|
Reference
|
|
| TEMP |
Pass
1
|
0
|
827
|
157
|
Downlog
|
| 3. LL3/NL/GR |
Main
|
142.6
|
822.3
|
157.5
|
|
|
Repeat
|
728.1
|
827.4
|
|
|
|
| 4. IEL/GR |
Main
|
0
|
825.1
|
152.3
|
|
|
Repeat
|
782
|
824.9
|
|
|
|
| 5. TEMP |
Pass
2
|
0
|
827
|
158
|
Downlog
|
Logging operations took place without major problems.
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 the main pass of the CDL/GR tool string, and were then shifted to the sea floor (- 3116 m). The sea floor depth was determined by the step in gamma ray values at 3116 mbrf observed on the IEL/GR main run. This differs by only 0.5 m from the "bottom felt" depth given by the drillers (3115.5 mbrf).
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.
Sonic data: The transit time data were converted into velocities.
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). Correlation between the tools from the different runs is generally good; the logs clearly show the sediment-basement interface at 747.5 mbsf; this is marked by a steep increase in density, resistivity, and acoustic velocity. There is a discrepancy of 6 m between this depth and the boundary inferred from the cores (741.5 mbsf); the first core recovered in the basement, however, had a low recovery, only 23 %, which would not allow for a precise definition of the top of the basement.
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 NL porosity can sometimes be used qualitatively through the BHA and pipe, but most of the other logs will not give usable data.) The different tool string entered the drill pipe at deferent depths (see table above).
A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (CDL, NL). Hole diameter was recorded by the hydraulic caliper on the CDL tool and by the 3-arm CL tool. The caliper measurements correlate well; they indicate a generally good hole with diameter ranging from 10 to about 13 inches.
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