Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 462
Leg: 61
Location: Nauru Basin (sequatorial NW Pacific)
Latitude: 7° 14.25' N
Longitude: 165° 1.83' E
Logging date: June 1978
Sea floor depth ("bottom felt"): 5189 mbrf
Total penetration: 617 mbsf
Total core recovered: 376.8 m (61 % of cored section)
Oldest sediment cored: Brown clay and chert (Cenomanian)
Lithologies: calcareousand radiolarian ooze and chalk, cherts, chalk, limestone, volcanogenic sandstone, mudstone, clay (sediments), basaltic sill (basement)
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 February 2005.
|
Tool string
|
Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
|
1. TEMP
|
Pass
1
|
0
|
608.3
|
|
Downlog (4 sections)
|
|
2. BHC/CL/GR
|
|
|
|
|
Failed
|
|
3. TEMP
|
Pass
2
|
0
|
603
|
96
|
Downlog
|
|
CDL/GR
|
Main
|
152.4
|
597.5
|
|
|
|
|
Repeat
|
557.9
|
606.4
|
|
|
|
4. IEL/GR
|
Main
|
137.6
|
587.9
|
|
Reference
|
|
|
Repeat
|
541
|
587.7
|
|
|
|
5. LL3/NL/GR
|
|
|
|
|
Invalid data
|
At Hole 462, only three logging strings were successful. The LL3/NL/GR and the BHC/CL/GR strings could only log the washed-out portion of the hole, above 350 mbsf; only the NL was successful, but because the hole was washed-out the data is of no use. The Temperature log was recorded downhole in two passes as part of the CDL/GR tool string. From the logs available on tape it appears that some intervals may be missing, particularly in Pass 1. No blueprint was available and therefore these intervals could not be digitized.
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 IEL/GR tool string, and were then shifted to the sea floor (- 5189 m). Because the temperature log was the only one to cross the mudline but typically does not show any significant change, the sea floor depth corresponds to the "bottom felt" depth given by the drillers.
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 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). The logs recorded at Hole 462 correlate quite well, clearly indicating the sediments-basement interface at about 562 mbsf, 3 meters higher than the contact inferred from the cores. The discrepancy is attributed to the low core recovery above and below the boundary, which would not allow for an accurate identification of the contact.
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. With the exception of the temperature log, all logs were recorded open-hole. The second pass of the temperature logs show a small step where the string entered the bottom hole assembly at 96 mbsf.
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. The hole conditions were not ideal to obtain high quality results; the hole was completely washed out above 350 mbsf, where the tool reached its maximum span at 13.9 inches, and is locally very enlarged below that depth.
Temperature is expressed as degF.
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 61. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia