Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 501
Leg: 68
Location: Costa Rica Rift (equatorial NE Pacific)
Latitude: 1° 13.63' N
Longitude: 83° 44.06' W
Logging date: July 1979
Sea floor depth ("bottom felt"): 3466.9 mbrf
Total penetration: 337.1 mbsf
Total core recovered: 74.53 m (50.6 % of cored section)
Oldest sediment cored: Cherty limestone (no paleontological age)
Lithologies: siliceous and calcareous ooze, chert and cherty limestone (sediments); basalt (basement)
The logging data was recorded by Gearhart-Owen in GO format. The data was digitized by Centerline Data and subsequently 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. BHC/CL/GR |
Main
|
131.4
|
326.8
|
136.5
|
BHC failed
|
|
Repeat
|
217.8
|
324.7
|
|
BHC failed
|
|
| 2. CDL/GR |
Main
|
139.1
|
329.1
|
140.5
|
GR data bad
|
|
Repeat
|
234.5
|
7
|
321.3
|
GR data bad
|
|
| 3. Magnetometer |
|
256.1
|
300.1
|
|
Vertical component
|
| 4. TEMP |
Pass
1
|
0
|
325.1
|
136.1
|
Downlog
|
|
Pass
2
|
0
|
267.7
|
156.1
|
Uplog
|
|
| 5. LL3/NL/GR |
Main
|
129.1
|
323.1
|
129.3
|
|
|
Repeat
|
234.4
|
322.8
|
|
|
The logging program at Hole 501 was generally successful, though some of the tool string components did not wok (BHC) or failed to record reliable data (GR from CDL/GR tool string and GR from LL3/NL/GR below 275 mbsf). The 3-component Russian Borehole Magnetometer acquired good magnetic field data and poor magnetic susceptibility data. Only the vertical component (Z) has been digitized, lacking information to identify the other two components on the blueprints for digitization. According to the log header, the first pass of the temperature data was acquired on the trip downhole while the second pass was logged uphole, due to technical difficulties during the lowering of the tool.
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 conductivity log from the main pass of the LL3/NL/GR tool string, and were then shifted to the sea floor (- 3466.9 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.
The sea floor depth used to depth shift the data was the "bottom felt" depth (-3466.9 mbrf) given by the drillers. As a result, the sediment-basalt interface falls at 265 mbsf, which is consistent with the depth inferred from the cores.
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 bolk density log). All logs, with the exception of the gamma ray, show excellent correlation in the basement section, where they clearly show massive versus pillow basalt units.
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 CNL porosity can sometimes be used qualitatively through the BHA and pipe, but most of the other logs will not give usable data.)
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. While all four caliper curves (main and repeat) are generally similar in shape and value range, they differ above 230 mbsf; the caliper form the CDL tool saturates at about 20.5 inches, the 3-arm CL at 14.5 inches. The both agree, however, in showing that the upper part of the hole was greatly enlarged, which in turn may have affected the quality of the log measurements (fpr example the gamma ray).
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 68. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail; Cristina Broglia