Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 504B
Leg: 69
Location: Costa Rica Rift (tropical NE Pacific)
Latitude: 1° 13.611' N
Longitude: 83° 43.818' W
Logging date: October 1983
Sea floor depth ("bottom felt:): 3474 mbrf
Total penetration: 489 mbsf
Total core recovered: 76.66 m (33.7 % of cored section)
Oldest sediment cored: Chert and Siliceous limestone (Late Miocene)
Lithologies: massive and pillow basalts (basement).
The logging data was recorded by Gearhart-Owen in GO format and subsequently translated into LIS format. Data were processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory in April 2005.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. CDL/GR/TEMP |
Downlog
|
0
|
483.4
|
274.5
|
Only TEMP
Pass 1 |
|
Main
|
216.1
|
477.4
|
|
|
|
|
Repeat
|
191.4
|
291.2
|
|
|
|
| 2. LL3/NL//GR |
|
228.8
|
477
|
274.5
|
|
| 3. Magnetometer |
|
|
|
|
Unsuccessful
|
| 4. BHC/GR |
|
|
|
|
Failed
|
| 5. Magnetometer |
Main
|
275.7
|
482
|
|
Good data
|
|
Repeat
|
392.6
|
449.3
|
|
Good data
|
|
| 6. TEMP |
Downlog
|
0
|
482.2
|
|
Pass 2
|
Through-pipe logging was attempted but abandoned when the pipe became stuck. The DIT/LSS/GR/MCD tool string met an obstruction just below the bit, but was able to pass through it. The pipe was lowered below the obstruction for the FDC/CNL/GR runs.
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 first depth shifted 3475.5 mbsf to match the sediment-basalt contact and the casing shoe at 274.5 mbsf. Using the DLL curve from ODP Leg 111 as reference, they were then depth matched to it. The Leg 111 log was chosen as the reference run because all subsequent ODP logging of Hole 504B (Legs 140 and 148) had been previously tied to it.
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 given by the drillers ("bottom felt" depth) was 3474 mbrf; none of the logs recorded during Leg 70 crossed the mudline or clearly showed its exact depth (temperature logs). In order to match the sediment-basement contact and the casing shoe at 274.5 mbsf, the logs had to be shifted 3475.5 m, a discrepancy of 1.5 m with the depth given by the drillers.
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). In Hole 504B, most logs show similar response to lithologic and physical property changes; for example, massive basalt units are well characterized by their uniformity, with high resistivity and density values correlating well with low neutron porosity readings. On the other hand, the Gamma Ray log shows very little character throughout the logged interval. This tool responds largely to the potassium content in the rocks, and is not a very helpful in basalts, which have a very low potassium content.
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 (CALI).
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 69. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia