Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 417D
Leg: 51
Location: Bermuda Rise (tropical NW Atlantic)
Latitude: 25° 6.63' N
Longitude: 68° 2.48' W
Logging date: January 1977
Sea floor depth ("bottom felt"): 5489 mbrf
Sea floor depth (to match basement-sediment contact in log and core): 5482 mbrf
Total penetration: 708.5 mbsf
Total core recovered: 320.8 m (59 % of cored section)
Oldest sediment cored: Lower Aptian
Lithologies: clay, ooze, chalk, pillow basalt, massive basalt, breccia.
The logging data was recorded by Schlumberger in LIS format on tape. Data were processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in June 2004.
| Tool string | Pass |
Top depth (mbsf) |
Bottom depth (mbsf) |
Bit depth (mbsf) |
Tool string |
| 1. HRT |
Main
|
0
|
46
|
44
|
|
| 2. BHC/GR/MCD |
Main
|
0
|
449
|
145
|
|
| 3. FDC/CNL/GR |
Main
|
115
|
447
|
115
|
|
|
Repeat
|
320
|
369
|
open hole
|
FDC only | |
| 4. DIL/LL8/GR |
Main
|
115
|
434
|
115
|
After an abortive logging run with the HRT temperature tool, the drill pipe was lowered and the hole was successfully logged with three toll strings.
The depths in the table are for the processed logs. 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 shifted to the sea floor (-5482 m). The GR logs from the three tool strings were compared and common features were found to be within 0.5 m depth of each other. The good depth match was confirmed with the DT, RHOB, and ILD logs.
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 was determined by the step in the physical property logs at 5822 mbrf, the sediment-basement contact. This is given as 340 mbsf in the DSDP results volume on the basis of core lithology, and therefore the sea floor depth shift was set at 5482 m to enable comparison to the core data. This sea floor depth differs by 7 m from the sea floor depth given by the drillers (see above). The sea floor depth is usually given by the step in gamma radiation at the sea floor. There is a step at 5488 mbrf in the GR log from the BHC/GR tool string, however, there is very little data above this depth to be sure of the pick and it leads to an offset at the sediment-basement contact, so we prefer the sea floor depth of 5482 mbrf.
Sonic data: The DT slowness data have been converted to velocity. The velocity log is of good quality.
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).
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 (FDC, CNL). The caliper log for this hole indicates a fairly constant borehole diameter of about 10 inches, with thin narrower hole in some places in the sediments.
The main pass of the FDC tool has miscalibrated low values in the basement section of the hole; the repeat pass has properly calibrated values.
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 51B. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia