DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 396B
Leg: 46
Location: Mid Atlantic Ridge (central tropical N Atlantic)
Latitude: 22° 59.14' N
Longitude: 43° 30.90' W
Logging date: February 1976
Sea floor depth ("bottom felt"): 4465 mbrf
Sea floor depth (step in GR log): logs did not reach the mudline
Total penetration: 405.5 mbsf
Total core recovered: 63.56 m (23.3 % of cored section)
Oldest sediment cored: Middle Miocene (nannofossil foraminifer ooze)
Lithologies: nannofossil and foraminifer oozes (sediments), basalts (basement)
The logging data was recorded by Schlumberger in analog form only. Data were digitized by Centerline Data in 2004 and processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in July 2004. Only LL8 was digitized among the resistivity curves because of the difficulty in telling the three resistivity curves recorded apart.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Casingdepth (mbsf) | Notes |
| 1. BHC |
uplog
|
160
|
338
|
161
|
|
| 2. FDC/CNL/GR |
uplog
|
157
|
389
|
161
|
GR run?
|
| 3. DIL/LL8/GR |
uplog
|
157
|
375
|
161
|
GR run?
|
Note: according to the log report, the GR log was recorded with both the DIL/LL8 and FDC/CNL tool strings. The blueprints, however, display the same GR curve, without specifying with which tool string it was recorded.
Depth match and depth shift to sea floor: Depth-matching is typically done by choosing as reference (base) log the gamma ray log from the run with the greatest vertical extent and no sudden changes in cable speed; then, the features in the equivalent logs from the other runs are matched to it. 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.
No depth matching was carried out at Hole 396B because no gamma ray curve was recorded on each logging run: the logs, however, seem to correlate reasonably well.
The sea floor depth of 4465 mbrf corresponds to the "bottom felt" depth given by the drillers. Because none of the logs reached the mudline, the data was shifted by this amount.
Sonic data: The acoustic data show some correlation with the other logs to a depth of 337 mbsf. Below this depth, the data was degraded so badly that no digitization was performed. A velocity log has been derived from the acoustic slowness.
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).
The hole diameter was measured by the caliper tool on the FDC/CNL tool string. In the upper part of the hole, the caliper measured less than 11 inches; below 310 mbsf the hole becomes enlarged, with caliper readings above 11 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 46. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia
Trevor Williams
Phone: 845-365-8626
Fax: 845-365-3182
E-mail: Trevor Williams