Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 495
Leg: 67
Location: Mid America Trench (tropical NE Pacific)
Latitude: 12° 29.78' N
Longitude: 91° 2.26' W
Logging date: May 1979
Sea floor depth ("bottom felt"): 4150 mbrf
Total penetration: 446.5 mbsf
Total core recovered: 332.72 m (74.5 % of cored section)
Oldest sediment cored: Manganiferous chalk (Lower Miocene)
Lithologies: Diatomaceous mud, clay, manganiferous chalk and chert (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. CDL/GR |
Main
|
111
|
439
|
|
Reference
|
|
Repeat
|
391.7
|
439.4
|
|
|
|
| TEMP |
Downlog
|
0
|
448
|
|
|
The CDL/GR/TEMP tool string acquired temperature data during the lowering of the tool in the hole and density, caliper, and gamma ray data uphole .
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 density (RHOB) log from the main pass of the CDL/GR/TEMP tool string, and were then shifted to the sea floor (-4150 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.
Only the temperature log crossed the mudline and, as expected, did not show any step in value. Therefore, the sea floor depth used to depth shift the data corresponds to the "bottom felt" depth (-4150 mbrf) given by the drillers. As a result, the sediment-basalt interface is at 432 mbsf, 4 m deeper than the boundary based on the cores. It must be taken into account, however, that core recovery in basalt was very poor, and no sediment-basalt contact was preserved in the cores; on the other hand, logs provide a continuous record of the formation physical properties.
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 density, gamma ray, and caliper logs show excellent repeatability in the main and repeat pass. The sediment-basalt interface is clearly shown at 432 mbsf.
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 Hole 495 can be found in the Site Chapter in DSDP Initial Reports volume 67. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia