Standard Wireline Data Processing
DSDP operator
and logging contractor:
Scripps Institution of Oceanography
Hole: 492B
Leg: 66
Location: Mid America Trench (central NE
Atlantic)
Latitude: 16° 4.73' N
Longitude: 98° 56.72' W
Logging date: April 1979
Sea floor
depth (driller's):
1971.5 mbrf
Sea floor
depth (step in GR log):
1970 mbrf
Total
penetration: 290 mbsf
Total core
recovered: 6.4 m (68 %
of cored section)
Oldest
sediment cored: Sand and
muddy siltstone (Middle-Late Miocene)
Lithologies: Silt, siltstone, frozen sediments
(hydrate?)
The logging data
was recorded by Gearhart-Owen in Gearhart-Owen format. The data was digitized by Centerline
Data in 2005 and subsequently processed at the Borehole Research Group of the
Lamont-Doherty Earth Observatory.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. NL/GR/Temperature |
Downlog |
0 |
258 |
Thru pipe and BHA |
Temperature only |
| Uplog |
0 |
258 |
Thru pipe and BHA |
NL/GR only NL invalid |
Failure of the
bit release and the inability to remove the go-devil used for releasing the bit
forced to log through pipe and bottom hole assembly (BHA). The neutron log is of no use, due to
equipment malfunction, whereas the GR response is affected by the presence of
drill collars and drill pipe joints.
The depths in
the table are for the processed logs (after 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 shift to
sea floor: The original
logs were shifted to the sea floor (- 1970 m). run. The sea floor depth was
determined by the step in gamma ray values at 1970 mbrf. This differs by only
1.5 m from the sea floor depth given by the drillers (see above).
The quality of
the data is usually 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). This approach could not be used in Hole 492B, where only two logs could
be recorded through pipe.
The gamma ray
log 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 BHA extends from 258 to 158 mbsf;
above this depth, the drill pipe joints correspond to negative peaks evenly
spaced every 9.5 m on the GR log.
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 66. For
further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia