Geologic Magnetic Data Processing
ODP logging contractor: LDEO-BRG
Hole: 1127B
Leg: 182
Location: Great Australian Bight
(SW Pacific Ocean)
Latitude: 33° 28.45' S
Longitude: 128° 28.87' E
Logging date: November, 1998
Bottom felt: 490.6 mbrf (used for
depth shift)
Total penetration: 1001.8 mbsf
Total core recovered: 442.05 m
(86.6 %)
One pass was recorded with the GHMT tool
string. The NMRT component of the GHMT tool did not function during Leg 182,
therefore no MAGB data was recorded. The magnetic susceptibility (MAGS) record
is of good quality: processing was performed on the open-hole section
(82.3-505.8 mbsf). Wireline heave compensator was used to counter ship heave.
It was deactivated 119.4 mbsf during the GHMT run.
Hole conditions
The magnetic susceptibility data was
corrected for borehole variations using the HLDT caliper from 106 to 129 mbsf,
the FMS caliper from 129 mbsf to the bottom of the hole and a constant caliper
value of 17 inches above 139 mbsf.
Depth Shift
The GHMT logs have been interactively depth
shifted with reference to HNGS from DIT/HLDS/APS/HNGS run and to the sea floor
(490.6 mbsf). The program used is an interactive, graphical depth-match program
that allows to visually correlate logs and to define appropriate shifts. The
reference and match channels are displayed on the screen, with vectors
connecting old (reference curve) and new (match curve) shift depths. The total
gamma ray curve (HSGR or SGR) from the HNGS or NGT tool run on each logging
string is used to correlate the logging runs most often. In general, the
reference curve is chosen on the basis of constant, low cable tension and high
cable speed (tools run at faster speeds are less likely to stick and are less
susceptible to data degradation caused by ship heave). Other factors, however,
such as the length of the logged interval, the presence of drill pipe, and the
statistical quality of the collected data (better statistics is obtained at
lower logging speeds) are also considered in the selection. A list of the
amount of differential depth shifts applied at this hole is available upon
request.
Additional information about the logs can be
found in the "Explanatory Notes" and Site Chapter, ODP IR volume 182.
For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia