Geologic Magnetic Data Processing
ODP logging contractor: LDEO-BRG
Holee: 1123B
Leg: 181
Location: New Zealand Continental
Shelf (SW Pacific Ocean)
Latitude: 41° 47.1598' S
Longitude: 171° 29.9387' E
Logging date: September, 1998
Bottom felt: 3303.1 mbrf
Total penetration: 489 mbsf
Total core recovered: 451.4 m
(92.3 %)
Two passes were recorded with the GHMT tool string. The NMRT component of the GHMT tool did not function during Leg 181, therefore no MAGB data was recorded. The magnetic susceptibility (MAGS) record is of good quality: processing was performed on the first pass, open-hole section (84.9-486.8 mbsf).
Wireline heave compensator was used to counter ship heave resulting from the
rough sea conditions. Because the WHC reached its limit during GHMT/NGT pass 1,
the tool string had to be lowered to the bottom of the hole to reset the WHC.
Hole conditions
Magnetic susceptibility data from the first
pass was corrected for borehole variations using the FMS calipers from 97 mbsf
to the bottom of the hole and a constant caliper value of 11 inches above 97
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
(3303.5 m). This amount corresponds to the mudline as seen on the logs, which differs
2.4 m from the "bottom felt" depth given by the drillers (see above).
The program used is an interactive, graphical depth-match program, which 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 181.
For further questions about the logs, please contact:
Cristina
Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia