Geologic Magnetic Data Processing
ODP logging contractor: LDEO-BRG
Hole: 1119C
Leg: 181
Location: New Zealand Continental
Shelf (SW Pacific Ocean)
Latitude: 44° 45.33' S
Longitude: 172° 23.61' E
Logging date: August, 1998
Bottom felt: 407.2 mbrf
Total penetration: 494.8 mbsf
Total core recovered: 442.2m (89.4
%)
One pass was 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 open-hole section
(81.8-455.2 mbsf). Wireline heave compensator was used to counter ship heave.
Hole conditions
The magnetic susceptibility data was
corrected for borehole variations using the FMS calipers from 90 mbsf to the
bottom of the hole and a constant caliper value of 12.5 inches above 90 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
(406 mbsf). This amount corresponds to the mudline as seen on the logs, which
differs 1.2 m from the "bottom felt" depth given by the drillers (see
above). 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 181.
For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia