Geologic Magnetic Data Processing
ODP logging contractor: LDEO-BRG
Hole: 1172D
Leg: 189
Location: East Tasman Plateau (Tasman
Sea)
Latitude: 43° 57.5545' S
Longitude: 149° 55.7169' E
Logging date: 2-3 May 2000
Bottom felt: 2633 mbrf (used for
depth shift to sea floor)
Total penetration: 766.5 mbsf
Total core recovered: 237.09 m (79.6
%)
GHMT
Logging Runs
Two passes of the GHMT/DSI/NGT combination
were recorded open hole: a main pass from 165 to 759 mbsf and a repeat pass
from 259 to 379 mbsf. Processing was performed on the main pass.
Hole 1172D was logged under conditions of
high ship heave; it was not used for most of the GHMT runs.
Hole
conditions
The HLDS caliper shows a borehole varying in
size from 11 to 19 inches. The raw susceptibility has been corrected for hole
diameter using a mean value of 12 inches.
Depth
Shift
The SGR log from the GHMT/NGT/DSI run was
used as the reference for depth matching. The logs were then shifted to the sea
floor (-2633 m). The sea floor depth is determined by the step in gamma ray
values at the sediment-water interface. In this case it is the same as the
"bottom felt" depth given by the drillers (see above).
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 then the
features in the equivalent logs from the other runs are matched to it in turn.
This matching is performed automatically, and the result checked and adjusted
as necessary. 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.
Log
Quality
The total magnetic induction (MAGB) is of
good quality except the presence of about 5 spiky zones, which were removed
manually. The pipe effect is visible on the total induction signal and is
corrected as well as a weak linear drift of -0.03 nT/m.
The magnetic susceptibility signal is of good
quality.
Lithology
The GHMT crosses lithostratigraphic units IB
to IVB, which consist of a succession of nannofossil ooze and claystone.
Age
information
The processed section corresponds to Late
Miocene (top of the processed GHMT section) to Cretaceous (below 700 mbsf), as
inferred onboard from biostratigraphic data. The deepest recognized magnetochron
is C18n with its top at 383 mbsf (Hole 1172A).
Proposed
interpretation of GHMT data
Because of the weakness of the intensity of
magnetization of sediments, it is very hard to propose any magnetostratigraphic
interpretation from GHMT data. The local magnetic field (remanent + induced)
seems to mainly consist of the field due to the susceptibility.
Additional information about the logs can be
found in the "Explanatory Notes" and Site Chapter, ODP IR volume 189.
For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia