Geologic Magnetic Data Processing
ODP logging contractor: LDEO-BRG
Hole: 1168A
Leg: 189
Location: West Tasmania Slope (Tasman
Sea)
Latitude: 42° 36.58' S
Longitude: 144° 24.76' E
Logging date: March, 2000
Bottom felt: 2474 mbrf
Total penetration: 883.5 mbsf
Total core recovered: 837.1 m (94.7%)
GHMT
Logging Runs
Two passes of the GHMT/DSI/NGT combination
were recorded openhole: the main pass from 101 to 723 mbsf and the repeat pass
from 242 to 373 mbsf Processing was performed on the main pass.
The wireline heave compensator was used to
counter ship heave (typically 3 m).
Hole
conditions
The HLDS caliper shows strong variations,
with maximum readings of 19 inches.
The raw susceptibility has bee corrected for
hole diameter variations using an average value of 12.8 inches.
Depth
Shift
Since the depth match between the total gamma
of the two logging runs is somewhat ambiguous, the conductivity from the GHMT
(MAGC) was matched to the DIT from the DIT/APS/HLDS/HNGS main pass. This led to
an improved depth match. All the logs were then shifted to the sea floor (-2475
m). The sea floor depth is determined by the step in gamma ray values at the
sediment-water interface. It differs 1 m from 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.
The depth match between the main passes of
the two tool strings is fairly robust for most of the hole, but a lack of
correlateable features in the intervals 305-350 and 425-555 mbsf makes the
matches there more questionable.
Depth matching was also made difficult by the
variation in tool speed caused by the tool strings dragging on the bridges
while logging up the hole. While the cable speed (CS) at the surface remains
fairly constant, the actual tool speed is slowed, as indicated by increased
tension at the head of the tool (DF, down hole force). For example, density and
porosity measurements are offset from each other in intervals of high DF, for
example from 410-440 and 720-755 mbsf.
Log
Quality
The total magnetic induction (MAGB) is of
good quality, with the exception of about 20 spiky zones, which were removed
manually. The pipe effect is visible on the total induction signal down to
about 250 mbsf; it is corrected as well as a linear drift of -0.035 nT/m. A
variation of MAGB off the theoretical pipe effect line is observed at 140 mbsf
because of pipe movement during the recording.
The magnetic susceptibility signal is of good
quality except at 303 mbsf, where a spiky zone has been removed manually.
Lithology
The GHMT crosses lithostratigraphic units IB
to IIIB, which consist of a succession of ooze, chalk, claystone, and
siltstone.
Age
information
The processed section corresponds to Late
Miocene (top of the processed GHMT section: 135 mbsf) to Early Oligocene
(bottom of the processed GHMT section: 725 mbsf) periods, as inferred onboard
from biostratigraphic data.
Despite the good core recovery, the shipboard
magnetostratigraphy obtained from core is very poor, mainly because of the weak
intensity of magnetization of the sediments. In the studied depth section,
magnetochrons C5Cn to C6n normal polarity intervals are observed on core
between 250 and 350 mbsf.
Proposed
interpretation of GHMT data
Because of the weakness of the sediments
magnetization intensity, it is very hard to propose any magnetostratigraphic
interpretation from GHMT data at this site. 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