Logging Summary
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IODP Expedition 330: |
Louisville Seamount Trail
Expedition 330
Scientific Party
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Introduction |
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Figure 1.
Bathymetric map showing
Expedition 330 drill sites. The
green circles denote the
locations of the two logged
sites (U1374 and U1376).
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The Louisville Seamount Trail is a 4300
km long volcanic chain that is inferred
to have been built in the past 80
million years as the Pacific plate moved
over a persistent melt anomaly or
hotspot. It is the South Pacific
counterpart of the much better studied
Hawaiian-Emperor seamount chain.
Drilling during ODP Leg 197 in the
Emperor Seamounts documented a
substantial ~15° southward motion of the
Hawaiian hotspot prior to 47 Ma, calling
into question whether the Pacific
hotspots constitute a fixed frame of
reference.
The Expedition 330 drilling program was
designed to examine: 1) the possible
motion of the Louisville hotspot and its
geodynamical implications; and 2) the
eruptive cycle and geochemical evolution
of this seamount trail. This expedition
aimed to replicate the ODP Leg 197
drilling experiment by matching the ages
of the proposed drill sites to the ages
of Detroit, Suiko, Nintoku and Koko
seamounts in the Emperor Chain. This
would enable direct comparison of
paleolatitude estimates between the two
longest-lived hotspot systems in the
Pacific. Both objectives will provide
important data to investigate the
possible link between the Louisville
mantle plume and the formation of the
Ontong Java Plateau. Finally, the thin
cover of sediments on these seamounts
may provide additional information on
the subsidence history of the individual
Louisville seamounts.
The primary logging objectives of
Expedition 330 were to collect
high-resolution downhole physical
property data and integrate them with
core measurements. Wireline logging was
planned for all sites (with one hole
drilled per site). The purpose of this
was to obtain a continuous in situ
record of the formations encountered
through the seamount.
Five Louisville guyots were drilled and
cored - volcanic basement was reached at
four of these drilling targets. Downhole
logging measurements were taken at two
of the sites drilled, U1374 and U1376
respectively (Figure
1).
Logging Tools
The logging program on Expedition 330
was designed to obtain continuous in
situ physical property data
needed to assist in lithologic
identification and the recognition of
structural characteristics of cored
volcanic basement formations.
Combinations of four wireline logging
tool strings were deployed during the
expedition: (1) the triple combination
(triple combo) tool string, (2) the
Formation Micro-Scanner (FMS)-sonic tool
string, (3) the Ultrasonic Borehole
Imager (UBI), and (4) the third-party
Göttingen Borehole Magnetometer (GBM).
The third-party GBM allows fully
oriented component magnetic anomalies to
be determined by measuring three
orthogonal components of the magnetic
field. The GBM includes three optical
gyroscopes, which record the tool's
overall rotation from the start of the
measurement. This enables the cumulative
rotation of the tool to be undone by
retroactively unwinding the recorded
rotation of the GBM using a Matlab
software program (S. Ehmann, Technische
Universität Braunschweig, Germany).
Together, this allows for independent
determination of the intensity,
inclination, and declination of the
magnetization in the borehole lava flow
formations. For optimum data quality the
GBM was deployed with a centralizer and
a non-magnetic sinker bar directly above
the GBM.
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Logging
Operations
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Downhole logging data obtained from
U1374A included natural, total and
spectral gamma ray radiation, density,
neutron porosity and electrical
resistivity, electrical images, p-wave
velocity (Figure
2), three-component magnetic field
(Figure 3)
and acoustic images (Figure 4).
Following some initial issues with rock
bridges in the borehole, and an
extensive borehole preparation, an open
hole section of 393.9 m was logged. The
downhole log measurements (resisitivity,
density, velocity and neutron porosity)
were used to identify a total of nine
log units in Hole U1374A with two in the
section covered by the Bottom Hole
Assembly (BHA)and seven in the volcanic
sequences in the open hole interval. The
third party GBM collected good quality
magnetic data, which will be reoriented
post-expedition. The raw data correlates
well to the lithological changes
observed in the core (Figure 3).
Lithological and structural features are
well imaged with the Formation
MicroScanner (FMS) (Figure 4).
Site U1376: Burton Guyot
At Hole U1376A three logging tools
stings were deployed: the triple combo (Figure 5)
and FMS-sonic tool strings (Figure 6)
made two full passes, and the GBM
performed one full pass (starting and
ending at the rig floor, as described
above) (Figure
7). Measurement depths were
adjusted to match across different
logging runs, obtaining a wireline
matched below seafloor (WMSF) depth
scale.
The logged depth interval for Hole
U1376A was 80.4 - 182.3 m WMSF.
Resistivity, density, compressional
velocity and neutron porosity derived
from downhole logging measurements
(triple combo) were used to identify a
total of thirteen Log Units in Hole
U1376A. Three of these Log Units were in
the section covered by the BHA and ten
in the volcanic sequences in the open
hole interval. These defined Log Units
correlated to changes from massive
basalt flows to more brecciated units,
and interlayered aphyric and
olivine-phyric flow units.
The GBM was run once in Hole U1376A
and collected good quality magnetic
data, which will be reoriented
post-expedition. The GBM data shows that
a massive lava flow is not as
homogeneous as it appears in the
palaeomagnetic data obtained from the
recovered cores at Hole U1376A (Figure 7).
Additionally, in the unrecovered section
of the hole, between ~130 and 140 mbsf,
the GBM data shows strong variations.
Post-expedition work, which will split
the horizontal component into north and
east components, should provide further
insight into these observed variations.
Lithological and structural features
are well imaged with the FMS (Figure 6),
in particular fractures, clast size,
shape and distribution and areas of
solid, massive basalt versus brecciated
material. Of particular importance is
FMS coverage over the unrecovered
section between ~130 and 140 mbsf
because it will provide valuable
information in reconstructing the
lithology over this interval of the
hole.
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Louise Anderson:
Logging Staff Scientist, Borehole
Research Group University of Leicester,
University Road, Leicester, LE1 7RH, UK
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