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IODP
Expedition 303: |
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Noth Atlantic Climate 1
Expedition
303 Shipboard Scientific Party
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| Introduction |
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Figure
1. Map of the North Atlantic showing the
location of Expedition 303 sites.
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The primary objective of Expedition 303 was to place late
Neogene–Quaternary climate proxies in the North Atlantic
into a PAC (Paleointensity Assisted Chronology), a chronology
based on a combination of geomagnetic paleointensity, stable
isotope, and detrital layer stratigraphies. Sites drilled
during Expedition 303 are located off Orphan Knoll (Newfoundland),
on the Eirik Drift (southeast Greenland), on the southern
Gardar Drift, and in the central Atlantic “ice-rafted
debris (IRD) belt” (Fig. 1).
The primary logging objective of Expedition 303 was to provide
detailed core-log integration to allow assessment of core
expansion and to provide a quality control check of the spliced
core record. Given the high sedimentation rates at most of
the Expedition 303 sites, a secondary objective was to examine
cyclicity within the logging data. It was hoped that millennial
scale changes would be identifiable in Formation MircoScanner
(FMS) data. However, because of operational difficulties
and deteriorating weather conditions it was only possible
to deploy the “triple combination” tool string
at one site, Site U1305. Unfortunately, this meant that the
highest-resolution tools (the Lamont Multi-sensor Gamma ray
Tool [MGT] and the FMS-sonic) were not deployed during Expedition
303.
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| Results from
Site U1305 |
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Figure
2. Caliper, main and repeat pass gamma ray and
core recovery records for Hole U1305C. gAPI = American
Petroleum Institute gamma ray units. |
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Figure
3. Caliper, density, porosity, electrical resistivity
and photoelectric effect (PEF) data for the interval
95 to 250 mbsf in Hole U1305C.
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| Figure
4. Caliper, total gamma ray and spectral gamma ray
data (K, Th, U) for the interval 95 to 250 mbsf in Hole
U1305C. HCGR = computed gamma ray headspace, HSGR =
total spectral gamma ray, gAPI = American Petroleum
Institute gamma ray units, in. = inches. |
The caliper data show that the diameter of the borehole
ranged from ~13.6 to 18.0 in (Fig. 2),
resulting in data of variable quality. Reproducibility of
data is high between passes (see gamma ray example in Fig.
2).
The density and porosity tools require good borehole contact.
Thus, intervals with a large borehole diameter are characterized
by high porosities and low densities (Fig.
3). Density and porosity data are also less reliable
when the caliper is not open (i.e., above ~107 mbsf during
the main pass).
The downhole logging data suggest that the formation is
fairly uniform in the open hole (Fig.
3). As expected, the density and porosity data are generally
inversely related to each other and show downhole trends
of increasing density and decreasing porosity. Resistivity
values are low reflecting the generally moderate to high
porosity sediments. Photoelectric effect (PEF) values range
between 1.0 and 3.3 b/e-, consistent with the clay-rich lithologies.
Extremely low PEF values (>1.8, the PEF value of pure
quartz) may be the result of poor contact with the borehole
wall or extremely porous intervals (seawater has a PEF value
of 0.807).
Gamma-ray values increase slightly with depth, possibly
as a result of increasing clay content. The low uranium content
of the formation results in very similar HSGR (Total gamma
ray) and HCGR (summation of Th and K gamma rays only) values
(Fig. 4). The uranium data suggest
that total organic carbon values in the logged interval are
consistently very low, as shown by discrete samples. Potassium
and thorium display very similar trends downhole, suggesting
that there are no major downhole changes in mineralogy (Fig.
4).
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| Core-Log
Comparisons |
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Figure
5. Comparison of core and log physical properties
from Hole U1305C. A: Gamma ray activity for the interval
95 to 250 mbsf. B: Density for the interval 95 to
250 mbsf. C: Gamma ray activity for the interval
190 to 215 mbsf. cps = counts per second; gAPI =
American Petroleum Institute gamma ray units.
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| Figure
6. Correlation of spliced core gamma-radiation data
(corrected counts) in red with logging data in black,
for the depth interval of 100 to 200 m from Site U1305.
In the left-hand panel is the spliced data in mcd; the
right hand panel shows the corrected spliced data (in
meters equivalent logging depth or meld) and the logging
data (mbsf). Note that spliced core record in the right-hand
panel has been smoothed in Sagan to allow easier correlation.
cps = counts per second; gAPI = American Petroleum Institute
gamma ray units. |
All the downhole data sets display meter to decimeter scale
variability that are most likely the result of subtle changes
in lithology. A comparison of log- and core-derived natural
gamma radiation and density records shows close agreement
in downhole trends and patterns (Fig
5). Measured density values are very similar in both
core and log data. Closer inspection of the gamma ray data
suggests that 5-meter scale patterns can be recognized in
both the core and log records (Fig 5).
Using the downhole log records as a depth reference, and
the software program Sagan, it was possible to correlate
the core measurements to equivalent logging depths to more
precisely determine the amount of core expansion.
Figure 6 shows some of the tie points
used to integrate core and log data. By recognizing similar
patterns in the composite core record and the logging data,
it was possible to convert the depths for core data from
mcd (meters composite depth) to meld (meters equivalent logging
depth). Using this method of core-log integration it will
be possible to compare various physical properties measured
in core and downhole. This allows us to more fully utilize
and integrate measurements that were only made either downhole
(such as spectral gamma and resistivity) or on core (such
as color).
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Stuart Robinson: Logging Staff Scientist, School
of Human & Environmental Sciences, University of Reading,
Whiteknights, PO Box 227, Reading, RG6 6AB, UK.
email: s.a.robinson@reading.ac.uk
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Additional Leg-related
publications:
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