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IODP
Expedition 301: |
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The Hydrogeologic Architecture of Basaltic
Ocean Crust: Compartmentalization, Anisotropy, Microbiology,
and Crustal-scale Properties on the Eastern Flank of Juan
de Fuca Ridge
Expedition
301 Shipboard Scientific Party
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| Introduction |
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Figure
1. Bathymetric map showing the sites drilled
during IODP Expedition 301 (black stars) and ODP
Leg 168 (red dots).
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Figure 2. Two-way
travel time seismic reflection profile showing the location
of Site 1301 that was drilled during IODP Expedition
301 (Expedition
301 Preliminary Report, 2004). This seismic data
was acquired with the 2000 ImageFlux survey (Sonne, SO149).
Vertical lines on this figure represent approximate total
depth of Holes 1301A (shallower hole) and 1301B (deeper
hole).
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During Integrated Ocean Drilling Program (IODP) Expedition
301, sediments and oceanic crust were drilled at Site 1301,which
is located on the Endeavour segment of the Juan de Fuca Ridge
(Figure 1). The primary objectives
of the IODP Expedition 301 were to evaluate the formation-scale
hydrogeologic properties within oceanic crust; determine how
fluid pathways are distributed within an active hydrothermal
system; establish linkages between fluid circulation, alteration,
and microbiological processes; and determine relations between
seismic and hydrologic anisotropy (Expedition
301 Preliminary Report, 2004).
Site 1301 is located above a buried basement ridge
(Second Ridge), where sediment thins to 250-265
m (Figure 2). Site 1301
was positioned ~1 km SSW of Site 1026, which was
drilled during ODP Leg 168 (Figure
1). Three holes were drilled at Site 1301 (Hole
1301A, 1301B, and 1301C) and logging operations
were carried out along the deeper oceanic crust
interval penetrated in Hole 1301B. For further
information and geological setting, please refer
the Expedition
301 Preliminary Report.
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| Logging Operations |
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| Figure 3. Schematic
showing the configuration of the wireline logging tool
strings that were used during IODP Expedition 301. |
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Figure
4. Results of Triple Combo measurements and sonic
tool string measurements from Hole 1301B. Black dots
in the porosity, density, and P-wave velocity panels
represent shipboard measurements on core samples.
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The wireline logging operations consisted of four tool string
deployments (Figure 3): (1) the Triple
Combo, (2) Ultrasonic Borehole Imager (UBI), (3) Formation
MicroScanner (FMS)/Sonic, and (4) the Well Seismic Tool (WST).
Brief descriptions of the operations and tool string configurations
are as follows:
(1) The Triple Combo tool string consisted of the logging
equipment head - mud temperature (LEH-MT) cable head with
sensors for measuring spontaneous potential (SP), temperature
and tension, the hostile environment natural gamma-ray sonde
(HNGS), the hostile environment lithodensity sonde (HLDS),
the accelerator porosity sonde (APS), and the slimextreme
array induction tool (QAIT). This tool string measured the
basic physical properties from 350 mbsf to 578.2 mbsf, without
problems (Figure 4). The caliper
arm shows that the borehole is almost in gauge below ~ 464
mbsf but very irregular and oversized between 352 and 464
mbsf, reaching >18 inches between 395-405 mbsf.
(2) The UBI tool string consists of the LEH-MT with sensors
fo r measuring SP, temperature
and tension, scintillation gamma ray tool (SGT), the general-purpose
inclinometry tool (GPIT), and the UBI. An obstruction was
found in the hole at 428.2 mbsf. Several attempts to get
past the obstruction failed thus, UBI data were only acquired
from 350 mbsf to 428.2 mbsf.
(3) FMS/sonic tool string consisted of the LEH-MT with sensors
for measuring SP, temperature and tension, the SGT, the dipole
sonic imager (DSI), the GPIT, and the FMS. The obstruction
was found at the same depth of 428.2 mbsf resulting in a
shortened logged interval. The sonic velocity logs contain
several isolated intervals of with noisy data, but most of
the logged interval seems to have reliable results (Figure
4).
(4) Rigging up procedures began with the deployment of a
generator injector seismic source (GI-gun) consisting of
a 45 in3 generator chamber volume and a 105 in3 injector
chamber volume and by placing several observers around the
ship for compliance with the IODP marine mammal policy. The
procedure included a 1-hr observation period prior to the
use of the seismic source where the Mate on watch and the
marine mammal observers on the aft end of the ship began
observations. Observations continued throughout the duration
of the seismic experiment and no marine mammals were sighted
within the 700 m safety zone. After the initial observation
period, the “soft start” procedure began with
the seismic source being fired at 30-sec intervals starting
at a pressure of 500-psi and gradually increasing the pressure
to the “operational” pressure 2000 psi over
a 30-minute period. During the WST experiment, the GI-seismic
source was operated at 2000 psi air pressure with a time
delay between the generator and injector shots of 40 ms.
As in the case of UBI and FMS/sonic deployments, the tool
could not pass the obstruction located at 428.2 mbsf and
the WST stations were spaced at 20-m from the top of the
obstruction to the bottom of the bottom hole assembly.
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| Logging Results |
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Figure
5. (A) WST stacked waveforms and (B) determination
of interval velocities for the 100-m section of open
hole that was logged.
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Resistivity, Porosity, Density, Gamma ray, and Spontaneous
potential
Most of the resistivity curves show values within the basement
ranging between 0.27 and 146 Wm. Among the set of resistivity
curves, the 10-in depth of investigation curves show lower
electrical resistivity values than the other resistivity
curves of the same vertical resolution. In general, all curves
follow similar trends.
Values of neutron porosity show a large range from 4 to
100%. Neutron porosity values are particularly high above
462 mbsf where the borehole is enlarged and lower in the
bottommost part of the hole where values are mostly between
5 and 20%, which represent values that are closer to porosities
measured on core samples (~2 to 9%).
Density values range from 1.23 to slightly over 3.00 g/cm³ over
the entire logged section of the borehole. Below 462 mbsf,
density values are between 2.5 and 3.0 g/cm³ and close
to the range of values measured on core samples, which average
2.78 ± 0.08 g/cm³. Above 460 mbsf density values
are lower due to the irregular and enlarged shape of the
borehole.
Total gamma ray values (HSGR) range from 5.3 to 13.2 gAPI.
Potassium values are low with values between 0 and 0.48 wt.%.
Thorium and uranium values are mostly between 0 and 1 ppm.
The slight increase in gamma ray values below 515 mbsf may
be caused by slightly higher alteration, which is the only
depth interval with highly altered rocks identified in the
cores samples. Typical secondary minerals in this interval
are saponite, iron hydroxides, and celadonite. Celadonite
may contain potassium and thus could increase the gamma ray
values.
SP values vary between –170 and 24 mV and tend to increase
within the enlarged borehole intervals while decreasing within
more massive intervals. Particularly, low-values are observed
above 378 mbsf.
Sonic Velocity
P-wave velocities range from 4000 m/s to 6000 m/s, and correlate
well with the average laboratory velocity measurements of
~5300 m/s that were obtained from core samples (Figure
4). S-wave velocities range from 2000 m/s to 3000 m/s.
However, several sections have anomalous velocities, especially
below 385 mbsf where both P- and S-wave velocities are low
because the borehole is enlarged and irregular. Although
the tool worked well, the processing of the waveforms was
not straightforward and further processing had to be done
onshore to improve the quality of the results.
Logging Units
A preliminary interpretation of the geophysical logs yielded
the identification of 21 logging units (Figure
4). Most of the logging units seem to be characterized
by massive sections bounded by fractured intervals. In Figure
4, the yellow shading parts represent the massive intervals
and the white shading parts represent mostly fractured intervals.
A few of the massive flow units (purple shading on the Figure
4) can also be identified in the downhole logs as slight
increases in electrical resistivity, low neutron porosity,
and high density values. Above 462 mbsf, pillow basalt units
are characterized by the enlarged borehole intervals however,
a pillow basalt unit below 474 mbsf is characterized by low-porosity
and high-density values with small spikes of high-porosity
and low-density that may be related to thin fractured intervals.
Vertical Seismic Profile (VSP)
The waveforms at the each WST station were stacked and a
travel times were determined from the first breaks of the
waveforms acquired at four stations (Figure
5). In some instances it was difficult to determine the
first break therefore, the median of the first break for
each stacked trace were also used to determine interval velocities.
The gradient of the travel time first break used to estimate
an interval velocity produced a result of ~5220 m/s whereas
the median yielded an interval velocity of 4990 m/s. Core
sample and sonic log measurements show a slightly higher
range of velocities and the difference may reflect the different
scales of core measurements, sonic logs, and seismic experiments.
Borehole Images (FMS and UBI)
The quality of the FMS and UBI images were poor because
of two main reasons. The section of the borehole that was
imaged is characterized by washouts and irregularities that
hinder the acquisition of high-resolution images. In addition,
the new heave compensating system used during Expedition
301 may have not been working properly.
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| Summary |
Overall, the logging data expand upon core-based observations
and provide in situ measurements at Site 1301. The Triple
Combo was the only tool string that provided reliable data
of the entire borehole revealing ideal places for Packer
experiments and allowing for the interpretation of several
logging units that correlated to physical and lithological
changes identified from core-based observations. The downhole
coverage obtained with the other tool string deployments
consisted of only 1/4 of the borehole’s total depth
because of a borehole obstruction. The VSP experiment obtained
the best results of any subsequent tool deployment allowing
for the estimation of the shallow basement velocity profile
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Gerardo J Itturino: Logging Staff Scientist, Borehole
Research Group, Lamont-Doherty Earth Observatory of Columbia
University, PO Box 1000, 61 Route 9W, Palisades NY 10964,
USA.
email: iturrino@ldeo.columbia.edu
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Additional Leg-related
publications:
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