Rock &
Soil Laboratory
At
Berkeley Lab's Soil and Rock Properties Lab, electrical resistivity,
ultrasonic
wave propagation and hydraulic conductivity can be measured
in a triaxial cell equipped to measure all these parameters simultaneously.
Confining and axial stresses are set independently to
represent in-situ states of stress. The cell is designed to handle
samples
from 3" diameter Shelby tubes, using sample transfer techniques
developed from geotechnical practice. The sample is jacketed
with a flexible membrane, either latex, viton or teflon, depending
upon the sample texture and fluid composition. Sample length
is determined by considerations of ultrasonic wave attenuation
and the extent of stratification of the core. Typically, samples
of approximately 5 cm lengths are used, although different lengths
can be accommodated. The advantage of making these measurements
simultaneously on the same sample is that disturbance from
sample transfer between test cells is avoided; this a particular
concern
for unconsolidated samples. The endcaps of the test cell contain
1 MHz piezo-electric crystals for P- and S-wave transmission
and receiving, flow ports and pressure ports. Porous aluminum
plates between the sample and the endcaps provide even flow
distribution over the sample cross-section.
Electrical
resistivity is measured by the four-electrode technique. Both
faces of the aluminum plates are gold coated. Electrical current
is
driven through the outside faces and voltage drop is measured from
the inside faces; a GenRad 1692 RLC Digibridge supplies current
and measures voltage drops at five test frequencies varying from
100 Hz to 100 kHz. P- and S- wave propagation (velocity and attenuation)
is measured by the pulse-transmission technique. Voltage
pulses are generated by Cober Model 605P High Power Pulse
Generator (Cober Electronics, Stamford, Conn) and data is acquired
to a 40 MHz Gagescope data acquisition board (Gage Applied
Sciences Inc., Montreal, Quebec) installed in a PC. Hydraulic
conductivity is measured either by the constant head method
for more permeable samples, or by the falling head method for
tighter samples. Differential pressure across the column is measured
with variable reluctance transducers. When possible, site water
is used for the hydraulic
conductivity measurements to avoid dispersion of clays. Otherwise,
test water is made-up based upon a chemical analysis of the
site water. This apparatus has been used to measure the relationship
between hydraulic
conductivity and seismic wave propagation in undisturbed cores
of unconsolidated sands and clays (as well as the sensitivity of
seismic
wave propagation to the presence of organic liquid contaminants
in homogeneous sands.
In
addition to the above facilities in 2000 an high resolution X-ray
facility, (linear x-ray and CAT
scan) and NMR imaging facility was added for detailed core studies
and
simultaneous flow and transport studies.
In
2001 further capability will be added by linking these studies with
ultra-high resolution tomographic
work at the LBNL advanced light source (ALS). This will bring micron-resolution
imaging
capability to core scale investigation (10 nanometer to smaller
scale samples).
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