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Photo by: Roy Kaltschmidt
Kurt Nihei analyzes a core sample in the Rock & Soil Laboratory at Berkeley Lab Earth Sciences Division
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).