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The development of effective exploitation strategies requires scientifically sound
characterization of geothermal reservoirs. Surface geophysical
data are potentially the most powerful and cost effective means of exploration
and characterization, providing images of the subsurface structures prior
to extensive drilling.
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Complex Electrical Impedance
Resistivity can be measured as a function of frequency, stress, and temperature. The AutoLab system and software allows for both two electrode (four terminal pair) and true four electrode resistivity measurements. AutoLab processing software allows for corrections to be applied for changes in pore fluid temperature (which effects pore fluid conductivity) and for the effects of parasitic impedances that result from the cabling and electronics.Two electrode measurements can be made using either NER's Zmeter electrical impedance system or a Hewlett Packard HP4192A impedance analyzer. Two electrode measurements are best performed using an HP4192A impedance analyzer that connects to the coreholder via a connector and feed-through system that maintains the HP4192A's excellent guard driving capabilities, minimizing parasitic impedances at high frequencies. This allows measurements in our standard system at frequencies up to 4 MHz.
NER's Zmeter impedance analysis system is used to perform true four electrode measurements. A computer controlled function generator provides the input to a circuit including a switchable reference resistor in series with the sample. Two matched FET input amplifiers measure the voltage across the reference resistor and the sample. A digital oscilloscope records the signals from these amplifiers. The amplitude ratio and phase shift between the two signals is used to measure the complex electrical impedance of the sample. Integral computer control of the function generator and oscilloscope allows for automated sweeps through user defined frequency ranges in a single capture. Typical ZMeter system configuration allows for measurements to be performed at frequencies from 0.02 Hz to 100 kHz.
331 Olcott Drive Suite L1 White River Jct., VT 05001 USA tel: +1 802.296.2401 fax: +1 802.296.8333