Abstract
As a basis for the electrical tomography of laboratory-scale rock samples (~ 10 cm), we developed a procedure for stable, multi-point, electrical measurement on rock samples that is effective even at high contact and sample resistance. Electrodes were strongly attached to the surface of high-resistivity rock using conductive and adhesive epoxy. Sustained current injection for long periods into high-resistance rocks was fulfilled using a constant direct current source with high internal resistance. Accurate voltage measurement across the high-resistance rock was accomplished by differential measurement using two high input resistance voltmeters. Measurements of high resistance also require a stable measurement environment: the temperature and humidity in the laboratory were controlled using an air conditioner, a humidifier, a dehumidifier, and a vinyl tent. Signal noise arising from human activities was eliminated by the remote operation of the measuring equipment and switching terminal. The proposed measurement procedure was evaluated in terms of the stability and accuracy of measured values and its applicability to electrical tomography. To assess measurement stability, we performed multiple measurements of a dry granite sample at various levels of absolute humidity. Our procedure recorded highly reproducible measurements under each humidity condition. The observed changes in measured values with absolute humidity indicate the importance of stabilising the temperature and humidity conditions in the laboratory. Applying our technique to multiple plastic samples with known resistivity confirmed its accuracy. To evaluate its applicability to electrical tomography, we measured the potential distribution on a dry granite surface in response to an injected current using a simple 40-electrode array. The potential distribution measured by our procedure agreed well with that predicted by forward modelling, demonstrating the robustness of our procedure in array measurements, and thus indicating its potential applicability to tomographic measurements for a variety of targets even under severe conditions including the relative dryness of ambient humidity.
Highlights
Electrical resistivity estimated through geo-electromagnetic observations is crucial to understanding underground strata and their compositions
Data obtained at seven timings (i.e. 60, 100, 200, 300, 400, 500, and 600 s) in the 600 s time-series were plotted: their invariance confirms the stability of the time-series data after the inrush current; we used the average of the last 100 s of the 600 s of data, as the end of the time-series is expected to have the least noise due to terminal switching
We propose a reliable procedure for stable multi-point electrical measurements on a rock sample with high contact resistance
Summary
Electrical resistivity estimated through geo-electromagnetic observations is crucial to understanding underground strata and their compositions. Electrical measurements of a variety of rock samples across a wide range of conditions and spatial scales are essential. Previous studies have measured the resistivities of various rock samples in a variety of conditions. Fuji-ta et al (2004) studied the electrical resistivity of granulite at 1.0 GPa and 300–890 K, and Fuji-ta et al (2007) investigated the electrical resistivity of gneiss at 1.0 GPa and up to 1000 K. These previous studies focused only on the bulk resistivity of rock samples. Few experiments have measured the internal resistivity structure of rocks, despite its potential usefulness in providing important information for the interpretation of electromagnetic survey results
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