62_CEG 4012 Notes Fall 2011 CEG 4012 Notes Fall 2011

62_CEG 4012 Notes Fall 2011 CEG 4012 Notes Fall 2011 -...

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X3 p. LAVER. v a. LAYERED EARTH INTERMEDIATE LAVERS an ,..... ----1111----< J')-----, Resistivity is affected by soil porosity, water content, type (clay/sand), mineral content, salinity and grain size distrib. Measured "apparent resistivity" (pa) may include different soil types. For equally spaced electrodes (Wenner configuration, distance "a" between 2rr.aV electrodes): Pa =-1--:* Use to determine layering, find water table or anomalies (cavities, objects, gravel). Move array with fixed spacing over an area - develop contour plot. Increase spacing to use as a vertical sounding tool. Depth to layer P,I---~-~ interface(s) somewhat subjective = 0.67a - 1.0a. o Resistivity Apply source current (I, amps), measure voltage drop (V, volts), calculate
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Unformatted text preview: ohms) which changes with length (L) and cross-sectional area (A) of (3-D) medium. (figures from USACOE manual) Calculate resistivity (ohm-cm,ohm-ft)- an intrinsic soil RA ~ property: p = L ELECTRODE ~PACING b. APPARENT RESISTIVITY Typical resistivity values: Wet to moist clay Wet to moist silty clays &amp; silts Moist to dry silts &amp; sands Well fractured to slightly fractured bedrock wI moist soil filled cracks Sand &amp; gravel wlsilt Slightly fractured bedrock wI dry soil filled cracks, Sand &amp; gravel wlsilt layers Massive bedded and hard bedrock, Coarse dry sand and gravel 5-100hm-ft 10-50 ohm-ft 50-500 ohm-ft 500-1,000 ohm-ft 1,000 ohm-ft 1,000-8,000 ohm-ft 8,000+ ohm-ft...
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