Monday, November 13, 2006 - 1:30 PM
100-3
Estimating Layered-Earth Conductivity and Thickness with Multiple-Array Electromagnetic Instruments.
Since 2000, dual-array electromagnetic (EM) instruments have been used to identify layering of electrical conductivity in the upper-few metres of the earth. For example, instruments with short EM-array lengths (e.g. 1 m) have been used to monitor the dynamics of conductivity within and underlying the root-zone, while longer instruments (e.g. 4 m) have been used to delineate conductivity plumes in shallow groundwater beneath well-drained soil. Newly introduced instruments with quadruple- and sextuple-arrays can provide more detailed information about conductive layering. The sextuple-array instrument has dual receivers at 1-, 2- and 4-m array lengths. The quadruple-array instruments have dual receivers at two of these array lengths. The detailed measurements from the multiple-array instruments can provide estimates of the conductivity and thickness of a surficial layer along with the conductivity of the underlying earth. Two cases show the quantitative potential of these estimates, (i) where a non-conductive layer from 0- to 2-m thick overlies a loam of moderate conductivity, and (ii) where well-drained loam of variable depth overlies carbonate bedrock. The estimates of layer conductivity, layer thickness, and underlying conductivity are obtained by adjusting these parameters automatically to minimize the discrepancy between theoretical responses and actual measurements. The contrast in conductivity between the surficial layer and the underlying earth is roughly one order-of-magnitude in the two cases. This suggests there is broad applicability for the multiple-array technique, for example, monitoring fluctuations in the concentration and depth of nutrients and moisture. The technique should be particularly effective in high-contrast situations, such as where well-drained soil overlies saline soil.
Recorded presentation