Measurement of Inverted Electrical Conductivity Profiles Using Electromagnetic Induction1

Due to sharp discrepancies in the relative response of the horizontal and vertical coil configurations of the Geonics Limited EM38 electromagnetic induction soil conductivity meter for the top 15 cm of soil, the accuracy of measurement of bulk electrical conductivity for soil having a high surface electrical conductivity relative to deeper depths (i.e., an inverted electrical conductivity profile) has been unreliable. Two new approaches have been developed to compensate for these discrepancies, either through compensations in the vertical coil configuration response curve or the reestablishment of EM0>H adjustment curves utilizing data solely from inverted conductivity profiles. Both approaches yield more consistently reliable calculated bulk soil electrical conductivities when compared to measured electrical conductivities using the four-electrode probe. The latter approach, however, appears to be more accurate. Additional Index Words: soil salinity, soil resistivity, electro-magnetic conductivity. Corwin, D. L., and J. D. Rhoades. 1984. Measurement of inverted electrical conductivity profiles using electromagnetic induction. Soil Sci. Soc. Am. J. 48:288-291. T POTENTIAL for the use of electromagnetic induction (EM) techniques as a means of performing reconnaissance surveys of soil salinity is self-evident. Williams and Baker (1982) measured apparent soil electrical conductivities (o-a) to depths ranging from 7.5 to 60 m for an area of 10 000 km using EM techniques. From these measurements areas of apparent high salinity were inferred. Others have demonstrated that soil bodies of widely differing salinity can be delineated by EM techniques (DeJong et al., 1979). Since plant root activity occurs primarily within the top 0.9 m of soil, the electrical conductivity of this portion is extremely important in assessing soil salinity from the standpoint of agricultural productivity. Therefore, once areas of potential soil salinity hazard are delineated, it is necessary to be able to survey these areas more intensively within relatively shallow soil depths (i.e., 0-0.9 m). In situ methods of measuring soil electrical conductivity within such depths are available (Rhoades, 1978; Rhoades, 1979), but remote measurements would offer advantages of reduced labor, time, and cost. We have previously shown that bulk soil electrical conductivities by depth increments through the soil profile can be determined with reasonable confidence from remote EM measurments using the Geonics Limited EM-38 instrument (Rhoades and Corwin, 1981; Corwin and Rhoades, 1982). Initially this required the solution to a complex system of simultaneous equations obtained for each general site from multiple regression analysis relating electromagnetic conductivity measurements to <ra (Rhoades 1 Contribution from the U.S. Salinity Laboratory, USDA-ARS, 4500 Glenwood Drive, Riverside, CA 92501. Received 21 Mar. 1983. Approved 14 Nov. 1983. 2 Soil Scientist and Supervisory Soil Scientist, respectively. 3 The citation of particular products or companies is for the convenience of the reader and does not imply any particular endorsement, guarantee, or preferential treatment by the USDA or its agents. and Corwin, 1981). Subsequently, we developed a simplified and more general method (Corwin and Rhoades, 1982) in which incremental depth response curves for the Geonic EM-38 instrument and "adjustments" to the readings to compensate for inequalities in profile volumes of measurement between vertical and horizontal coil configurations were used to calculate the distribution of <ra through the soil. The use of this new method on a wider variety of electrical conductivity profiles revealed, however, that the predicted bulk soil conductivities for inverted conductivity profiles (i.e., profiles where the electrical conductivity decreases rapidly with increased depth) consistently deviated from the corresponding "ground truth" conductivities as measured with the four-electrode probe (Rhoades and van Schilfgaarde, 1976). This fact pointed out an obvious insufficiency in the newly developed method. It is the purpose of this paper to present an alternative approach for the measurement of inverted conductivity profiles using electromagnetic induction. METHODS AND MATERIALS