An Improved Technique for Determining Soil Electrical Conductivity-Depth Relations from Above-ground Electromagnetic Measurements1

An improved method has been developed for determining the distribution of bulk soil electrical conductivity, EC0, through the soil from electromagnetic measurements taken at the soil surface with the Geonics Limited EM-38 device. Induced electromagnetic conductivity readings taken with the EM-38 device's coil configuration oriented parallel and then perpendicular to the soil surface provided sufficient information, when used with equations derived from geophysical instrumentation data, to produce a soil electrical conductivity-depth profile. The simplicity of this method further enhances the praiticability of the newly developed electromagnetic technique for field measurements of salinity and for saline seep diagnosis. Additional Index Words: soil salinity, soil resistivity, electromagnetic conductivity. Convin, D. L., and J. D. Rhoades. 1982. An improved technique for determining soil electrical conductivity-depth relations from above-ground electromagnetic measurements. Soil Sci. Soc. Am. J. 46:517-520. R Rhoades and Corwin (1981) have shown that bulk soil electrical conductivity, ECfl, of incremental depth intervals within the soil profile can be obtained from above-ground electromagnetic measurements of apparent soil electrical conductivity, EM, using multiple regression coefficients which relate electromagnetic conductivity to ECa. This initial method required the solution of a complex system of simultaneous equations. The coefficients of these equations were determined by multiple regression analyses of EM readings taken at five incremental heights (0, 0.3, 0.6, 0.9, and 1.2 m) above the soil and of ECa values measured (using a four-electrode probe) at corresponding depths (0 to 0.3, 0.3 to 0.6, 0.6 to 0.9, and 0.9 to 1.2 m) in the soil. It is the purpose of this paper to describe a less complicated method for obtaining ECa-depth relations of any soil from only two EM measurements taken at the soil surface. In 1 Contribution from the U. S. Salinity Laboratory, USDA, Riverside, CA 92501. Received 9 Sept. 1981. Approved 27 Jan. 1982. 2 Soil Scientist and Supervisory Soil Scientist, respectively. 3 Geonics Limited, 1745 Meyerside Drive, Mississauga, Ontario. Canada. this approach the EM readings are related to ECa by a series of simple equations derived from instrumentation data provided by the manufacturer of the EM device (EM-38).