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In situ measurement of apparent soil electrical conductivity (ECa) is an important precision agriculture tool useful for determining spatial changes in soil properties. Three near-surface geophysical methods are available for rapid, continuous measurement of ECa in agricultural fields. These methods are electromagnetic induction (EMI), capacitively coupled resistivity (CCR), and galvanic contact resistivity (GCR). Acceptance for using geophysical methods to gauge spatial changes in soil properties hinges to a significant degree on there being consistency of the measured ECa spatial pattern between geophysical methods. Testing of all three methods was conducted on two adjacent test plots having fine-grained soils and during two time periods with dissimilar shallow hydrologic conditions. Different operational modes for each of the three geophysical methods were evaluated, including three primary electromagnetic field frequencies (8190, 14610, and 20010 Hz) used for the EMI method, four spacing distances (0.625, 1.25, 2.5, and 5.0 m) between the two dipoles employed with the CCR method, and two different Schlumberger electrode array lengths (0.7 and 2.1 m) utilized for the GCR method. Therefore, a total of nine geophysical method - operational mode combinations were tested.

Based on spatial correlation analysis, the areal ECa patterns measured by the nine method - operational mode combinations showed substantial similarity to each other, with one exception. The exception was the short electrode array mode of the GCR method that when paired with the various modes of the other two geophysical methods, exhibited an average correlation coefficient, r, that ranged between only 0.30 and 0.45. All other average r values for pairs of different geophysical method - operational mode combinations ranged between 0.62 and 0.97. Spatial correlation coefficients, for both test plots, between the same method - operational mode combination, but at two different times in which hydrologic conditions varied, ranged from 0.55 to 0.95 for eight of the nine method - operational mode combinations, with the GCR short electrode array having values of 0.32 and 0.58. Regarding the test plot EC average or median, there were substantial differences in values obtained by the three geophysical methods. Electromagnetic vertical sounding measurements along with results obtained by CCR, and GCR surveying, when combined, indicate that for both test plots, from the surface to a depth of a little over 2 m, soil electrical conductivity generally increased first and then decreased. Most importantly, although the measured ECa magnitudes vary between the three geophysical methods, results show that EMI, CCR, and GCR all provide useful and consistent information on soil electrical conductivity spatial patterns.