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Comparison of Three Near-Surface Geophysical Methods
Used to Map Soil Electrical Conductivity

Summary:  Apparent soil electrical conductivity (ECa), mapped in situ with near-surface geophysical methods, can potentially be used to gauge spatial changes in soil fertility, since both are influenced by soil profile properties that include salinity, organic matter content, cation exchange capacity, grain size distribution, clay mineralogy, claypan/fragipan depth, etc.  Electromagnetic induction and two different pulled electrode array resistivity methods were used to map ECa on two different Columbus, Ohio agricultural test plots having fine-grained glacially derived soils.  The data collected showed that all three near-surface geophysical methods worked equally well when evaluated on these two test plots.  Although the average test plot ECa values differed between the three methods, spatial ECa patterns, when compared, were quite similar.  In addition, changing field conditions were found to impact the magnitude of the ECa values but not their spatial pattern.  Overall, these results suggest that comparable ECa maps can be obtained regardless of which near-surface geophysical ECa measurement method is used.  With more research, it is expected the near-surface geophysics will gain wider agricultural acceptance in the future.

 

Figure Descriptions  (a) Geophex, Ltd. GEM-2 multi-frequency ground conductivity meter, (b) Geometrics, Inc. OhmMapper TR1 pulled resistivity array, (c) Veris 3100 Soil EC Mapping System pulled resistivity array, (d) Soil electrical conductivity map based on Geophex, Ltd. GEM-2 measurements at 14610 Hz, (e) Soil electrical conductivity map based on Geometrics, Inc. OhmMapper TR1 pulled resistivity array measurements with dipoles of 5 m in length, spaced 2.5 m apart, (f) Soil electrical conductivity map based on Veris 3100 Soil EC Mapping System pulled resistivity array measurements - 90 cm depth.  (Note: All three soil electrical conductivity maps were from the same test plot, and the contour interval used was 4 mS/m.)