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SOIL PHYSICS

Comparing the EM38DD and DUALEM-21S Sensors for Depth-to-Clay Mapping

Research Group Soil Spatial Inventory Techniques, Dep. of Soil Management, Faculty of Bioscience Engineering, Ghent Univ., Coupure 653, B-9000 Gent, Belgium

^* Corresponding author (Timothy.Saey{at}UGent.be).

Geophysical instruments show great potential for the detailed quantification of soil stratigraphy. In this study, two electromagnetic induction sensors were evaluated on their capacity to map small-scale variations of the depth to the interface (z_in) in a two-layered soil. On a 2-ha study site, z_in between the silty topsoil and the contrasting clayey subsoil was modeled first by relating the two apparent electrical conductivity (EC_a) measurements of the EM38DD sensor to observations of z_in obtained by augering. A substantial number of these calibration observations was needed, however, to account for the modeling parameters. To avoid this step, an entirely noninvasive procedure was proposed based on one survey with the DUALEM-21S sensor. This sensor simultaneously records four EC_a values with different coil configurations. These measurements correspond to four different depth response functions that allow modeling z_in without calibration observations. The only assumption was a two-layered soil profile. The z_in predictions were validated with 24 independent depth observations. Both procedures resulted in equal correlation coefficients (0.85) between predicted and measured z_in and average estimation errors (0.26 m). This indicated that both sensors allowed the accurate mapping of the depth to a contrasting textural layer. With the EM38DD, calibration observations are needed, whereas the four different coil conurations of the DUALEM-21S sensor provided sufficient information to predict the interface depth without augering.

Abbreviations: DOE, depth of exploration • EC_a, apparent electrical conductivity • EMI, electromagnetic induction • RMSEE, root mean squared estimation error