Geophysical Sensors for Mapping Soil Layers – A Comparative Case Study Using Different Electrical and Electromagnetic Sensors

In agriculture, site-specific field management is based on several components including information regarding soil heterogeneity. Mobile geophysical sensors are useful tools to efficiently map the spatial distribution of physical parameters (e.g., electrical conductivity) for large areas (i.e., several hectares and more). In combination with the analysis of soil samples collected at selected points, these maps represent a database for decision-making, e.g., for programming and controlling fertilizer spreaders. In addition, multi-channel instruments not only provide data regarding lateral changes within a certain depth range, rather they also allow for reliable imaging of possible soil layers within the depth of investigation. The reliability of the geophysical parameter models (e.g., electrical conductivity model) is controlled by the conductivity and their contrasts and by the used sensor. A case study is presented in which the electromagnetic sensor DUALEM-21 is used at an agricultural field characterized by sandy soils (i.e., low electrical conductivity) including a relative homogeneous topsoil (i.e., with only minor differentiation). At our test site, the final geophysical parameter maps generated using DUALEM-21, are compared to the results obtained using a rolling electrode system (Geophilus) and ground-penetrating radar (GPR). All methods reveal similar patterns of soil heterogeneity. In addition, the conductivity-depth models resulting from kinematic surveys are in good agreement with those from conventional static electrical measurements and with the data from soil analysis of borehole samples. Thus, the results of our case study demonstrate that the DUALEM-21 can also be used successfully on sandy soils to map the lateral conductivity variation and possible layers within the root zone (i.e., the upper 1.5 m).