Electromagnetic induction (EMI)

EMI is an established, non-invasive geophysical method to map near subsurface features in engineering and environmental science (e.g. McNeil, 1980; Doolittle and Brevik, 2014; Altdorff et al., 2016). The working principle of EMI based on a two-coil system, that generates a primary magnetic field and induces eddy currents in the soil and creates a secondary magnetic field. The ratio between the two magnetic fields can be related to the electric conductivity (EC) of the subsurface (Keller and Frischknecht, 1966). As EMI recorded integral EC values over a certain depth, its unit called apparent EC (ECa). The ease of use and the wide range of possible applications have made the use of EMI very attractive.

Figure 1 Land use and ECa: Example of the ECa pattern as they reflect the former land use (Pynn’s Brook agricultural Research Station, Pasadena, Newfoundland, Canada)

However, the measured ECa value is an integral over a depth integral and effected by a large range of different soil properties, such as clay content, soil water content, soil water electric conductivity and bulk density (Doolittle and Brevik, 2014; Altdorff et al., 2016). Also land use could have an active effect on the ECa readings because by alternating the effecting soil properties (Altdorff et al., 2018) (Figure 1). The interpretation of the ECa maps remains hence complex and depends on the test site and the application. The example of ECa maps in Figure 2 shows the vertical variation of the ECa; roughly the deeper the integral reached into the soil, the higher ECa values (the more electric conductive the soil).

Figure 2 Example of the ECa pattern from different pseudo depths (Pynn’s Brook agricultural Research Station, Pasadena, Newfoundland, Canada)

Measuring on different field days over a certain period of time can help to better understand the dynamic changes in the soil. As the ECa value is influenced by several soil properties, it is also able to record these soil properties non-invasively. Therefore, during calibration campaigns, the ECa values must be adjusted to the desired soil property, e.g. to the water content or the storage density of the soil. The EMI maps can only then be interpreted correctly.

References

Altdorff, D., Bechtold, M., van der Kruk, J., Vereecken, H., Huisman, J. A. (2016). Mapping peat layer properties with multi-coil offset electromagnetic induction and laser scanning elevation data. Geoderma, 261(Supplement C), 178-189. https://doi.org/10.1016/j.geoderma.2015.07.015.

Altdorff, D., Galagedara, L., Nadeem, M., Cheema, M., Unc, A. (2018). Effect of agronomic treatments on the accuracy of soil moisture mapping by electromagnetic induction. CATENA, 164, 96–106. https://doi.org/10.1016/j.catena.2017.12.036.

Doolittle, J.A., Brevik, E.C., (2014). The use of electromagnetic induction techniques in soils studies. Geoderma 223–225, 33–45. https://doi.org/10.1016/j.geoderma.2014.01.027.

Keller, G.V., Frischknecht, F.C. (1966). Electrical Methods Of Geophysical Prospecting. International Series of Monographs in Electromagnetic Waves. Pergamon Press, Oxford, New York. https://archive.org/details/electricalmethod00kell/page/n7/mode/2up