EGU General Assembly 2020
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Mapping cation exchange capacity using a quasi-3d joint-inversion of EM38 and EM31 data

Dongxue Zhao and John Triantafilis
Dongxue Zhao and John Triantafilis
  • University of New South Wales, Faculty of Science, School of Biological, Earth and Environmental SciencesSchool of Biological, Earth and Environmental Sciences, Australia (

The cation exchange capacity (CEC, cmol(+)/kg) is a measure of soil’s capacity to retain exchangeable cations. However, it is expensive to collect CEC across a heterogenous field and at different depths. To value-add to limited data, proximal sensed electromagnetic (EM) data has been coupled to CEC through linear regression (LR) models, because they measure apparent soil electrical conductivity (ECa, mS/m). However, these LRs have been depth-specific. This approach was compared with one universal LR between estimates of true electrical conductivity (s, mS/m) and CEC from various depths, including topsoil (0-0.3 m), subsurface (0.3-0.6 m), shallow subsoil (0.6-0.9 m) and deeper subsoil (0.9-2.1 m). We estimated s from inversion of EM38 and EM31 ECa either alone or in combination (joint-inversion), in horizontal (ECah) and vertical (ECav) modes, using a quasi-3d (q3-d) inversion software (EM4Soil) and various parameters, including EM38 at two different heights (i.e. 0.2 or 0.4 m). In terms of performance, the LR correlation (R2 > 0.60) was largest between deeper subsoil CEC and EM38 ECah at 0.2 m. However, the LR was unsatisfactory for CEC calibration in the topsoil (0.31), subsurface (0.37) and shallow subsoil (0.52). In comparison, a universal LR between CEC and σ was well correlated (0.72), when both EM38 (0.2 m) and EM31 ECa in both modes, were inverted using a forward model (CF), inversion algorithm (S2) and small damping factor (λ = 0.03). The calibrations tested using a leave-one-out cross validation, showed CEC prediction was precise (RMSE, 2.35 cmol(+)/kg), unbiased (ME, -0.002 cmol(+)/kg) with good concordance (Lin’s, 0.83). To improve areal prediction, closer spaced transects need to be collected, while improved vertical resolution of CEC prediction we recommend DUALEM-421 ECa data be acquired. 

How to cite: Zhao, D. and Triantafilis, J.: Mapping cation exchange capacity using a quasi-3d joint-inversion of EM38 and EM31 data , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-918,, 2019

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