Saturated water conductivity estimation based on X-ray CT images - Evaluation of the impact of thresholding errors

Abstract

X-ray computed tomography soil studies rely on image analysis procedures that commonly begin with a thresholding step which is prone to errors and leads to uncertainty in the deduced values of soil characteristics, e.g., total porosity, specific surface or simulated saturated water conductivity. In this paper, four 3D images of soil cores were thresholded using two different algorithms. Total porosity and specific surface were determined for binarized images whereas saturated water conductivity was numerically estimated using the Navier-Stokes equation-based modelling. The study shows that an erroneous thresholding step leads to uncertainty in the determination of soil pore system characteristics and saturated water conductivity estimation. The lowest relative error in the total porosity determination, which was obtained in our study, was 15%, and the highest 40%. The results of this study demonstrate that errors related to thresholding may have a huge impact on the estimation of saturated hydraulic conductivity in soils, easily reaching a relative error of 50% of the saturated water conductivity reference value. Even small shifts in the threshold level can cause huge changes in saturated water conductivity estimation (a threshold shift by 6.7% for sample 2 analysed in the study caused more than a two-fold increase in the estimated value of saturated hydraulic conductivity).