AN INTEGRATIVE APPROACH FOR MONITORING WATER MOVEMENT IN THE VADOSE ZONE

Electrical resistivity tomography (ERT) during the past few years has emerged as a potentially cost-effective, non-invasive tool for imaging changes of the moisture content in the vadose zone. Accuracy of the ERT survey, however, is subjected to debate because of uncertainty in its inverse modeling and spatially varying resistivity-moisture content relation. In this paper, an integrative inverse approach for ERT was developed to derive the best unbiased estimate of the moisture content distribution. Unlike classical ERT inverse approaches, this new approach assimilates prior information about the geological structure of a given geological medium, and sparse point measurements of the moisture content, electrical resistivity, and electric potential to directly estimate three-dimensional moisture content distributions, instead of the changes in moisture content in the vadose zone. The new approach also considers spatial variability of resistivity-moisture content relation. Numerical experiments were first conducted to investigate the effect of uncertainties in the prior information on the estimate. The study then examined the effect of spatially varying resistivity-moisture content relation on the interpretation of change of moisture content based on the change in electrical resistivity from the ERT survey. Finally, usefulness of the integrative approach was demonstrated by directly estimating moisture distribution in three-dimensional, heterogeneous vadose zones. Results show that the integrative approach can produce images of the true moisture content distributions, and that incorporation of point measurements of moisture content is essential to improve the estimate.