Calibrating electrical measurements of vadose zone properties
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NO FULL TEXT AVAILABLE. Access is restricted indefinitely. ----- Electrical resistivity imaging has the capacity to map changes in soil water content and quality in the vadose zone. However, interpretation of the images is difficult because of the numerous variables that influence the measured resistivity. The dominant variables include porosity, degree of saturation, pore water salinity, clay content and clay type. There have been numerous models published that combine these variables in order to predict the bulk electrical conductivity of any mixture. All these equations need to be calibrated against controlled samples in order to determine parameters that account for geometrical effects and grain surface conduction. Recently Glover et al. (2000) published an extension to Archie's equation to account for the nonlinear behavior seen in the relationship between pore water salinity and soil/rock bulk conductivity. In this thesis the Glover et al. (2000) model is extended to partial saturation conditions. Both the saturated and partially saturated models are then calibrated against published data. From the calibration, it is shown that both the electrical conduction along the surface of the soil particles (surface conductions) and the geometrical variables are correlated with the cation exchange capacity of the sediments. At two case study sites the correlation of the bulk soil electrical conductivity to the cation exchange capacity of the soils is used to constrain the predicted pore water salinity and soil moisture content. At Second Ponds Creek, Sydney, NSW, EM38 and resistivity imaging are used to map the location of stored salts in the soil profile at a housing development site that has a high risk of salt mobilization and potential urban salinity problems. The new partially saturated extension to the Glover et al. (2000) model is used to predict pore water salinity. At Reef Hills Park, Benalla, Victoria resistivity imaging is used to map changes in soil moisture due to a tree. Multiple resistivity images were recorded over two days to show the changes in water uptake by the tree throughout the day. At this location the partially saturated electrical conductivity mode is used to quantify the changes in soil moisture. The predicted changes from the resistivity imaging are compared to neutron logging measurements and show good agreement. The application of the new model in the evaluation of salinity and changes of soil moisture is possible wherever the required criteria are met. Furthermore, more, this relation can also be taken to estimate the changes in soil moisture caused by root uptake.
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