Electrical resistivity imaging for characterizing dynamic hydrologic systems.

Electrical resistivity imaging (ERI) is widely used in hydrogeophysical studies for monitoring spatiotemporal variations in hydrologic properties and processes. Its applications to hydrologic settings found in sandy and other coarse-grained soils have been demonstrated. However, there has been limited use of the method for characterization of dynamic hydrologic systems such as those found in Vertisols (typical heavy-clay soils) and water layers in lakes. One reason for this is that principles that work well in sandy and loamy soils often produce erroneous results in clay soils. In addition, because of the dynamic nature of such systems, detailed empirical and computational studies are required to fully understand various properties, which vary spatially within a few meters or less, and temporally in less than few days. This dissertation investigates the effectiveness of ERI for characterizing dynamic hydrologic systems. Two specific questions are addressed: 1) Can spatiotemporal hydrologic variations in such systems be effectively characterized using ERI? 2) How accurately can the true resistivity distribution in the systems be determined? To address the first question, geoelectric studies of seasonal wetting and drying of a Texas Vertisol were carried out. Data processing involved inversion, temperature corrections and time-lapse analysis. In addition, a van Genuchten water retention function was incorporated into the study to estimate moisture flux. To answer the second question, theoretical and field geoelectric data from Lake Whitney, Texas, USA, were analyzed. Following an introduction to the research in chapter one, results of geoelectric studies of seasonal wetting and drying of the Texas Vertisol are presented in chapter two. Results reveal the seasonal hydrodynamics of the soil as they are controlled by micro-relief topography (gilgai) and cracks. In chapter three, time-lapse analysis and computations of the apparent moisture flux are discussed. This study shows that integrative hydrogeophysical and hydropedological method is a viable approach for visualizing moisture flux in soils. In chapter four, results of geoelectric studies in Lake Whitney are discussed with recommendations for advancing the ERI as a tool in limnological research for mapping freshwater zones within impacted lakes and water reservoirs. Chapter five presents brief summary and conclusion of the research.