Collaborative Research: Hydrogeophysical Quantification of Hydraulic Conductivity from Electrical Measurements of the Effective Properties of Porous Media

We will conduct research on quantification of hydraulic conductivity (K) from complex conductivity (sigma*) measurements. We will study (1) coarse alluvial deposits of the Boise Hydrogeophysical Research Site (BHRS), and (2) finer glacial melt deposits. The (sigma*) contains information on (a) the interconnected pore volume, (b) the interconnected pore surface area, and (c) the pore throat size controlling flow. We will explore whether low frequency electrical parameters can provide proxies of these pore geometrical parameters used in K prediction based on percolation theory, as well as capillary tube models. Soils with a narrow grain size distribution exhibit a low-frequency peak in polarization theoretically related to a pore length scale. Models for K prediction based on percolation theory utilize a characteristic length scale. Our work will explore the effectiveness of K prediction based on percolation type theory using the pore length scale given by (sigma*) measurements. Soils that exhibit a broad grain size distribution are typically devoid of a polarization peak and instead exhibit a constant polarization over the frequency range of (sigma*) measurements. Models for K prediction based on capillary tube models rely on a proxy measure of the hydraulic radius of tubes, usually the measurable specific surface area per unit pore volume (Spor). Our research will explore whether the magnitude of the polarization can also be used to develop electrical models of K prediction. Laboratory studies will examine candidate petrophysical relationships linking (sigma*) to measures of the effective pore radius and (Spor). We will examine (1) the (r-) pore radius relation, where (r) is a relaxation time related to the peak in frequency (w) of the (sigma*(w)) polarization, and (2) the single frequency (sigma')-(Spor) relation. A theoretical framework for interpretation of (sigma*(w)) in terms of a complex surface conductivity ((sigma*)surf(w)), will be derived and its predictive capability evaluated by comparison with Darcy flow tests. Upscaling will be examined at the BHRS. Hydraulic conductivity estimates based on borehole (sigma*) profiles will be compared with K estimates from multi-level slug tests. Two strategies for inverting (sigma*) datasets for tomographic estimates of K are: (1) direct conversion of (sigma*) images to K images assuming a stationary K prediction equation, and (2) a structural inversion whereby the K zonation is estimated. These strategies will be assessed via comparison with spatial K distribution at the BHRS estimated from kriging of borehole-based K measurements, and available hydraulic tomography datasets. A Hydrogeophysics Workshop will be offered to Ph.D. students during Yr 3 of this project. We will also develop Honors student UnderGraduate (HUG) research experiences in Hydrogeophysics on the Rutgers-Newark (R-N) campus. This initiative, run in collaboration with the R-N Honors College, will provide 2-3 HUG stipends per semester. We will selectively target the unique minority population of the R-N campus. We will also accelerate ongoing efforts to make the BHRS a test bed for hydrogeophysics. Equipment purchased to conduct this research will be made available to the hydrological community via the Hydrologic Measurement Facility (HMF)-Geophysics module of the Consortium of Universities for the Advancement of Hydrologic Science (CUAHSI).