A Computer Study of the Correlation Between Aquifer Hydraulic and Aquifer Electric Properties

Abstract

Correlations between aquifer resistivity and aquifer permeability are examined as an improved method for freshwater aquifer exploration. Layered aquifer models were developed where permeabilities for each layer were obtained from a random distribution between reasonable limits. The permeabilities of the layers were then converted to resistivity layers by using a previously developed semi-empirical relationship between permeability and resistivity at the small sample level. Hence, the hydraulic model with layered permeabilities was converted to an electrical model with layered resistivities. Resistivities and permeabilities for the entire aquifer model were then calculate with analytical equations for linear flow parallel and perpendicular to layering. Trends were plotted from three hundred models for the four possible combination of these properties with respect to flow paths. Results showed that the best predictor of horizontal aquifer permeability in a horizontally layered aquifer, is the vertical or transverse aquifer resistivity. Horizontal or longitudinal aquifer resistivity can be used effectively to predict horizontal aquifer permeability only if the electric or hydraulic anisotropy is known. To compute aquifer properties for the spacially mixed case, where permeabilities were distributed as monomodal probability density functions, a finite difference computer program was developed. Trends of aquifer resistivity versus aquifer permeability were developed for the uniforms, exponential and lognormal permeability distributions. Flow geometry was approximately linear (quasi-linear). To relate the results of linear flow aquifer property trends more to the field situation, where pump tests determine aquifer permeabilities based on radial flow, and the current from an electric sounding moves from point source to point sink, radial and point to point flow geometries were used to compute aquifer properties where the aquifer was considered to be isolated from surrounding strata. Results showed that flow geometry does not make a significant difference in computing aquifer properties in spacially mixed isolated aquifers, yet may be very important for the layered case. For non-isolated aquifers, where current is refracted by surrounding strata, methods of obtaining linear flow aquifer resistivities by interpreting sounding curves for various formation resistivity stratifications are discussed. Results indicate that good correlations between aquifer resistivity and aquifer permeability are possible when formation stratifications are such that the aquifer sounding curve interpretations.