UNCONSOLIDATED SANDY AQUIFER PROPERTIES INFERRED FROM INDUCED POLARIZATION MEASUREMENTS

Electrical measurements (0.1-1000 Hz) were made on clay-free unconsolidated sediments from a sandy, alluvial aquifer in the Kansas River floodplain. The sensitivity of induced polarization (IP) measurements to lithological parameters, fluid chemistry and degree of saturation was assessed. IP measurements exhibit power law dependence on surface area and grain size within one order of magnitude of lithologic parameter variation. The strength of the grain size-IP relationship is effectively frequency independent between 0.1-100 Hz but depends on the definition of representative grain diameter. In this study d90, which defines the grain diameter of the coarsest sediments in a sample, is well correlated with imaginary conductivity determined from IP. Fluid conductivity of a NaCl solution (0.06 - 10 S/m) exerts only a weak control on IP in these sediments. The dependence of imaginary conductivity on fluid conductivity is consistent with competing effects of surface charge density and surface ionic mobility on interfacial polarization. Preliminary IP measurements suggest power law dependence on saturation. However, the imaginary conductivity saturation exponent is approximately only half that of the Archie bulk conductivity exponent. Application of cross-borehole IP imaging at the study site illustrates the potential of IP in lithologic characterization of unconsolidated sandy aquifers. A fining upward sequence of sediments is correlated with an upward increase in normalized chargeability (MN), a field IP parameter proportional to imaginary conductivity. Layers in the MN image correlate well with layers in the lithologic sequence as confirmed from drilling records.