An Investigation of the Ability of Induced Polarization to Resolve Aquifer Heterogeneity in an Unconsolidated Sedimentary Aquifer (2011)

Numerous recent studies have sought to interpret induced polarization (IP) parameters as predictors of hydraulic conductivity (K). The majority of this work has been laboratory based, and focused on sediments that exhibit variations in surface area and K over orders of magnitude. Such studies demonstrate that orders of magnitude estimates of K are achievable from site‐specific K‐IP relationships. However, there is an increasing need to resolve K heterogeneity in near surface unconsolidated sedimentary aquifers that are vulnerable to contamination by anthropogenic activities. Laboratory and field‐scale induced polarization measurements have been performed at the Boise Hydrogeophysical Research Site (BHRS), a fluvial depositional environment located on a gravel bar adjacent to the Boise River, near Boise, Idaho. Hydrostratigraphy of this well characterized site, as defined from high resolution porosity logs and grain‐size analysis from multiple wells, consists of a sequence of four cobble‐dominated units overlain by a channel sand unit. Hydraulic conductivity determined from high resolution multi‐level slug tests varies by just over one order of magnitude. Spectral Induced polarization (0.001–1000 Hz) and constant head K lab measurements were performed on twelve samples obtained from locations spanning the five units. A meticulous packing procedure was developed whereby samples were reconstituted in 4″ diameter vessels that facilitated inclusion of the cobble framework. Cross‐borehole single frequency IP imaging was performed between borehole pairs radially distributed around a central borehole. Laboratory measurements showed that the reconstituted samples retained the porosity and electrical properties of aquifer volumes sampled with field‐scale measurements. Inclusion of the cobble‐framework in the reconstituted samples resulted in a strong porosity dependence and a significant (R2 = 0.7) Archie relation with a cementation exponent of 1.4. However, K measurements in the laboratory were not well representative of field‐scale measurements and no significant relationship between laboratory spectral IP parameters and K was observed. Cross‐borehole electrical images from the site exhibit resistivity contrasts that coincide with the interfaces between the cobble‐dominated units, and with the sand unit, when the regularization includes an anisotropy constraint to favor vertical contrasts between horizontal layers. Some of these interfaces also coincide with contrasts in the phase angles imaged across the site. However, as with the laboratory measurements the electrical sensitivity to hydrostratigraphy is mostly manifested in the resistivity, which is consistent with a predominantly porosity dominated hydrogeological system. Such observations have initiated development of stochastic geophysical inversion procedures that incorporate local (borehole) observations in an attempt to maximize the information content in field geophysical data. Our findings highlight the challenges to K estimation from IP measurements in such near surface depositional environments characterized by limited (yet important for defining flow and transport) K variations that are consistent with porosity changes within and across hydrostratigraphic units in places.