ALBERT

Description: Accurate estimation of the hydrological properties of near-surface aquifers is important because these properties strongly influence groundwater flow and solute transport. Laboratory-based investigations have indicated that induced polarization (IP) properties of porous media may be linked, through either semiempirical or fully mechanistic models, to hydrological properties including hydraulic conductivity. Therefore, there is a need for field assessments of the value of IP measurements in providing insights into the hydrological properties of aquifers. A cross-borehole IP survey was carried out at the Boise Hydrogeophysical Research Site (BHRS), an unconsolidated fluvial aquifer that has previously been well-studied with a variety of geophysical and hydrogeologic techniques. High-quality IP measurements were inverted, with careful consideration of the data error structure, to provide a 3D distribution of complex electrical conductivity values. The inverted distribution was further simplified using k -means cluster analysis to divide the inverted volume into discrete zones with horizontal layering. Identified layers based on complex electrical conductivity inversions are in broad agreement with stratigraphic units identified in previous studies at the site. Although mostly subtle variations in the phase angle are recovered through inversion of field data, greater contrasts in the IP data are evident at some unit boundaries. However, in coarse-grained aquifers, such as the BHRS, the discrimination of mildly contrasting lithologic units and associated changes in hydraulic conductivity of one or two orders of magnitude are unlikely to be achieved through field IP surveys. Despite the difficulty of differentiating subtle differences between all units, overall estimates of hydraulic conductivity purely from our field IP data are typically within an order of magnitude of independently measured values.

Description: We used a recent ground-penetrating radar (GPR) methodology, early-time amplitude analysis, with the goal of monitoring changes in soil water content (SWC) in response to irrigation in clayey soils. We hypothesized that early-time analysis could be used to monitor changes in SWC in clay-rich soil where ground wave and reflection-based GPR methods traditionally fail. An overnight irrigation experiment was performed in a 20- by 14-m section of natural grassland at the Samford Ecological Research Facility in southeastern Queensland, Australia. Both GPR reflection surveys and ground wave velocity analysis were ineffective at the site due to the signal attenuation associated with the clay-rich soil. We collected daily GPR and time-domain reflectometry (TDR) data sets during a 5-d period in August 2014, with soil samples collected for gravimetric analysis at the conclusion of data collection. The GPR data display a clear response of the early-time signal amplitude to changes in SWC. The GPR data sets exhibit a strong correlation with SWC, as measured by TDR and gravimetric analysis of soil cores, which is consistent with the dependence of GPR early-time amplitude on relative permittivity. The results suggest that the early-time method can be used to obtain spatially distributed information on subsurface moisture content in clay-rich soils.