W. Steven Holbrook

Estimating the water holding capacity of the critical zone using near-surface geophysics

Land and Water, Commonwealth Scientific Industrial Research Organisation (CSIRO), Adelaide, South Australia, Australia Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming Department of Geosciences, Virginia Tech, Blacksburg, Virginia Department of Earth Sciences, Dickinson College, Carlisle, Pennsylvania Correspondence Brady A. Flinchum, Land and Water, Commonwealth Scientific Industrial Research Organisation (CSIRO), Adelaide, South Australia, Australia. Email: brady.flinchum@csiro.au Funding information Division of Earth Sciences, Grant/Award Number: 1531313; Office of Experimental Program to Stimulate Competitive Research, Grant/Award Number: 1208909; Society of Exploration Geophysicists (SEG)

Spatial delineation of riparian groundwater within alluvium deposit of mountainous region using Laplace equation

Department of Civil and Architectural Engineering, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, USA Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA Department of Urban Science, Meijo University, 4‐102‐9 Yataminami, Higashi, Nagoya 461‐8534, Japan Department of Geology and Geophysics, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, USA Correspondence Noriaki Ohara, Department of Civil and Architectural Engineering, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, USA. Email: nohara1@uwyo.edu Funding information National Science Foundation

Geophysics and Geologic Hazards

Sinkholes in Florida pose significant geotechnical, engineering, and hydrogeological challenges for using the land in constructive ways. In some instances, the sinkholes may prove unstable, thus limiting the overburden stress that can be applied. Additionally, the sinkholes may provide a conduit for accelerated contaminant transport from surface activities. In this case study, we use electrical resistivity tomography (ERT) to understand the scope of sinkhole activity under a planned landfill. As part of their application, the landfill permit applicant submitted a dense network of parallel, twodimensional electrical resistivity profiles as described in the following. We provided an alternative, three dimensional analysis of this data set to enhance detection of subsurface sinkhole targets. Eighty five parallel resistivity lines spaced 6m (20ft) apart were coalesced into a large three-dimensional resistivity model to map the 14 hectare (35 acre) site. The results revealed that resistive sand-filled sinkholes could extend at least 30m (100ft) below ground surface with a diameter that ranged from 30 to 100m (100-300ft). The host conductive limestone was shown to have a complex undulating topography with eroded pinnacles. Using cone penetrometer technology (CPT), the edge of the limestone pinnacles were also shown to have significant raveling, which coincided with a narrow range of resistivity values. The implications of the correlation between direct characterization using CPT and indirect characterization with ERT suggest that raveling could cover as much as 17% of the site. Based on these findings, the site was determined to be ill suited for landfill construction.

NMR for Near-surface Investigations (Development and Applications)

In porous materials, susceptibility contrasts between the matrix and the pore fluid generate pore-scale inhomogeneities in the magnetic field that are referred to as internal gradients. Internal gradients impact NMR measurements, and can cause large errors in the calculated diffusion coefficient if they are not accounted for. The magnitude of the internal gradients is determined by the susceptibility contrast, the strength of the background magnetic field, and the pore geometry. We use statistical analysis to look for correlation between measured internal gradients and properties of sediment samples. The primary goal of this analysis was to identify parameters that could be used as predictors of internal gradient magnitudes. We measured internal gradients using two different NMR methods: Method 1 estimates an average effective gradient, and Method 2 calculates a distribution of effective gradients. The sediment properties that we consider are magnetic susceptibility, iron content, specific surface area, grain size, and measured NMR parameters including the mean log T2 and the T1/T2 ratio. In our preliminary analysis, conducted with data from 20 sediment samples, we observe linear trends between iron content and measured gradients, and between magnetic susceptibility and measured gradients. We also see that the mineral form of iron appears to impact the relationships between iron content, magnetic susceptibility, and internal gradients. The correlation observed between gradients measured with Method 1 and both the specific surface area and T2 could indicate that this method is biased by relaxation time; this relationship was not observed for the gradients measured with Method 2. We plan to collect data on more sediment samples to better understand these relationships and develop a model for the estimation of internal gradients. Such a model will enable us to include internal gradient values in diffusion coefficient calculations for a range of nearsurface applications.