Brady Flinchum

Near-surface geophysics for informed water-management decisions in the Aṉangu Pitjantjatjara Yankunytjatjara (APY) lands of South Australia

The Aboriginal population of the Aṉangu Pitjantjatjara Yankunytjatjara (APY) lands in South Australia is dependent on groundwater for nearly all water needs. In that region, placement of wells in productive aquifers of appropriate water quality is challenging because of lack of hydrologic data and variable aquifer properties. It is desirable to have an improved ability to identify and evaluate groundwater resources in this remote region with cost-effective methods that make minimal impact on the environment. A project supported by the Society of Exploration Geophysicists program Geoscientists Without Borders tested a combined geophysical approach with airborne and ground-based data sets to locate a potential aquifer, confirm water content, and estimate the subsurface extent of the water-bearing zone. This hydrogeophysical approach was an effective means for exploration and evaluation of groundwater resources in APY lands generally, and it characterized a specific aquifer as a case study.

Validating Nevada ShakeZoning Predictions of Las Vegas Basin Response against 1992 Little Skull Mountain Earthquake Records

Over the last two years, the Nevada Seismological Laboratory has devel- oped and refined Nevada ShakeZoning (NSZ) procedures to characterize earthquake hazards in the Intermountain West. Simulating the ML 5.6-5.8 Little Skull Mountain (LSM) earthquake validates the results of the NSZ process and the ground shaking it predicts for Las Vegas Valley (LVV). The NSZ process employs a physics-based finite- difference code from Lawrence Livermore Laboratory to compute wave propagation through complex 3D earth models. Computing limitations restrict the results to low frequencies of shaking. For this LSM regional model the limitation is to frequencies of 0.12 Hz, and below. The Clark County Parcel Map, completed in 2011, is a critical and unique geotechnical data set included in NSZ predictions for LVV. Replacing default geotechnical velocities with the Parcel Map velocities in a sensitivity test produced peak ground velocity amplifications of 5%-11% in places, even at low frequencies of 0.1 Hz. A detailed model of LVV basin-floor depth and regional basin-thickness mod- els derived from gravity surveys by the U.S. Geological Survey are also important components of NSZ velocity-model building. In the NSZ-predicted seismograms at 0.1 Hz, Rayleigh-wave minus P-wave (R − P) differential arrival times and the pulse shapes of Rayleigh waves correlate well with the low-pass filtered LSM recordings. Importantly, peak ground velocities predicted by NSZ matched what was recorded, to be closer than a factor of two. Observed seismograms within LVV show longer du- rations of shaking than the synthetics, appearing as horizontally reverberating, 0.2 Hz longitudinal waves beyond 60 s after Rayleigh-wave arrival. Within the basins, the current velocity models are laterally homogeneous below 300 m depth, leading the 0.1 Hz NSZ synthetics to show insufficient shaking durations of only 30-40 s.

Critical Zone Structure Under a Granite Ridge Inferred From Drilling and Three‐Dimensional Seismic Refraction Data

1Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming, USA 2Wyoming Center for Environmental Hydrology and Geophysics, University of Wyoming, Laramie, Wyoming, USA 3Department of Earth Sciences, Dickinson College, Carlisle, Pennsylvania, USA 4Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA 5Earth, Planetary, and Space Sciences, University of California Los Angeles, Los Angeles, California, USA 6Idaho National Laboratory, Idaho Falls, Idaho, USA 7CSIRO Land and Water, PMB 2, Glen Osmond, Adelaide, SA 5064, Australia

Comparing Measurement Response and Inverted Results of Electrical Resistivity Tomography Instruments

ABSTRACT In this investigation, we compare the results of electrical resistivity measurements made by six commercially available instruments on the same line of electrodes to determine if there are differences in the measured data or inverted results. These comparisons are important to determine whether measurements made between different instruments are consistent. We also degraded contact resistance on one quarter of the electrodes to study how each instrument responds to different electrical connection with the ground. We find that each instrument produced statistically similar apparent resistivity results, and that any conservative assessment of the final inverted resistivity models would result in a similar interpretation for each. We also note that inversions, as expected, are affected by measurement error weights. Increased measurement errors were most closely associated with degraded contact resistance in this set of experiments. In a separate test we recorded the full measured waveform for a sing...

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

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.

Near-surface geophysics for informed water-management decisions in the Anangu Pitjantjatjara Yankunytjatjara (APY) lands of South Australia

The Anangu Pitjantjatjara Yankunytjatjara (APY) Lands of South Australia is an arid environment and the population relies largely on groundwater resources for potable water and agricultural needs. Historically, locating productive wells in the region has been hit-and-miss and even if a water source was found, the quality may be unreliable. In this project, we seek to improve the water security in the APY lands by demonstrating that surface Nuclear Magnetic Resonance (NMR) and Time-Domain Electromagnetic (TEM) geophysical measurements are able to map local aquifers and quantify ground water resources, thereby optimizing site selection for potential future wells. Surface NMR is directly sensitive to water and TEM measurements detecting the electrical conductivity structure and able to image the subsurface over large areas - all entirely non-invasively and with minimal risk of disturbing sites of importance to the local Aboriginals.

Surface NMR to Image Aquifer Properties in a Magnetic Subsurface

Surface Nuclear Magnetic Resonance (NMR) is a noninvasive geophysical technique providing the ability to image and investigate aquifer properties. In order to produce reliable images and interpretations of subsurface properties accurate modelling of the underlying physics is required. In magnetic environments, where the background magnetic field varies spatially, challenges can arise that lead to difficulty accurately modelling the excitation process and interpreting the signal’s time dependence. We demonstrate using field data collected in the Anangu Pitjantjatjara Yankunytjatjara (APY) Lands of South Australia that neglecting the influence of the magnetic environment can significantly alter the final images and interpretation of the subsurface structure and properties.

USING ERT TO LOCATE A HISTORICAL MINE TUNNEL

Existing tunnels in historical mine sites often serve as conduits for acid mine drainage thus, delineating them can be important in remediating these sites. In a recent field study, electrical resistivity tomography was successfully used to locate a tunnel in a former mining district in the Rocky Mountains of Colorado. Prior to the ERT survey, a combination of historical mine data and surface geophysical surveys were used to find the approximate location of the tunnel. This culminated in drilling of boreholes on both sides of the location of the tunnel. Cross-borehole ERT was then used to provide a final estimate of the tunnel location. Surface-to-hole ERT surveys were conducted near a second, known portion of the tunnel to determine the feasibility of finding the tunnel using only a single borehole. In addition to the ERT surveys, an electrode was placed inside the entrance to the tunnel and used to perform mise-a-la-masse (MALM) surveys with receiving electrodes at the surface and in the borehole.