Bradley J. Carr

Geophysical imaging reveals topographic stress control of bedrock weathering.

Bedrock weathering runs to the hills Fractures in bedrock drive the breakdown of rock into soil. Soil makes observations of bedrock processes challenging. St. Clair et al. combined a three-dimensional stress model with geophysical measurements to show that bedrock erosion rates mirror changes in topography (see the Perspective by Anderson). Seismic reflection and electromagnetic profiles allowed mapping of the bedrock fracture density. The profiles mirror changes in surface elevation and thus provide a way to study the critical zone between rock and soil. Science, this issue p. 534; see also p. 506 Geophysical survey data and stress modeling connect surface topography to Earth’s critical zone. [Also see Perspective by Anderson] Bedrock fracture systems facilitate weathering, allowing fresh mineral surfaces to interact with corrosive waters and biota from Earth’s surface, while simultaneously promoting drainage of chemically equilibrated fluids. We show that topographic perturbations to regional stress fields explain bedrock fracture distributions, as revealed by seismic velocity and electrical resistivity surveys from three landscapes. The base of the fracture-rich zone mirrors surface topography where the ratio of horizontal compressive tectonic stresses to near-surface gravitational stresses is relatively large, and it parallels the surface topography where the ratio is relatively small. Three-dimensional stress calculations predict these results, suggesting that tectonic stresses interact with topography to influence bedrock disaggregation, groundwater flow, chemical weathering, and the depth of the “critical zone” in which many biogeochemical processes occur.

DC Resistivity Monitoring of Potassium Permanganate Injected to Oxidize TCE In Situ

The U.S. Department of Energys Office of Science and Technology selected the X‐701B Site at the Portsmouth Gaseous Diffusion Plant, Piketon, Ohio, for a demonstration of in situ chemical oxidation. The goal was to oxidize trichloroethylene (TCE), present in both the soil and groundwater of the local Gallia aquifer, by circulating a solution of potassium permanganate (KMnO4) between two horizontal wells.Potassium permanganate is a salt. Laboratory measurements showed that the addition of 1.0% of potassium permanganate increased the fluid conductivity of a Portsmouth groundwater sample from 339 to 7,250 mS/m, or equivalently, decreased the electrical resistivity from 2.9 ohm‐m to 0.14 ohm‐m. Although the contaminated Gallia aquifer is only about 1.5 m thick at this site and is overlain by nearly 8 m of Minford clay, geophysical modeling showed that if the injection flooded a sufficient portion of the aquifer then the change in resistivity would be detectable from the surface using DC resistivity. Field mea...

Geophysical imaging of shallow degassing in a Yellowstone hydrothermal system

The Yellowstone Plateau Volcanic Field, which hosts over 10,000 thermal features, is the worlds largest active continental hydrothermal system, yet very little is known about the shallow “plumbing” system connecting hydrothermal reservoirs to surface features. Here we present the results of geophysical investigations of shallow hydrothermal degassing in Yellowstone. We measured electrical resistivity, compressional-wave velocity from refraction data, and shear wave velocity from surface-wave analysis to image shallow hydrothermal degassing to depths of 15–30 m. We find that resistivity helps identify fluid pathways and that Poissons ratio shows good sensitivity to saturation variations, highlighting gas-saturated areas and the local water table. Porosity and saturation predicted from rock physics modeling provide critical insight to estimate the fluid phase separation depth and understand the structure of hydrothermal systems. Finally, our results show that Poissons ratio can effectively discriminate gas- from water-saturated zones in hydrothermal systems.

Transformation of four-component vertical seismic profiling records from Kola superdeep borehole, Russia

Abstract Multicomponent vertical seismic profiling (VSP) provides valuable and reliable information about the geologic structure of the subsurface and is one of the best ways to study seismic anisotropy. However, some VSP tools have no orientation instrumentation, and additional preprocessing steps may be required. Using the example of the 1992 VSP survey on the Kola Superdeep Borehole, Russia, we describe a general approach to the data reduction for a VSP tool with any number of channels. Due to data redundancy provided by the four-component Kola VSP tool, we are able to increase signal-to-noise ratio, estimate coupling variations, perform geophone gain corrections, and carry out an additional quality control. We describe the computer implementation of the method based on the seismic processing system currently being developed. The program is able to apply tool-rotation correction based on the polarization properties of the first break. This rotation technique works well in the case of offset VSPs; whereas for zero-offset VSPs, we recommend more robust rotation procedure based on the use of the direct shear wave. We find that plotting of instantaneous polarization azimuths provides a useful device to control the performance of tool-correction approaches.

P- and SV-wave separation by polarization-dependent velocity filtering: application to vertical seismic profiles from Kola superdeep borehole, Russia

Abstract In multicomponent seismic surveys, separation of interfering compressional (P) and shear-wave (S) energy provides valuable information for the interpretation of mode conversions and seismic anisotropy. We present a technique for such a separation using a polarization-dependent velocity filtering. At the first step of the procedure, a linear two-channel filter is designed in the frequency-wavenumber domain, with filter coefficients determined by the angles of incidence of respective seismic waves. Due to several physical reasons, the filter is unstable near the high end of the P-wave spectrum. We stabilize the filter with the use of S-wave spectrum balancing. This operation results in a robust decomposition into P- and SV-responses, while retaining the full energy of the vector wavefield. We implement this method in a module of our seismic processing system. The module employs a versatile parametrization scheme, as well as structured data input/output. It is able to process VSP or surface data, with velocities varying by either depth or offset. Application of the module to the processing of an offset VSP from the Kola Superdeep Borehole demonstrates that the program successfully separates P- and S-wave phases in crustal VSP data.

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)

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.

Geophysical Imaging at the U.S. Critical Zone Observatories

Over the past two years, the Wyoming Center for Environmental Hydrology and Geophysics (WyCEHG) has imaged the subsurface at five CZO¹s: Calhoun, Boulder Creek, Eel River, Reynolds Creek, and Southern Sierra. Techniques applied include seismic refraction, electrical resistivity, downhole logging, ground-penetrating radar, magnetic gradiometry, EMI, and surface NMR. We will present results from these sites.