Stephen Moysey

Improved extraction of hydrologic information from geophysical data through coupled hydrogeophysical inversion

Improved extraction of hydrologic information from geophysical data through coupled hydrogeophysical inversion A.C. Hinnell 1 , T.P.A. Ferre 1 , J.A. Vrugt 2 , J.A. Huisman 3 , S. Moysey 4 , J Rings 3 , and M.B. Kowalsky 5 Hydrology and Water Resources, University of Arizona, Tucson, AZ, 85721-0011 Center for Nonlinear Studies (CNLS), Mail Stop B258, Los Alamos, NM 87545 ICG 4 Agrosphere, Forschungszentrum Julich, 52425 Julich, Germany Environmental Engineering and Earth Sciences, Clemson University, Clemson, S.C. 29634 Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720 Abstract There is increasing interest in the use of multiple measurement types, including indirect (geophysical) methods, to constrain hydrologic interpretations. To date, most examples integrating geophysical measurements in hydrology have followed a three-step, uncoupled inverse approach. This approach begins with independent geophysical inversion to infer the spatial and/or temporal distribution of a geophysical property (e.g. electrical conductivity). The geophysical property is then converted to a hydrologic property (e.g. water content) through a petrophysical relation. The inferred hydrologic property is then used either independently or together with direct hydrologic observations to constrain a hydrologic inversion. We present an alternative approach, coupled inversion, which relies on direct coupling of hydrologic models and geophysical models during inversion. We compare the abilities of coupled and uncoupled

Chlorine-36, bromide, and the origin of spring water

Natural ratios of chlorine-36 (36Cl) to stable chlorine (i.e., 36Cl/Cl×10−15) vary in shallow groundwater of the United States from about 50 in coastal areas to about 1400 in the northern Rocky Mountains. Ratios lower than these indicate the presence of chloride (Cl−) that has been isolated from the atmosphere for hundreds of thousands of years, if not longer. Higher ratios, which can exceed 5000, usually originate from fallout from testing thermonuclear devices in the western Pacific in the 1950s. Natural mass ratios of chloride to bromide (Cl−/Br−) in precipitation vary in the United States from about 250 in coastal areas to about 50 in the north-central states. Lower ratios may suggest contamination from human sources. Higher ratios, which may exceed 2000, commonly reflect the dissolution of halite. Seawater has a Cl−/Br− ratio of 290. Both 36Cl and Cl−/Br− ratios have been measured in 21 samples of spring water collected from springs in 10 different states. Brackish water from Saratoga Springs area in New York has low values for both 36Cl and Cl−/Br− ratios. This indicates that a large component of the water has a very deep origin. Brackish water from Alexander Springs in Florida has a low 36Cl ratio but a high Cl−/Br− ratio similar to seawater. This suggests the addition of ancient seawater that may be trapped in the aquifer. Big Spring in Iowa discharges water with a very high Cl−/Br− ratio but a moderate 36Cl ratio. The high ratio of Cl−/Br− may be produced by dissolution of road salt or agricultural chemicals. Of the 21 springs sampled, only 10 appeared to have potable water not significantly affected by human activity. Chlorine-36 from testing of nuclear devices is still being flushed out of four of the spring systems that were sampled. Thus, more than 45 years have passed since 36Cl was introduced into the aquifers feeding the springs and the systems, as yet, have not been purged.

Accounting for spatially variable resolution in electrical resistivity tomography through field-scale rock-physics relations

A number of issues impact electrical resistivity tomography (ERT) inversions: how ERT measurements sample the subsurface, the nature of subsurface heterogeneity, the geometry selected for data collection, the choice of data-misfit criteria, and regularization of the inverse problem. Lab-scale rock-physics models, typically used to estimate solute concentration from ERT, do not accommodate or account for these issues and therefore produce inaccurate geophysical estimates of solute concentrations. In contrast, the influence of measurement sensitivity and inversion artifacts can be captured by pixel-based rock-physics relationships, determined using numerical analogs that recreate the field-scale ERT experiment based on flow and transport modeling and a priori data. In the 2D synthetic example presented here, where ERT is used to monitor the transport of a saline tracer through the subsurface, improved estimates of concentration are obtained when field-scale rock-physics relationships based on numerical analogs are used.

A Stochastic Approach to Model Dynamic Systems in Life Cycle Assessment

This article presents a framework to evaluate emerging systems in life cycle assessment (LCA). Current LCA methods are effective for established systems; however, lack of data often inhibits robust analysis of future products or processes that may benefit the most from life cycle information. In many cases the life cycle inventory (LCI) of a system can change depending on its development pathway. Modeling emerging systems allows insights into probable trends and a greater understanding of the effect of future scenarios on LCA results. The proposed framework uses Bayesian probabilities to model technology adoption. The method presents a unique approach to modeling system evolution and can be used independently or within the context of an agent‐based model (ABM). LCA can be made more robust and dynamic by using this framework to couple scenario modeling with life cycle data, analyzing the effect of decision‐making patterns over time. Potential uses include examining the changing urban metabolism of growing cities, understanding the development of renewable energy technologies, identifying transformations in material flows over space and time, and forecasting industrial networks for developing products. A switchgrass‐to‐energy case demonstrates the approach.

Hydrologic trajectories in transient ground-penetrating-radar reflection data

A variable-rate infiltration experiment was conducted in a sandbox to demonstrate that distinctive patterns are produced in transient ground-penetrating-radar (GPR) data collected during wetting and drying events. The observed GPR response was found to be very consistent with the results of numerical simulations performed using finite-difference time-domain modeling of GPR coupled with a 1D unsaturated flow model (HYDRUS-1D) for which the sand hydraulic properties were determined independently using core samples. Despite this agreement, few methods are available that can efficiently analyze transient GPR data to make a quantitative link between observed responses and the hydraulic properties of soils. To address this problem, a computationally efficient method is proposed that is analogous to coherency analysis used in multioffset surveys. The new method isbased on the calculation of semblance along trajectories through transient GPR data. Each trajectory represents a specific GPR arrival, e.g., the groun...

Texture-based classification of ground-penetrating radar images

Image texture is one of the key features used for the interpretation of radar facies in ground-penetrating radar (GPR) data. Establishing quantitative measures of texture is therefore a critical step in the effective development of advanced techniques for the interpretation of GPR images. This study presents the first effort to evaluate whether different measures of a GPR image capture the features of the data that, when coupled with a neural network classifier, are able to reproduce a human interpretation. The measures compared in this study are instantaneous amplitude and frequency, as well as the variance, covariance, Fourier-Mellin transform, R-transform, and principle components (PCs) determined for a window of radar data. A 50-MHz GPR section collected over the William River delta in Saskatchewan, Canada, is used for the analysis. We found that measures describing the local spatial structure of the GPR image (i.e., covariance, Fourier-Mellin, R-transform, and PCs) were able to reproduce human interp...

Relating Geophysical and Hydrologic Properties Using Field-Scale Rock Physics

Understanding how the parameters estimated in a geophysical investigation are related to hydrologic properties of interest is an important part of a hydrogeophysical study. This problem is often tackled using rock physics to determine how the pore-scale properties of a medium, such as mineralogy, fluid content, and grain geometry, affect the geophysical response of a rock or sediment. It is then often assumed that these pore-scale rock physics insights can be used to interpret field-scale data. However, upscaling rock physics information is not straightforward. For example, the data obtained from an individual geophysical measurement in the field represents an average of local variations in pore-scale properties. Preferential sampling in heterogeneous environments can cause the field-scale relationship between geophysical and hydrological parameters to shift away from that determined at the pore scale, particularly in cases where property variations occur on spatial scales between the pore scale and the geophysical measurement scale. In addition to the sampling physics of individual measurements, spatial variations in the resolution of a geophysical survey can also impact the relationship between geophysical and hydrologic properties at the field scale; resolution is impacted by a number of factors, including the parameters defining the design and inversion of a field-scale geophysical survey. As a result, there is a need for methods that integrate variations in pore-scale rock properties with an understanding of geophysical sampling at the field-scale. To address this problem, we are using numerical analog models as tools to build field- scale rock physics relationships. Our approach allows for the flexible analysis of how factors like variations in geologic heterogeneity, changes in survey design, and uncertainty in subsurface properties and processes impact the relationship between geophysical and hydrologic properties. In this work we place an emphasis on describing how increasing non-linearity of geophysical estimation problems affects our ability to predict field-scale rock physics relationships. Specific examples discussed include the use of cross-borehole radar tomography for estimating water content and electrical resistivity tomography for monitoring the migration of a saline plume.

High-Resolution 4D Preclinical Single-Photon Emission Computed Tomography/X-ray Computed Tomography Imaging of Technetium Transport within a Heterogeneous Porous Media

A dynamic 99mTc tracer experiment was performed to investigate the capabilities of combined preclinical single photon emission computed tomography (SPECT) and X-ray computed tomography (CT) for investigating transport in a heterogeneous porous medium. The experiment was conducted by continuously injecting a 99mTc solution into a column packed with eight layers (i.e., soil, silica gel, and 0.2-4 mm glass beads). Within the imaging results it was possible to correlate observed features with objects as small as 2 mm for the SPECT and 0.2 mm for the CT. Time-lapse SPECT imaging results illustrated both local and global nonuniform transport phenomena and the high-resolution CT data were found to be useful for interpreting the cause of variations in the 99mTc concentration associated with structural features within the materials, such as macropores. The results of this study demonstrate SPECT/CT as a novel tool for 4D (i.e., transient three-dimensional) noninvasive imaging of fate and transport processes in porous media. Despite its small scale, an experiment with such high resolution data allows us to better understand the pore scale transport which can then be used to inform larger scale studies.

Evaluation of Surface Sorption Processes Using Spectral Induced Polarization and a (22)Na Tracer.

We investigate mechanisms controlling the complex electrical conductivity of a porous media using noninvasive spectral induced polarization (SIP) measurements of a silica gel during a pH dependent surface adsorption experiment. Sorption of sodium on silica gel surfaces was monitored as the pH of a column was equilibrated at 5.0 and then successively raised to 6.5 and 8.0, but the composition of the 0.01 M NaCl solution was otherwise unchanged. SIP measurements show an increase in the imaginary conductivity of the sample (17.82 ± 0.07 μS/cm) in response to the pH change, interpreted as deprotonation of silanol groups on the silica gel surface followed by sorption of sodium cations. Independent measurements of Na(+) accumulation on grain surfaces performed using a radioactive (22)Na tracer support the interpretation of pH-dependent sorption as a dominant process controlling the electrical properties of the silica gel (R(2) = 0.99) and confirms the importance of grain polarization (versus membrane polarization) in influencing SIP measurements of silicate minerals. The number of surface sorption sites estimated by fitting a mechanistic, triple-layer model for the complex conductivity to the SIP data (13.22 × 10(16) sites/m(2)) was 2.8 times larger than that estimated directly by a (22)Na mass balance (5.13 × 10(16) sites/m(2)), suggesting additional contributions to polarization exist.

Recognizing stakeholders in the design of effective community-level geophysics programs

Despite the challenges involved in implementing “humanitarian geophysics” projects, the potential impact of these projects is vast—particularly for engaging the public and encouraging a new and diverse generation of students to pursue careers in geophysics. Established programs like Engineers without Borders and emerging programs like Geoscientists Without Borders (GWB), which is administered through the SEG Foundation, provide unprecedented opportunities for geoscientists to become engaged in projects with a humanitarian focus. Many of us are excited by the prospect of using our knowledge and skills to make impacts on communities in a direct and immediate way. As scientists, it is easy for us to focus on addressing the technical issues of a project, such as selecting appropriate measurement methods and overcoming the daunting logistics of field operations—particularly if the project is in a foreign country. To create effective programs, however, it is equally important for us to consider nontechnical iss...