Judith Robinson

Imaging Pathways in Fractured Rock Using Three-Dimensional Electrical Resistivity Tomography

Major challenges exist in delineating bedrock fracture zones because these cause abrupt changes in geological and hydrogeological properties over small distances. Borehole observations cannot sufficiently capture heterogeneity in these systems. Geophysical techniques offer the potential to image properties and processes in between boreholes. We used three-dimensional cross borehole electrical resistivity tomography (ERT) in a 9 m (diameter) × 15 m well field to capture high-resolution flow and transport processes in a fractured mudstone contaminated by chlorinated solvents, primarily trichloroethylene. Conductive (sodium bromide) and resistive (deionized water) injections were monitored in seven boreholes. Electrode arrays with isolation packers and fluid sampling ports were designed to enable acquisition of ERT measurements during pulsed tracer injections. Fracture zone locations and hydraulic pathways inferred from hydraulic head drawdown data were compared with electrical conductivity distributions from ERT measurements. Static ERT imaging has limited resolution to decipher individual fractures; however, these images showed alternating conductive and resistive zones, consistent with alternating laminated and massive mudstone units at the site. Tracer evolution and migration was clearly revealed in time-lapse ERT images and supported by in situ borehole vertical apparent conductivity profiles collected during the pulsed tracer test. While water samples provided important local information at the extraction borehole, ERT delineated tracer migration over spatial scales capturing the primary hydrogeological heterogeneity controlling flow and transport. The fate of these tracer injections at this scale could not have been quantified using borehole logging and/or borehole sampling methods alone.

Scenario Evaluator for Electrical Resistivity Survey Pre-modeling Tool

Geophysical tools have much to offer users in environmental, water resource, and geotechnical fields; however, techniques such as electrical resistivity imaging (ERI) are often oversold and/or overinterpreted due to a lack of understanding of the limitations of the techniques, such as the appropriate depth intervals or resolution of the methods. The relationship between ERI data and resistivity is nonlinear; therefore, these limitations depend on site conditions and survey design and are best assessed through forward and inverse modeling exercises prior to field investigations. In this approach, proposed field surveys are first numerically simulated given the expected electrical properties of the site, and the resulting hypothetical data are then analyzed using inverse models. Performing ERI forward/inverse modeling, however, requires substantial expertise and can take many hours to implement. We present a new spreadsheet-based tool, the Scenario Evaluator for Electrical Resistivity (SEER), which features a graphical user interface that allows users to manipulate a resistivity model and instantly view how that model would likely be interpreted by an ERI survey. The SEER tool is intended for use by those who wish to determine the value of including ERI to achieve project goals, and is designed to have broad utility in industry, teaching, and research.

On Permeability Prediction From Complex Conductivity Measurements Using Polarization Magnitude and Relaxation Time

Geophysical length scales determined from complex conductivity (CC) measurements can be used to estimate permeability k when the electrical formation factor F is known. Two geophysical length scales have been proposed: (1) the specific polarizability cp normalized by the imaginary conductivity r00 and (2) the time constant s multiplied by a diffusion coefficient D1. The parameters cp and D1 account for the control of fluid chemistry and/or varying minerology on the geophysical length scale. We evaluated the predictive capability of two CC permeability models: (1) an empirical formulation based on r00 or normalized chargeability mn and (2) a mechanistic formulation based on s. The performance of the CC models was evaluated against measured k; and further compared against that of well-established k estimation equations that use geometric length scales. Both CC models predict permeability within one order of magnitude for a database of 58 sandstone samples, with the exception of samples characterized by high pore volume normalized surface area Spor . Variations in cp and D1 likely contribute to the poor model performance for the high Spor samples, which contain significant dolomite. Two observations favor the implementation of the r00-based model over the s-based model for field-scale k estimation: (1) a limited range of variation in cp relative to D1 and (2) r00 field measurements are less time consuming to acquire relative to s. The need for a reliable field-estimate of F limits application of either model, in particular the r00 model due to a high power law exponent associated with F.

Scenario Evaluator for Electrical Resistivity (SEER) Survey Design Tool

The USGS Scenario Evaluator for Electrical Resistivity (SEER) is a quick and simple Excel-based decision support tool practitioners can use to assess the likely outcome of using two-dimensional (2D) electrical resistivity imaging for site characterization and remediation monitoring. SEER features a graphical user interface (GUI) that allows users to manipulate a resistivity model and instantly view how that model would likely be interpreted by an electrical resistivity imaging survey. The SEER tool is intended for use by practitioners who wish to determine the value of including electrical resistivity imaging to achieve project goals, and is designed to have broad utility in industry, teaching, and research. This is the initial release.

Borehole Nuclear Magnetic Resonance (NMR): a valuable tool for environmental site management

Borehole nuclear magnetic resonance (NMR) is an emerging geophysical method being applied to hydrogeology investigations. NMR is a quantitative geophysical method that can be used to make in situ assessments of porosity, water content, mobile and immobile water fraction, and estimates of permeability. While borehole NMR is commonly used in the oil and gas industry, it is only recently that NMR tools have been designed for use in small-diameter boreholes that are typically used in groundwater studies. This video presents an overview of borehole NMR and example applications for environmental site management.