USGS Groundwater Information: Branch of Geophysics
Lanbo Liu, University of Connecticut;
Frederick D. Day-Lewis, Bucknell University and U.S. Geological Survey; and
John W. Lane, Jr., U.S. Geological Survey
Computer forward modeling of borehole radar data for a series of synthetic discrete fracture network (DFN) models provides a conceptual framework for interpretation of field experimental data. A series of synthetic examples demonstrates the utility of single-hole reflection-mode and cross-hole transmission-mode radar data for (1) identification of fracture location and orientation, and (2) identification of the fracture pore-fluid, which might change as a result of tracer tests or remediation processes. A two-dimensional, synthetic DFN was generated statistically based on assumed distributions of fracture length, orientation, aperture, permeability, and inter-connectivity. The DFN includes a zone of permeable fractures embedded within a network of lowerpermeability fractures and low-permeability rock matrix. We modeled the isolated, non-permeable fractures as being filled with freshwater. To simulate tracer experiments, contaminant releases, or engineered-remediation processes, we alternately considered the inter-connected, permeable fractures to be filled with freshwater, saline water (tracer), dense non-aqueous phase liquid (contamination), air (steam), or hydrogen releasing compound. Synthetic radar data sets for both single-hole reflection and cross-hole transmission modes were generated by simulating electromagnetic (EM) wave propagation using a finite-difference time-domain (FDTD) forward model. The features in synthetic radargrams were then examined and compared to the DFN model to evaluate the likelihood of identifying fracture location, orientation, and pore fluid in field situations. This comparison demonstrates that (1) the replacement of freshwater with saline water in permeable fractures generally diminishes the amplitude of radar waves; (2) the replacement of freshwater with air, rather than saline water, results in stronger geophysical fracture signatures, thereby facilitating identification of permeable fractures in the radargrams; and (3) in general, radar reflection-mode data contain more information about fracture properties than transmission-mode data.
Using insights from our synthetic examples, we interpret single-hole reflection-mode field radar data from the U.S. Geological Survey Fractured Rock Hydrology Research Site near Mirror Lake, in Grafton County, NH. Radar data were collected before and during saline tracer experiments conducted to illuminate permeable fractures for geophysical identification. The combination of computer modeling and field data reduction is shown to be an effective approach to the identification of the locations and orientations of permeable fractures.
Final copy as submitted to 2004 U.S. EPA/NGWA Fractured Rock Conference for publication as: Liu, Lanbo, Day-Lewis, F.D., and Lane, J.W., Jr., 2004, Fracture characterization using borehole radar: computer simulation and field calibration, in 2004 U.S. EPA/NGWA Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 13-15, 2004, Portland, Maine. Proceedings: National Ground Water Association, CD-ROM, p. 523.
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