Fracture Characterization Using Borehole Radar: Numerical Modeling |
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Authors: | Lanbo Liu |
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Institution: | (1) Department of Civil & Environmental Engineering, University of Connecticut, Storrs, CT 06269-2037, USA;(2) Snow and Ice Branch, Cold Regions Research and Engineering Laboratory, Hanover, NH 03755-1290, USA |
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Abstract: | Forward modeling of borehole radar data for a series of synthetic discrete-fracture network (DFN) models provides a conceptual
framework for interpreting experimental field data at fractured rock sites. A finite-difference time-domain (FDTD) radar wave
propagation model was developed for this purpose. Synthetic examples demonstrate the utility of single-hole reflection-mode
and cross-hole transmission-mode borehole radar for (1) identification of fracture location and orientation, and (2) identification
of fracture pore-fluid properties, which might change as a result of tracer tests or flooding exercises in support of resource
development or site remediation. A two-dimensional, synthetic DFN was generated statistically based on hypothetical distributions
of fracture length, orientation, aperture, permeability, and inter-connectivity. The DFN includes a zone of permeable fractures
embedded within a network of lower-permeability fractures and a low-permeability rock matrix. We modeled the unconnected and
non-permeable fractures as being filled with freshwater. To simulate tracer experiments, contaminant releases, or engineered-remediation
processes, we considered alternately the inter-connected, permeable fractures to be filled with freshwater air, or saline
water (tracer). Synthetic radar data sets for both single-hole reflection and cross-hole transmission modes were generated.
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 increases the amplitude of reflections from permeable, connected
fractures; and (2) in general, radar reflection-mode data contains more information about fracture properties than transmission-mode
data. |
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Keywords: | borehole radar discrete-fracture network finite-difference time domain method single-hole reflection mode |
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