Precise characterization of subsurface fractures (their orientation, aperture, distribution,<br>hydraulic conductivity, etc.) is critical to many geoscience sub-disciplines such as water resources<br>exploration and management, contamination remediation, underground construction, as well as<br>subsurface energy resource (hydrocarbon, geothermal, etc.) exploration and management. This paper<br>presents the forward modeling results of borehole radar signature of fractures generated by the<br>hydrogeological model of a fractured rock aquifer environment. A fracture system consists of interconnected,<br>permeable fractures filled with either freshwater, air, or brine and the isolated, nonpermeable<br>ones filled with only freshwater forms hydrogeological test model. The fracture groups were<br>statistically generated with different and desired features in space density, length, orientation and<br>aperture. Electromagnetic (EM) wave was generated with the finite-difference time-domain (FDTD)<br>forward modeling technique and propagated through the fractured rock aquifer models to form the<br>synthetic radar data sets. The features in radar syntheses were then examined and corresponded to<br>predefined fracture models. Based on the comparison, it shows that (1) the amplitude of radar waves<br>was generally diminished when brine replacing freshwater in the permeable fractures; (2) replacing<br>freshwater with air significantly increases the fracture fluid property contrast and results in significant<br>changes in some time records at certain transmitter-receiver configurations, depending on their relative<br>position to the permeable fractures; (3) in general, radar syntheses in reflection mode contains more<br>information on fracture properties than its transmission counterpart; (4) it is easier to identify more<br>fractures when air replacing water than brine replacing water. Combination of computer forward<br>simulation and field data reduction bears the hope to successfully characterize fractures for various<br>scientific and engineering purposes.


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