Mapping Hydraulically Permeable Fractures Using Directional Borehole Radar And Hole-To-Hole Tomography With A Saline Tracer

Reflection-mode borehole radar and transmission-mode radar tomograms image heterogeneity in the<br>electromagnetic properties of rock. Heterogeneity may be produced by interfaces between different rock<br>types, foliation, and fracturing. In crystalline rock, hydraulic flow is primarily through fracture networks<br>rather than through the rock matrix. Borehole radar methods have been applied to help map flow paths<br>in crystalline rock. Correlation of features identified in borehole radar reflection records and tomograms<br>with hydraulic flow paths is generally uncertain because the records show responses to heterogeneity of<br>all- kinds, not just to hydraulically permeable fractures. Even in lithologically uniform rock, it is often<br>not possible to distinguish fractures of high hydraulic permeabilities from those with low permeabilities.<br>It is possible to “erase” signatures from lithologic interfaces and rock fabric to identify the signatures of<br>hydraulically permeable fractures by using a saline tracer in fractured crystalline rock because the<br>electrical properties of the rock, except for the fractures that are open to infiltration by the brine solution,<br>remain the same after the injection of the brine and may be removed by examining differences. Saline<br>tracer experiments were carried out in 1995, 1996, and 1997 in the FSE well field at the Mirror Lake<br>fractured-rock hydrology research site in Grafton County, New Hampshire. Comparisons of results from<br>directional radar reflection surveys to well-to-well difference attenuation tomography in the same pairs<br>of wells show generally good correspondence between the location of radar reflections and attenuation<br>anomalies. Our results demonstrate the advantage of using a saline tracer for before-and-after difference<br>mapping of hydraulically permeable fractures in lithologically heterogeneous rock and the utility of the<br>coordinated use of directional borehole radar and hole-to-hole radar tomography.