Mapping hydraulically permeable fractures using directional borehole radar and hole-to-hole tomography with a saline tracer

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Abstract

Reflection-mode borehole radar and transmission-mode radar tomograms image heterogeneity in the electromagnetic properties of rock. Heterogeneity may be produced by interfaces between different rock types, foliation, and fracturing. In crystalline rock, hydraulic flow is primarily through fracture networks rather than through the rock matrix. Borehole radar methods have been applied to help map flow paths in crystalline rock. Correlation of features identified in borehole radar reflection records and tomograms with hydraulic flow paths is generally uncertain because the records show responses to heterogeneity of all- kinds, not just to hydraulically permeable fractures. Even in lithologically uniform rock, it is often not possible to distinguish fractures of high hydraulic permeabilities from those with low permeabilities.

It is possible to “erase” signatures from lithologic interfaces and rock fabric to identify the signatures of hydraulically permeable fractures by using a saline tracer in fractured crystalline rock because the electrical properties of the rock, except for the fractures that are open to infiltration by the brine solution, remain the same after the injection of the brine and may be removed by examining differences. Saline tracer experiments were carried out in 1995, 1996, and 1997 in the FSE well field at the Mirror Lake fractured-rock hydrology research site in Grafton County, New Hampshire. Comparisons of results from directional radar reflection surveys to well-to-well difference attenuation tomography in the same pairs of wells show generally good correspondence between the location of radar reflections and attenuation anomalies. Our results demonstrate the advantage of using a saline tracer for before-and-after difference mapping of hydraulically permeable fractures in lithologically heterogeneous rock and the utility of the coordinated use of directional borehole radar and hole-to-hole radar tomography.

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Additional publication details

Publication type Conference Paper
Publication Subtype Conference Paper
Title Mapping hydraulically permeable fractures using directional borehole radar and hole-to-hole tomography with a saline tracer
Year Published 1998
Language English
Publisher Environmental and Engineering Geophysical Society
Contributing office(s) Office of Ground Water
Description 10 p.
Larger Work Type Book
Larger Work Subtype Conference publication
Larger Work Title Proceedings of the symposium on the application of geophysics to engineering and environmental problems
First page 379
Last page 388
Conference Title Symposium on the Application of Geophysics to Engineering and Environmental Problems
Conference Location Chicago, IL
Conference Date March 22-26, 1998
Country United States
State New Hampshire
County Grafton County
Other Geospatial Mirror Lake
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