Hydrologic Imaging of Fractured Rock
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HYDROLOGIC IMAGING OF FRACTURED ROCK KENZI KARASAKI, ANDREW COHEN, PAUL COOK, BARRY FREIFELD, KEN GROSSENBACHER, JOHN PETERSON, AND DON VASCO Lawrence Berkeley Laboratory, 1 Cyclotron Rd., Berkeley, California, USA 94720.
ABSTRACT Various geophysical and hydrologic tests were conducted in a cluster of nine wells to image the hydrologic connections of a fractured rock mass. Results of intra-borehole flow surveys and cross-hole radar and seismic tomography surveys correlated very well, and indicated that there is a major feature at a depth of 30m. Systematic injection tests were conducted in all nine wells. Three to four intervals in each well were isolated using pneumatic packers. Each interval was equipped with a high resolution pressure transducer. Some 130 injection tests were conducted, and more than 4,100 cross-hole transient pressure measurements were obtained. A computer algorithm was developed to analyze such massive interference data systematically. As a result of the analysis, an image of the fracture connections emerged which is consistent with the geophysical data.
INTRODUCTION Yucca Mountain, Nevada, USA is a potential repository site for high-level radioactive wastes. The geology of the site consists of sequences of ash-flow and ash-fall tuffs. These rocks are, in general, extensively fractured. Characterization of a large volume of such rocks is extremely difficult because fractures are often highly heterogeneous and discontinuous. Conducting in-situ tests in such rocks is problematic. Test artifacts often obscure the real information that is being sought. For example, boreholes themselves may provide shortcircuiting pathways for pressure and tracers, or present significant amount of storage. Furthermore, the theory of flow and 0.0 transport in fractures is not yet well established. Analysis of test results and .5 SSE modeling are difficult, primarily because Sw2
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the flow geometry is not known a priori.
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To develop field testing techniques and analysis methods for characterizing
30Dm
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flow and transport properties of fractured Californial,2. rocks, a dedicated field site was
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Ninetheboreholes established near town ofwere Raymond, drilled
in a reverse V pattern with increasing spacing between boreholes (Figure 1). sw4 SE4 Questions that are being addressed at this ESD-2 0....site include: 1) How many boreholes and Figure 1 Well configuration at the Raymond tests are sufficient to characterize a given volume of rock? 2) Can we predict Field Site. Large dots and small dots indicate 25 radionuclide transport based on fracture cm and 15 cm diameter wells, respectively. 0
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geometry? 3) How can we average, simplify, and scale-up observations made at a smaller scale? 4) How can we improve field test methods so that test artifacts are minimized? and 5) How can we relate geometric fracture information from outcrops and boreholes to hydrology? In the present paper we will discuss the results of hydrologic imag
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