Hydraulic tomography in fractured bedrock aquifers using high-resolution borehole flowmeter measurements

Geophysical soundings and tomographs can provide two- and sometimes three-dimensional images of the structure of fractured rock masses, but regions of anomalous geophysical properties are only indirectly related to the hydraulic properties of rocks. Hydraulic tomography based on cross-borehole flow tests cannot rigidly constrain all possible fracture flow connections, but such tests provide direct measurements of hydraulic connections between boreholes. The combination of flow tomographs based on high-resolution flowmeter measurements and conventional geophysical tomography can provide information about the hydraulic connections and large-scale flow paths that cannot be obtained by either method alone. The effectiveness of this combination of flowmeter and geophysical measurements is illustrated by results obtained in cross-borehole flow measurements at Mirror Lake, New Hampshire, and a hydraulic fracture experiment near Boulder, Colorado.