Geophysical Tomography for Imaging Water Movement in Welded Tuff-

Alterant tomography h*s been evaluated for its ability to de­ lineate itfsitu water flow paths in a fractured welded-tuff rock mass, The evaluation involved a field experiment in which tomographs of electromagnet attenuation factor (or attenuation rate) at 300 MHz were made before, during, and after the introduc­ tion to the rock of two different water-based tracers: a plain water and dye solution, and salt water and dye. Alterant tomographs were constructed by subtracting, cell by cell, the attenuation factors derived from measurements before each tracer was added to the roc!, mass from the attenuation factors derived after each tracer was -iuded. The alterant tomographs were compared with other evidence of water movement in the rock: borescope logs of fractures, and postexpenment cores used to locate the dye tracer on the fractured surfaces. These comparisons indicate that alterant tomography is suitable for mapping water flow through fractures and that it may be useful in inferring which of the fractures are hydrologjcally con­ nected in the image plane. The technique appears to be sensitive enough to delineate flow through a single fracture and to define fractures with a spatial resolution of about 10 cm on an imaging scale of a few meters.