Electromagnetic measurements have been performed at 300 MHz to evaluate the applicability of alterant geophysical tomography to delineate in situ flow paths in a welded tuff rock mass. The evaluation has involved a field experiment, which was supplemented by a test bed experiment and a computer simulation study. The field measurements were made before, during, and after a water-based dye tracer flowed through the rock mass. Alterant geophysical tomographs are compared with independent evidence: maps of fractures intersected by the measurement boreholes and dyed rock samples. Anomalies present in the tomograph match fractures mapped with a borescope. The locations of tracer-stained fractures coincide with the locations of most image anomalies. The core data show that some of the image anomalies represent flow through single fractures. Other geophysical anomalies required further evaluation because they existed where tracer-stained fractures were not observed. The field images were also evaluated using evidence from a computer simulation study and from a test bed experiment where the technique was evaluated under controlled conditions. The results from these supplementary tests suggest that the field test image anomalies without corresponding dyed fractures are unlikely to be artifacts and thus probably represent flow paths of the tracer through the rock. ¿ American Geophysical Union 1987