"The Lawrence Livermore National Laboratory is evaluating high resolution geophysical techniques for characterization of nuclear waste repository sites. This report presents the results of the first phase of this project. We describe the evaluation of a new geophysical technique used to map fractures remotely between boreholes: electromagnetic geotomography used in conjunction with water tracers. Salt water is forced into the fractured rock mass. This attenuates the high frequency electromagnetic waves used for probing. The locations in the rock where the salt water has induced signal losses are then mapped by geotomography. An experiment using this technique has been performed near Oracle, Arizona. A granitic rock mass was probed. The data obtained were reduced to gray level images, which show the calculated signal transmission properties of the rock mass. We analyzed these images and compared them with borehole geophysical data: neutron logs, acoustic velocity logs, caliper logs, and acoustic televiewer logs. Comparisons between the images and the borehole geophysical data suggest that geotomography has merit when used to map fractures in granite. Image anomalies, which can be indicative of fracturing along the borehole walls, usually coincide with geophysical log anomalies. Under the conditions of the Oracle experiment, available data indicate that clusters of fracture zones were detected. Single fractures were not detected. The thickness of the smallest recognizable fractured zone was 0.6 m (2 ft)."