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An experiment was conducted in the G tunnel complex at the Nevada Test Site to evaluate geotomography as a possible candidate for in situ monitoring of moisture content near a heater in densely welded tuff. Alterant tomographs of electromagnetic permittivity at 200 MHz were made of the densely welded tuff in two planes near a heater which was used to simulate the thermal load of a nuclear waste container. When the 1‐kW heater was turned on, the tomographs indicated a rapid and strong drying adjacent to the heater. Moisture loss was not symmetric about the heater but seemed to be strongly influenced by heterogeneity in the rock mass. The linear character of many tomographic features and their spatial correlation with fractures mapped in boreholes are evidence that drying was most rapid along certain fractures. When the heater was turned off an increase in moisture content occurred around the heater and along the dry fractures. However, this process is much slower and the magnitude of the moisture increase much smaller than the changes observed during heating of the rock. The interpretation of the tomographs is preliminary until they can be processed without the restrictive assumption of straight ray paths for the signals through the highly heterogeneous rock mass. However, from these preliminary results we conclude that geotomography can provide useful moisture content information near a heater in densely welded tuff. This conclusion is based on the fact that the experiment results and their interpretation are self consistent and in good agreement with behavior expected of the heated rock mass.
Daily et al. (1989) studied this question.