| Electrical resistivity methods were applied to characterize water-content distributions in the vadose zone of a fractured, ash-flow tuff sequence on the Apache Leap plateau near Superior, Arizona. Because fractures are conduits through which infiltration and drainage occur, the regional and local structural geometries were quantified by statistical analysis of field measurements and lineaments on aerial photos. Laboratory measurements of electrical resistivity and induced polarization were conducted on tuff cores equilibrated at suctions ranging from 0 to 500 kilopascals, and empirical coefficients of the Archie equation were calculated. Apparent resistivity profiles were also collected in the field using a Wenner array at electrode spacings of 0.9 and 1.8 meters. Laboratory resistivity values ranged from 50 to 250 ohm-meters, but field measurements locally reached values greater than 500 ohm-m where drained fractures interrupted the flow of electrical current. Field resistivities responded primarily to moisture distributions which were controlled by: (1) north and northwest-striking fracture zones, (2) topography, (3) depth, and (4) seasonal precipitation and infiltration. |
