Water content of unsaturated, fractured, crystalline rocks from electrical resistivity and neutron logging

Abstract

Changes in the water content of unsaturated, fractured, crystalline rocks were measured with electrical resistivity and neutron techniques. Boreholes drilled into a quartz monzonite near Lochiel, Arizona were logged periodically from October 1982 through August 1983; and a borehole drilled into a dacitic ash-flow tuff near Superior, Arizona was logged periodically from March 1983 through August 1983. The boreholes, drilled from existing mine tunnels, were situated above the water table and six meters below the surface at the Lochiel site and 80 meters below the surface at the Superior site. The resistivity and neutron data indicate changes in the water content of the quartz monzonite were in response to surface precipitation. The response time was approximately six weeks when the overburden was initially dry. The resistivity and neutron data also correlate with water level data measured in a nearby saturated borehole. Similar trends were not observed at the Superior site. The resistivity and neutron data from Superior indicate the water content of the dacitic ash-flow tuff did not change during this study.