Application of electric and electromagnetic geophysical methods to detect fractures in unsaturated tuff

Abstract

Three electrical and electromagnetic geophysical methods were used to examine the moisture and permeability distribution and identify fractures in a partially saturated, fractured ash-flow tuff near Superior, Arizona. Experiments using ground penetrating radar (GPR) to locate targets with known orientations in the tuff were used to assess the angular limits of fracture detection from radar profiles. Identification of fractures from radar records appears to be limited to those with dips of less than 45°. The application of a single well electrical tracer (SWET) technique proved effective in identifying relative permeability in a borehole drilled in the tuff when compared to air permeability logs of the same borehole. Laboratory resistivity measurements of electrical resistivity were conducted to determine electrical resistivity/moisture saturation relationships. Twenty-seven tuff cores were measured at equilibrated suctions of 0 to 500 kPa. Resistivity values ranged from 50 to 1600 ohm-m.