Multiscale Investigation of Fracture Permeability Within a Single Borehole

Permeability controls mass and energy transfer through rocks and soils. Studies conducted in different hydrogeological environments suggest that permeability varies with measurement scale. In many cases this scale effect is evident from the comparison of permeability data obtained from: 1) core to borehole tests and 2) single borehole tests to larger scale pumping tests conducted between multiple boreholes. A pronounced permeability scale effect has been previously observed from the analyses of data from single- and cross-hole pneumatic injection tests in unsaturated fractured tuff at the Apache Leap Research Site (ALRS) in central Arizona, USA (Illman and Neuman, 2001; 2003; Vesselinov et al. 2001a-b). Here, I ask the question whether such an effect is seen at the ALRS when one examines permeability data collected at multiple measurement scales (0.5, 1.0, 2.0, 3.0, and 20 m) within a single borehole. Results from the multiscale single-hole pneumatic injection tests show that permeability decreases with scale for the 0.5 through 3.0-m scale tests. However, permeability increases with scale when the data are compared against the 20-m scale data, but the increase is considerably lower than what has been previously surmised at the site. I hypothesize that the suppression in the magnitude of scale effect is due to the decreased connectivity of the fracture continuum when tests are conducted within a single borehole at this site.