Hydraulic tomography in fractured granite: Mizunami Underground Research site, Japan

Abstract

Two large‐scale cross‐hole pumping tests were conducted at depths of 191–226 m and 662–706 m in deep boreholes at the Mizunami Underground Research Laboratory (MIU) construction site in central Japan. During these two tests, induced groundwater responses were monitored at many observation intervals at various depths in different boreholes at the site. We analyze the two cross‐hole pumping tests using transient hydraulic tomography (THT) based on an efficient sequential successive linear estimator to compute the hydraulic conductivity (K) and specific storage (Ss) tomograms, as well as their uncertainties in three dimensions. The equivalent K and Ss estimates obtained using asymptotic analysis treating the medium to be homogeneous served as the mean parameter estimates for the 3‐D stochastic inverse modeling effort. Results show several, distinct, high K and low Ss zones that are continuous over hundreds of meters, which appear to delineate fault zones and their connectivity. The THT analysis of the tests also identified a low K zone which corresponds with a known fault zone trending NNW and has been found to compartmentalize groundwater flow at the site. These results corroborate well with observed water level records, available fault information, and coseismic groundwater level responses during several large earthquakes. The successful application of THT to cross‐hole pumping tests conducted in fractured granite at this site suggests that THT is a promising approach to delineate large‐scale K and Ss heterogeneities, fracture connectivity, and to quantify uncertainty of the estimated fields.