Mapping Three-Dimensional Hydraulic Heterogeneities in Fractured Granite through Transient Hydraulic Tomography

Friday, 19 December 2014
Yuanyuan Zha1,2, Tian-Chyi J Yeh3, Walter A Illman4, Tatsuya Tanaka5, Patrick Bruines5, Hironori Onoe6, Hiromitsu Saegusa6 and Deqiang Mao7, (1)University of Arizona, Tucson, AZ, United States, (2)Wuhan University, Wuhan, China, (3)Univ Arizona, Tucson, AZ, United States, (4)University of Waterloo, Waterloo, ON, Canada, (5)Obayashi Corporation, Tokyo, Japan, (6)JAEA Japan Atomic Energy Agency, Toki, Japan, (7)Colorado School of Mines, Golden, CO, United States
Between 2005 and 2010, three independent sequential pumping tests were conducted in a fractured granite formation at the Mizunami Underground Research Laboratory (MIU) site in central Japan. Additional pumping operations were undertaken in the course of excavation of two vertical shafts at the site. During these events, groundwater responses were monitored in multiple observation intervals in several shallow and deep boreholes. In this study, we first visually inspected these responses at all observation intervals induced by each pumping event. We found that some intervals at far-distance boreholes showed rapid, and strong responses, while intervals of boreholes near the vicinity of the pumping locations showed little responses. Moreover, the locations of the rapid and slow responses varied with pumping locations. This preliminary inspection suggested that some boreholes are likely connected via fractures and some are separated by flow barrier(s). Subsequently, these drawdown data sets were analyzed using a three-dimensional (3-D) transient hydraulic tomography (HT) code to estimate the hydraulic conductivity (K) and specific storage (Ss) distributions. Results of the analysis depicted several large-scale high K and low K zones and some small-scale features at the MIU site. The locations of these high and low K estimates explain the observed fast and slow groundwater responses, and corroborate with the locations of fractures and fault zones estimated based on geologic information. The HT analysis nevertheless provides a detailed description of the hydraulic characteristics of the fracture and fault zones.