CROSS-BOREHOLE FLOW ANALYSIS TO CHARACTERIZE HYDRAULIC CONNECTIONS IN FRACTURED GRANITE, BOHEMIAN-MORAVIAN HIGHLAND, CZECH REPUBLIC

Cross-borehole flow analysis was used to characterize the hydraulic connections between two boreholes completed in fractured granite in the Bohemian-Moravian Highland of the Czech Republic. Cross-borehole flow analysis is designed to determine the hydraulic characteristics of fracture connections between boreholes rather than those of discrete fractures that intersect a single borehole. The two boreholes, which were 200 meters deep and spaced 20 meters apart, were initially characterized through collection and analysis of a standard suite of geophysical logs and application of the borehole dilution method under ambient and pumped conditions. The cross-borehole flow test was conducted by measuring transient flow with a high-resolution flowmeter at depth stations located between the transmissive fractures in one of the boreholes during short-term pumping and recovery cycles in the other borehole. Insight into the character of the fracture connections was gained through comparison of measured flows and flows simulated by use of a numerical model with adjustable connection geometry and hydraulic properties. The presence of a primary and two secondary hydraulic connections, and their relative transmissivity and storage, inferred from the cross-borehole flow analysis were corroborated by tracer testing. The results of the cross-borehole flow analysis demonstrated that it is possible to evaluate the characteristics of a fracture-flow network despite being restricted to making measurements in boreholes in which a local population of discrete fractures controls hydraulic communication with the larger-scale aquifer system.