INTEGRATING BOREHOLE GEOPHYSICS AND HYDRAULICS IN THE CHARACTERIZATION OF FRACTURED AQUIFERS

Although there are numerous down-hole techniques for the characterization of fractures in situ, the local geophysical properties of fractures in the immediate vicinity of the borehole wall do not correlate very well with the hydraulic properties of fractures. High-resolution flowmeter logging combined with other geophysical measurements provides a way to define the hydraulic properties such as fracture-zone transmissivity and electrical conductivity of produced ground water for the individual fractures identified with the geophysical logs. Even when such combinations of logs provide unambiguous identification of water-producing fractures and effective estimates of fracture transmissivity, these results only apply to the immediate vicinity of the borehole. Previous studies show that the large-scale character of flow through fractured bedrock aquifers is controlled by the geometry of fracture connections rather than by the local hydraulic conductivity of individual fracture segments. These large-scale flow paths can be investigated by conducting cross-borehole flow experiments, where one borehole is repeatedly pumped for short periods, and the flow induced by that pumping measured in adjacent boreholes. The time-varying flow measured at stations above and between water-producing fractures in the observation borehole can be related to computed curves that are characteristic of parallel and series connections of fractures in the surrounding rock mass. In the case of parallel fracture connections, locations of response curves between limits computed for isolated and short circuited flow connections can indicate the presence of other cross-connecting fractures in the surrounding rock mass. Examples of cross-borehole flowmeter experiments used to characterize flow paths in fractured rock illustrate the effectiveness of this technique in defining fracture connects in situ, and in identifying the presence of otherwise undetected cross-connections in the surrounding rock mass.