Interdisciplinary characterization of fracture systems at the US/BK site, Grimsel Laboratory, Switzerland

Abstract

We have implemented a characterization methodology for fractured rock sites that combines geophysical and geologic information to aid hydrologic modelling. This approach concentrates on fracture zones, which commonly dominate the hydrologic behaviour of large rock masses. This approach contrast with approaches that focus on individual fractures and are largely statistically-based. The general philosophy is to identify and locate the major fracture zones, and then establish the essential and recurring patterns in which fractures are organized with the zones. We make a concerted effort to use this information on fracture systematics to link site-specific geologic and geophysical information. The procedure generally is applied to a specific site in a four-step sequence. First, information on the region encompassing the site is assembled and a model of the geologic structure in the vicinity of the target site is prepared. The major structures that might intersect the site are identified in this stage. Second, detailed geologic mapping is conducted to define the structural systematics of the major fracture zones near the site and to gain insight into how fluid might flow along the zones. Third, a preliminary model of the major structures at the site is prepared using the regional information together with geologic mapping and borehole surveys along the target site perimeter. Finally, the model is refined based on geophysical techniques [such as vertical seismic profiling (VSP) and geophysical tomography] that can image a rock mass at distances of at least several tens of meters from a borehole. Results to date from the US/BK site at the Grimsel Rock Laboratory in Switzerland indicate that this approach can contribute significantly to the hydrologic evaluation of a fractured rock mass.