A multidisciplinary fractured rock characterization study at Raymond fieldsite, Raymond, CA

A dedicated field site was developed and a suite of experiments were conduc ted in the Sierra Nevada foothills, near the town of Raymond, CA to develop and test a multi-disciplinary approach to the characterization of groundwa ter flow and transport in fractured rocks. A wealth of geologic, hydrologic and geophysical data was collected at the site using a variety of unique t ools. A cluster of nine approximately 90 m deep boreholes were drilled at t he site in a V-shaped pattern with an angle of 60 degrees, The boreholes ar e spaced 7.5, 15, 30 and 60 m from the central borehole. Various geophysica l and hydrologic tests were conducted in and between these boreholes. Integ ration of cross-hole radar and seismic tomography, borehole flow surveys an d images from a new digital borehole scanner indicated that groundwater flo w is mainly confined to a few sub-horizontal fracture zones. A unique suite of hydraulic tests were conducted, in which three to four intervals in eac h of the nine boreholes were isolated using pneumatic packers. Some 130 inj ection tests were conducted, and more than 4100 cross-hole transient pressu re measurements were obtained. A computer algorithm was developed to analyz e such massive interference data systematically. As a result of the analysi s, an image of the fracture connections emerged, which is consistent with t he geophysical data. High precision tiltmeters were effective in remotely c haracterizing the preferential flow path. Several radial convergent tracer tests were conducted by injecting a mixture of several conservative tracers and one sorbing tracer: deuterium, fluorescein, lithium bromide and polyst yrene micro-spheres. Some differences between the breakthrough curves are o bserved, which may be due to possible differences among so-called "conserva tive" tracers. Some characterization tools were found to be more effective than others in locating flowing fractures. However, no single tool was almi ghty. Characterization of fractured rock is extremely challenging and requi res a stepwise and well-thought approach, which is basically a good old sci entific approach. Prediction of transport based on the characterization res ults is even more challenging and one should always bear in mind that it is virtually impossible to uniquely characterize a fractured rock. (C) 2000 P ublished by Elsevier Science B.V.