In situ measurements of hydraulic properties of a shear zone in northwestern South Carolina

Subsequent to the initial impoundment of the Bad Creek Reservoir in northwe st South Carolina in January 1991, lake level fluctuations (up to 33 m d(-1 )) caused delayed (98 hours) correlative water level changes in an observat ion well (OW3) 250 m away. The bottom of the well is connected to the botto m of the reservoir by a shallowly dipping, 1 m wide shear zone. The amplitu de of water level changes in OW3 was 0.19 of that in the reservoir. The amp litude ratio, R, of the water level changes in OW3 to those in the reservoi r is a measure of the pore pressure transmitted through the shear zone. Ass uming one-dimensional diffusion of pore pressure in the shear zone between the reservoir and OW3, these values of time lag and amplitude ratio of the water level fluctuations changes yield estimates of 0.05 m(2) s(-1) for hyd raulic diffusivity and 1.1 x 10(-15) m(2) for permeability of the shear zon e. In subsequent years the time lag decreased to 72 hours and R increased t o 0.23, suggesting a nearly 60% increase in the two hydraulic parameters. W e interpret the increase to be owing to flushing of fines in the shear zone due to the pumping action of lake level fluctuations or due to dissolution . After 1994, there were no further changes in these parameters suggesting steady state conditions. Our analysis demonstrated that the fluid pressure flow is restricted to the shear zone and the pore pressure transmitted thro ugh the shear zone showed a lognormal relationship with the frequency of la ke level fluctuations, R 0.5 exp (-14.93/T), where T (days) is the period o f lake level changes. Spectral analyses of the data were used to obtain a f requency independent hydraulic diffusivity (similar to 0.076 m(2) s(-1)) of the shear zone after it had reached steady state conditions. Using the und rained response of the reservoir, an in situ estimate value 0.66 was obtain ed for the Skempton's coefficient. The hydraulic diffusivity similar to 0.1 m(2) s(-1) was found to be lower than that encountered at other locations of induced seismicity in the region.