Open AccessIn situ measurements of hydraulic properties of a shear zone in northwestern South Carolina
Author(s) -
Talwani Pradeep,
Cobb Jason S.,
Schaeffer Malcolm F.
Publication year - 1999
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/1999jb900059
Subject(s) - geology , pore water pressure , shear (geology) , shear zone , permeability (electromagnetism) , amplitude , flushing , water level , geotechnical engineering , geomorphology , hydrology (agriculture) , petrology , seismology , tectonics , chemistry , medicine , biochemistry , physics , cartography , quantum mechanics , endocrinology , membrane , geography
Subsequent to the initial impoundment of the Bad Creek Reservoir in northwest South Carolina in January 1991, lake level fluctuations (up to 33 m d −1 ) caused delayed (98 hours) correlative water level changes in an observation well (OW3) 250 m away. The bottom of the well is connected to the bottom of the reservoir by a shallowly dipping, 1 m wide shear zone. The amplitude of water level changes in OW3 was 0.19 of that in the reservoir. The amplitude ratio, R , of the water level changes in OW3 to those in the reservoir is a measure of the pore pressure transmitted through the shear zone. Assuming 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 hydraulic diffusivity and 1.1 × 10 −15 m 2 for permeability of the shear zone. In subsequent years the time lag decreased to 72 hours and R increased to 0.23, suggesting a nearly 60% increase in the two hydraulic parameters. We 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 through the shear zone showed a lognormal relationship with the frequency of lake level fluctuations, R = 0.5 exp (−14.93/ T ), where T (days) is the period of lake level changes. Spectral analyses of the data were used to obtain a frequency independent hydraulic diffusivity (∼0.076 m 2 s −1 ) of the shear zone after it had reached steady state conditions. Using the undrained response of the reservoir, an in situ estimate value 0.66 was obtained for the Skempton's coefficient. The hydraulic diffusivity ∼0.1 m 2 s −1 was found to be lower than that encountered at other locations of induced seismicity in the region.