HR: 0800h
AN: G51C-0625    [Abstracts]
TI: InSAR Identifies Mine-Dewatering Associated Bedrock Compaction and Subsidence in North- Central Nevada
AU: * Katzenstein, K W
EM: kkatzens@unr.nevada.edu
AF: University of Nevada, Reno, Department of Geological Sciences and Engineering, M/S 172, Reno, NV 89557, United States
AU: Bell, J W
EM: jbell@unr.edu
AF: Nevada Bureau of Mines and Geology, University of Nevada, Reno, M/S 178, Reno, NV 89557, United States
AU: Watters, R J
EM: watters@mines.unr.edu
AF: University of Nevada, Reno, Department of Geological Sciences and Engineering, M/S 172, Reno, NV 89557, United States
AB: During the last decade, InSAR has been used extensively for the delineation of aquifer-system response to heavy groundwater pumping. A number of studies have demonstrated the vastly improved spatial resolution afforded by InSAR relative to traditional surveying techniques in detecting groundwater-related effects, including subsidence. This has allowed for further understanding of the complexity of subsidence bowls and the role of secondary factors such as structure, aquifer material properties and other previously unforeseen factors. In the western U.S., ground subsidence related to mine dewatering is a common occurrence due to the very large volumes of water (as high as 100,000 acre-ft/yr) that are typically pumped in order to lower the local groundwater table to facilitate the excavation of open pit and underground mines. Several gold mines located along the Carlin Trend of Central Nevada have produced distinct InSAR-identified subsidence signals of greater aerial extent and magnitude than most municipal groundwater signals, including signals partly or entirely within bedrock. One signal in particular shows a minimum of 54 cm of cumulative dewatering related subsidence between June 1, 1992 and September 21, 2000. Our study has produced many (>50) interferograms, each covering different time intervals, allowing a better understanding of how the subsidence signal has evolved in response to varied pumping rates from dewatering wells. Since the spatial resolution of the InSAR is much better than that of the monitoring well locations, the complexity of the signal is better delineated. The aerial extent of the subsidence feature is impressive as it extends as far as 20 km away from the location of the extraction wells used for dewatering. The area of maximum subsidence correlates well with the area of maximum groundwater drawdown, however the subsidence signal extends well beyond (as much as 8-10 km) the observed groundwater drawdown pattern. This suggests a much deeper zone of compaction and/or subsidence. The large aerial extent is likely a result of the fact that the vast majority of the pumping is from the deeper bedrock aquifer, with very small amounts of pumping from shallower siltstones and unconsolidated basin fill. The geology within the deformation signal is very complex. The dewatering is occurring in deep carbonates which are overlain by varying thicknesses of basin fill, volcanics, siliceous siltstones and mudstones and other limestone units. Close inspection of these units in the main open pit as well as a nearby underground mine suggests that while many of these units are highly fractured, most of the fractures have been healed with silica or are so tight that minimal fracture closing is possible. This suggests another mechanism causing the ground surface to subside, including compaction of intact bedrock. Groundwater related bedrock subsidence of this scale is rarely, if ever, observed, and therefore, poorly understood. Ongoing work at this site is focused on better understanding the mechanics of the observed bedrock compaction/subsidence, and possible implications to other high volume groundwater pumping sites.
DE: 1240 Satellite geodesy: results (6929, 7215, 7230, 7240)
DE: 1822 Geomechanics
DE: 1855 Remote sensing (1640)
DE: 1859 Rocks: physical properties
DE: 5199 General or miscellaneous
SC: Geodesy [G]
MN: 2007 Fall Meeting