Bedrock Subsidence Associated with Mine-dewatering Identified by InSAR in Central Nevada

During the past decade, synthetic aperture radar interferometry, or InSAR, has proven to be an invaluable tool in the geosciences. However, it was not until recently that InSAR has been used extensively for the delineation of groundwater related subsidence bowls. Groundwater related InSAR studies such as Amelung et al. (1999) as well as others have demonstrated the vastly improved spatial resolution afforded by InSAR relative to traditional surveying techniques. 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. Ground subsidence related to mine dewatering is a common occurrence due to the large volumes of water that are typically pumped in order to lower the local groundwater table to facilitate the excavation of open pit and underground mines. Several mines along the Carlin Trend of Central Nevada have produced distinct InSAR signals of greater aerial extent and magnitude than most municipal groundwater signals. One signal in particular shows a minimum of 54 cm of dewatering related subsidence between June 1, 1992 and September 21, 2000. A small area of uplift also exists in the vicinity of where groundwater is being reintroduced into the subsurface. 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 deformation signal correlates somewhat well with the observed groundwater drawdown pattern. However, 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 large aerial extent is likely a result of the fact that the vast majority of the pumping in the area is from the deeper carbonate aquifer, with very small amounts of pumping from shallower siltstones and unconsolidated basin fill. Of most interest is the fact that this extensive subsidence bowl extends into bedrock. Groundwater related bedrock subsidence of this scale is rarely, if ever, observed, and therefore, poorly understood. Future work at this site will be focused on better understanding the mechanics of the observed bedrock subsidence, and possible implications to other high volume groundwater pumping sites.