INSAR-IDENTIFIED BEDROCK COMPACTION AND SUBSIDENCE ASSOCIATED WITH MINE-DEWATERING IN NORTH-CENTRAL NEVADA

Interferometric synthetic aperture radar, or InSAR, has proved an invaluable tool for ground motion studies in the geosciences. However, it was not until recently that InSAR has been used 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 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, etc. Ground subsidence related to mine dewatering is a common occurrence due to the large volumes of water that are pumped in order to lower the local groundwater table to facilitate open pit and underground mining operations. Several mines located along the Carlin Trend of Central Nevada have produced InSAR identified subsidence signals of greater aerial extent and magnitude than most municipal groundwater signals.

One dewatering signal shows a minimum of 54 cm of cumulative subsidence between June 1, 1992 and September 21, 2000. Our study has created many (>50) interferograms, allowing a better understanding of how the subsidence signal evolved in response to varied pumping rates from dewatering wells. 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 maximum aerial extent of the subsidence feature extends as far as 20 km away from the location of the extraction wells used for dewatering. This is likely due to the fact that the majority of the pumping in the area is from the deeper carbonate aquifer, with very little pumping from shallower siltstones and unconsolidated basin fill. Of most interest is the fact that this subsidence bowl exists mostly in areas of very shallow or exposed 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 and subsidence, and possible implications to other high volume groundwater pumping sites.