Radar interferometry measurement of land subsidence
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Radar interferometry is a remote sensing technique which has been developed over the past two decades for the measurement of earth-surface topography and deformation. This investigation considers the application of radar interferometry to the study of urban land subsidence. Radar data collected by the European Earth Resource Satellites (ERS-1 and ERS-2) over Phoenix, Arizona and Houston, Texas are used to create a time series of interferograms spanning the 1990s. Deformation maps are validated with traditional subsidence measurements where available. Comparisons between interferograms confirm the existence and continued development of several subsidence features, some of which were not identified with traditional measurement techniques. Appropriate land use, land cover as well as atmospheric conditions are all critical to the successful application of the interferometry technique. This ix investigation provides methods for assessing the atmospheric contamination associated with and the environmental conditions under which radar interferometry may be used to monitor slow, continuous deformation. Phoenix, Arizona and Houston, Texas have witnessed significant urban development over the past century. The residential, industrial and agricultural water supply of these metropolitan areas is provided primarily from groundwater pumped from vast aquifer systems underlying the regions. The pumping has depressurized and subsequently compacted the aquifers. Consequently, several meters of land subsidence has occurred since groundwater extraction began, resulting in millions of dollars of damage to urban infrastructure. Considering the vast urban development continuing in these metropolitan areas, the demand on the groundwater supply and the associated land subsidence remain critical problems to be addressed by city planners. Monitoring groundwater-use and subsidence patterns in these regions over time provides valuable information which can be used to mitigate infrastructure damage as well as infer subsurface properties. Three techniques have been used historically to monitor subsidence in these regions: releveling of benchmark surveys, the use of extensometer measurements, and the use of GPS measurements. However, these measurements provide spatially-limited point measurements of deformation. In contrast, spaceborne repeat-pass differential radar interferometry, a technique developed over the past decade, provides subtle earth-surface deformation measurements at significantly improved spatial resolution as compared with traditional subsidence monitoring techniques.