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GeoWorld February 2012

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monitoring. For one, it requires minimal expensive field work, if any. But the greatest advantage is that instead of a few scattered points, the result is a dense array (e.g., every few meters) of geocoded elevation-change pixels that yield a detailed map of surface displacement. To demonstrate the potential of this technique for operational subsidence monitoring, a study was con- ducted using a region in Arizona that lies above an aqui- fer known to experience subsidence (see Figure 1). This area has been studied and mapped for a number of years by the Arizona Department of Water Resources, so it's a well-quantified test area. In addition, Arizona's arid climate provides dry soil Figure 2. A radar image of McMullen Valley shows a subset of geocoded TerraSAR-X data. Note the rectangular cultivated areas along the river running across the center. and minimal vegetation, which are advantageous for radar interferometry. The presence of vegetation can result in vegetation-induced decorrelation, which can compromise or prohibit precise analysis. When underground aquifers are used as a source for municipal water supplies, the aquifer can collapse if the water is extracted from an aquifer faster than rainfall can recharge it. After it collapses, recharge may become impossible, causing permanent loss of a critical resource. Subsidence in urban areas can be caused by settlement load (i.e., uneven settling of the soil beneath a structure), which can result in costly damage to vital infrastructure such as water and sewer pipes, electrical conduits, and transportation corridors. Companies involved in tunneling through rock to build transportation infrastructure must monitor regional effects of their actions. For these appli- cations and many others, a regional mapping of surface deformation provides valuable information. Acquiring Subsidence Data Although such information can prevent expensive dam- age, it also can be costly to acquire. This is especially challenging as current economic conditions force organiza- tions to accomplish more with limited financial resources. Currently, subsidence monitoring is conducted using repeated surveys with conventional or GPS leveling. Other techniques use permanent compaction recorders or vertical extensometers. All these approaches are limit- ing, because they yield point data, which are expensive and can be unrepresentative and misleading. Satellite remote sensing using radar interferometric technology is offering an attractive option for subsidence DInSAR Technology For this study, detailed displacement maps were derived using a technique called differential radar interferometry (DInSAR). In simple terms, the radar satellite collects an image of the study area during two different overpasses. The radar signal-phase information from these two images is processed, for every pixel, to extract a phase difference called an interferogram. Depending on various factors, such as system noise and atmospheric changes, this difference can be quantified to sub-wavelength precision, perhaps one quarter of a wavelength. Thus, an X-band sensor operating at three centimeters could, under optimum conditions, map displacement at approximately 0.75 centimeters. A section of such an interferogram is seen in Figure 4. Figure 3. In the ERDAS IMAGINE DInSAR Wizard Workflow, most parameters and variables are intelligent or scene-specific for optimal results. FEBRUAR Y 2O12 / WWW . GEOPLA CE . COM 23

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