Scientific Investigations Report 2006–5218

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5218

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Limitations

The colors and patterns of these interferograms, or phase-difference maps, result from ground-surface deformation, different satellite positions during data acquisitions, different atmospheric conditions, and decorrelation or noise (Zebker and others, 1994; Ferretti and others, 2000). Phase changes due to different satellite positions have been corrected in these interferograms. The remaining phase changes not related to ground-surface deformation are considered phase noise for the purposes of land-deformation studies. Although the processing of these interferograms focused on accentuating phase differences resulting from ground-surface deformation and minimizing phase noise, some phase noise persists in the interferograms.

For land-deformation studies, interferometric results are best for relatively flat ground surfaces, such as the floor of Las Vegas Valley. Digital elevation-data errors, which often are present for steep terrain, and orographic effects can affect phase-difference data for steep slopes and high mountains. For this reason, interferometric patterns that appear near and on Frenchman Mountain, such as those in figure 5A, should not be included in deformation analyses, as these patterns likely are not the result of land deformation.

Limitations associated with the acquisition and processing of SAR data also include (1) the temporal resolution of interferometry is dependent on satellite stability/repeatability, (2) deformation depicted within an interferogram pixel is an average of the true deformation distributed across the land surface represented by the pixel, (3) algorithms used to unwrap SAR data are not 100-percent accurate, and (4) processing raw SAR data into interferograms is complex and requires a specialist. A limitation that arises from a simplifying assumption is that phase differences attributed to deformation are assumed to be purely vertical (zero horizontal deformation is assumed).

The limitations of interferometric measurements of land-surface deformation are complex and a detailed discussion is beyond the scope of this report. Hoffmann (2003) provides a more detailed discussion of the SAR-data acquisition, processing, and limitations of the interferograms presented in this report. Jónsson (2002) provides a more thorough discussion of errors and biases associated with interferometric-phase measurements and Hanssen (2001) provides a comprehensive discussion of errors associated with radar interferometry.

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