Scientific Investigations Report 2006–5218
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5218
These interferograms were developed from paired satellite SAR images that were processed to estimate vertical displacement. SAR emits pulses of microwaves along a track of the satellite’s orbit and receives reflected waves from the Earth’s surface. Specialized processing routines account for the forward motion of the satellite and allow for higher resolution images than traditional (unfocused) radar.
The phase components of reflected waves from two data acquisitions are differenced and processed to form interferograms, which are phase-difference maps (see “Limitations” section). Phase differences are caused by (1) displacement or motion between the satellite and reflectors on the Earth’s surface that occurs between data acquisitions; (2) viewing the Earth from two slightly different angles (satellite orbital geometry); (3) changes in atmospheric conditions; and (4) decorrelation or noise (Zebker and others, 1994, and Ferretti and others, 2000). The goal of processing interferograms for studies of land deformation is to accentuate phase differences resulting from changes in the land-surface position and minimize all other phase differences. Based on the assumption of zero horizontal deformation, the phase-difference data are projected into assumed vertical changes in reflector height and then smoothed. The computed vertical changes were referenced to the same small area, assumed stable for all the interferograms, by subtracting the mean vertical change of the stable area from the computed vertical changes.
The phase differences are measured in the repeating interval from zero to 2π, where every 2π cycle is equivalent to a line-of-sight change equal to one-half the radar wavelength, which is about 28 mm for SAR data collected by ERS satellites. The phase-difference data are unwrapped (Costantini, 1998; Chen, 2001) to create an unwrapped interferogram that represents a continuous surface of absolute displacement values. Vertical displacement can be estimated with an accuracy of about 5 mm for each 4 x 40 m interferogram pixel by using stable reflectors such as buildings, roads, or undisturbed ground surfaces, and under favorable atmospheric conditions. The stacked interferogram is a summation of the time-sequential individual interferograms 1, 3, 5, 14, 23, 36, 41, and 44 (table 1).
For this report, each interferogram is unwrapped and presented using different color schemes (figs. 2 and 3). Three-color individual interferograms depict uplift (purple), no displacement (yellow), and subsidence (green), for a range of 224 mm of vertical displacement, whereas the three-color stacked interferogram depicts uplift (blue), no displacement (yellow), and subsidence (red), for a range of 448 mm of vertical displacement. Multicolor interferograms have a repeating, five-color sequence, where each sequence represents 20 mm of vertical displacement. The order of the colors indicates subsidence (blue-green-yellow-orange-red) or uplift (red-orange-yellow-green-blue).
Additional background on interferometry methodology is in Gabriel and others (1989), Massonnet and Feigl (1998), and Hanssen (2001); a comprehensive presentation of SAR image processing is in Curlander and McDonough (1991). Galloway and others (1998) and Hoffmann (2003) provide more intensive discussions about interferometric applications for the study of land subsidence attributed to ground-water development. The limitations of interferometry and phase measurement are discussed in the “Limitations” section of this report.
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