Mojave Land-Subsidence Studies
Spatially detailed maps of interferometric synthetic aperture radar (InSAR) methods were used to characterize land subsidence associated with groundwater-level declines during various intervals of time between 1992 and 1999 in the Mojave River and Morongo groundwater basins Sneed and others, 2003). Concerns related to the potential for new or renewed land subsidence in the basins resulted in a cooperative study between the Mojave Water Agency (MWA) and the U.S. Geological Survey (USGS) in 2006. InSAR data were developed to determine the location, extent, and magnitude of vertical land-surface changes in the Mojave River and Morongo ground-water basins for time intervals ranging from about 35 days to 14 months between 1999 and 2000 and between 2003 and 2004. (interactive Google map) The results from many future land-subsidence studies, which are scheduled about every 10 years, will be available on this website.
Mechanics of Land Subsidence
Land subsidence can occur in valleys containing aquifer systems that are, in part, made up of fine-grained sediments and that have undergone extensive ground-water development. The pore structure of a sedimentary aquifer system is supported by a combination of the granular skeleton of the aquifer system and the fluid pressure of the ground water that fills the intergranular pore space (Meinzer, 1928). When ground water is withdrawn in quantities that result in reduced pore-fluid pressures and water-level declines, more of the weight of the overlying sedimentary material must be supported by the skeleton. A loss of fluid-pressure support increases the intergranular load, or effective stress, on the skeleton. With a change in effective stress, the skeleton is subject to deformation—an increase in effective stress causes some degree of skeletal compression and a decrease causes expansion. The vertical component of this deformation sometimes results in irreversible compaction of the aquifer system and land subsidence. An aquifer-system skeleton that consists of primarily fine-grained sediments, such as silt and clay, is much more compressible than one that consists of primarily coarse-grained sediments, such as sand and gravel.
Aquifer-system deformation is elastic (recoverable) if the stress imposed on the skeleton is smaller than the previous maximum effective stress. The largest historical effective stress imposed on the aquifer system, sometimes the result of the lowest ground-water level, is called the "preconsolidation stress". When stresses are greater than the preconsolidation stress, the pore structure (granular framework) of the fine-grained sediments undergoes rearrangement toward a configuration that is more stable at higher stresses. This rearrangement results in an irreversible reduction of pore volume and, thus, in inelastic compaction of the aquifer system. Deformation under stresses in excess of the preconsolidation stress is 20 to more than 100 times greater than deformation under stresses less than the preconsolidation stress (Riley, 1998).
For an aquifer-system skeleton that has an appreciable thickness of fine-grained sediments, or confining layers, a significant part of the total compaction may be residual compaction (compaction that occurs in the confining layers while hydraulic heads in those layers equilibrate with hydraulic heads in the adjacent aquifers). Depending on the thickness and the vertical hydraulic diffusivity of a confining layer, fluid-pressure equilibration—and thus compaction—lags behind hydraulic head declines in the adjacent aquifers; this lag results in concomitant compaction that may require decades or centuries to approach completion. The time required to attain about 93 percent of the total compaction after water levels have stabilized is referred to as the time constant (Riley, 1969). Ireland and others (1984) estimated that the time constants for aquifer systems at 15 sites in the San Joaquin Valley ranged from 5 to 1,350 years. Terzaghi (1925) described this delay in his theory of hydrodynamic consolidation. Numerical modeling based on Terzaghi's theory has successfully simulated complex histories of compaction caused by known water-level fluctuations (Helm, 1978; Hanson, 1989; Sneed and Galloway, 2000). For a more complete description of aquifer-system compaction, see Poland (1984), and for a review and selected case studies of land subsidence caused by aquifer-system compaction in the United States, see Galloway and others (1999).
Interferometric Synthetic Aperture Radar (InSAR)
View color scale and description
Interferometric Synthetic Aperture Radar (InSAR) is an effective way to measure changes in land surface altitude. InSAR is a satellite-based remote sensing technique that can detect ground-surface displacement on the order of about a third of an inch (centimeter) over an area approximately 60 x 60 miles (100 x 100 kilometers) with spatial resolution on the order of about 300 feet (90 meters) or less. This technique has been used to investigate deformation resulting from earthquakes (Massonnet and others, 1993), volcanoes (Massonnet and others, 1995), and land subsidence (Massonnet and others, 1997; Fielding and others, 1998; Galloway and others, 1999; Amelung and others, 1999, Hoffmann and others, 2001; Sneed and others, 2003). Synthetic Aperture Radar (SAR) imagery is produced by reflecting radar signals off a target area and measuring the two-way travel time back to the satellite. The SAR interferometry technique uses two SAR images of the same area acquired at different times and "interferes" (differences) them, resulting in maps called interferograms that show line-of-sight ground-surface displacement (range change) between the two time periods. The generation of an interferogram produces two components; the amplitude and the phase components. The amplitude component is the measure of the signal intensity returned to the satellite and shows buildings, roads, mountains, and other reflective features, and the phase component is proportional to range distance and shows the coherent displacements imaged by the radar. If the ground has moved away from (subsidence) or towards (uplift) the satellite between the times of the two SAR images, a slightly different portion of the wavelength is reflected back to the satellite resulting in a measurable phase shift that is proportional to displacement. The map of phase shifts, or interferogram, is depicted with a repeating color scale that shows relative displacement between the first and the second acquisitions, where, in this report, one complete color cycle (fringe) represents 1.1 inches (28.3 millimeters) of displacement. The indicated displacement is about 90-95 percent of true vertical ground motion, depending on the satellite look angle and location of the target area. The direction of displacement —subsidence or uplift—is indicated by the color progression of the fringe(s) toward the center of a deforming feature. For interferograms in this report, the color-fringe progression of blue-green-yellow-orange-red-purple indicates subsidence; the opposite progression indicates uplift.
InSAR signal quality is partly dependent on satellite position, atmospheric effects, ground cover, land use practices, and temporal separation of the interferogram. Strict orbital control is required to precisely control the look angle and position of the satellite. Successful application of the InSAR technique is contingent on looking at the same point on the ground from the same position in space, such that the horizontal distance between each satellite pass, or perpendicular baseline, is minimized. Perpendicular baselines greater than about 650 feet (200 meters) generally produce excessive topographic effects (parallax) that can mask real signal. Phase shifts can be caused by variable atmospheric mass that is associated with different elevations. A digital elevation model (DEM) is used in the interferogram generation process to reduce the atmospheric effects caused by elevation differences (and also to georeference the image). Phase shifts also can be caused by laterally variable atmospheric conditions such as clouds or fog, as the non-uniform distribution of water vapor differentially slows the radar signal over an image, which causes a phase shift (Zebker and others, 1997). Atmospheric artifacts can be identified by using several independent interferograms, which are defined as interferograms that do not share a common SAR image. When apparent ground motion is detected only in one interferogram, or a set of interferograms sharing a common SAR image, then the apparent motion likely is due to atmospheric phase delay and can be discounted. The type and density of ground cover also can significantly affect interferogram quality. Densely forested areas are prone to poor signal quality because the C-band wavelength (about 2.2 inches or 56 millimeters) cannot effectively penetrate thick vegetation, and is either absorbed, or reflected back to the satellite from varying depths within the canopy resulting in incoherent signal (shown as randomized colors on an interferogram). Sparsely vegetated areas and urban centers, however, generally have high signal quality because bare ground, roads, and buildings have high reflectivities and are relatively uniform during at least some range of InSAR timescales. Certain land use practices, such as farming, also cause incoherent signal return. The tilling and plowing of farm fields causes large and non-uniform ground-surface change that cannot be resolved with InSAR. Signal quality also is adversely affected by larger temporal separations, as there is more opportunity for non-uniform change to occur in both urban and non-urban areas. Many of these error sources were minimized by producing interferograms with perpendicular baselines less than about 650 feet (200 meters) and by examining several independent interferograms for the area of interest in the Mojave Desert, which is sparsely vegetated and fairly flat.
For this study, SAR data from the European Space Agency's (ESA) ENVISAT and European Earth Remote Sensing II (ERS-2) satellites were used to map and measure displacement. The two satellites are side-looking, orbit the Earth at an altitude of approximately 500 mi (800 km), and have 35-day repeat cycles. Ten interferograms with temporal baselines ranging from 70 days to 18 months were developed for this study using 10 synthetic aperture radar (SAR) scenes acquired by the ERS-2 satellite, and 16 interferograms with time periods ranging from 35 days to 14 months were developed for this study using 22 synthetic aperture radar (SAR) scenes acquired by the European ENVISAT satellite.