Summer  >>  2005

A Sinking Feeling In Arizona

Subsidence is Significant

JERRY SKAW
Marketing Communications Manager

Vexcel Corporation
Boulder, Colo.
www.vexcel.com

There’s a sinking feeling about land levels in the Phoenix valley of Arizona.

Ground subsidence — the sinking of land surface — has been documented over several decades throughout the Phoenix valley, with measurements as large as five meters reported in the vicinity of Luke Air Force Base, west of the city of Phoenix. Even at a more moderate level of five centimeters or less per year, the hazards posed to infrastructure are very real. Ground motion compromises building foundations, canals, and sewer systems, while ground fissures (a byproduct of subsidence) have resulted in damage to dams, roads, and buried pipelines (see Figures 1 and 2). The problem is a costly one: in excess of $125 million per year, according to the National Research Council.

Figure 1 >> Fissures are the primary surface expression of subsidence and threaten infrastructure such as roads, dams, canals and pipelines.

Figure 2 >> This photo demonstrates 5.5 meter elevation change observed near Luke Air Force Base over 34 years. Courtesy of Herb Schumann.

The root of the problem can be traced back to the early 1900s when agriculture expanded quickly across much of the western and southwestern United States. Large stretches of available land were ideal for growing the produce necessary to support the rapidly growing U.S. population. In Arizona, the agricultural industry has been a critical element of the economy with key exports that include cotton, vegetables, fruit and meat.

However, the annual precipitation of this arid region cannot support a thriving agricultural industry, and additional water sources for irrigation had to be identified. In the Phoenix valley, where the average annual precipitation is around three inches, technology has been leveraged since the early twentieth century to extract water naturally deposited in aquifers — underground geological stores. The fine-grained sediments that constitute the alluvial basins of south-central Arizona are ideal natural storage structures for vast quantities of fresh water. These geological structures are essentially composed of familiar materials like sand, clay and gravel. Water naturally deposited in these structures occupies the spaces between particles and is available for removal via pumping. This historically rich natural resource has been the impetus for much of the growth in the Phoenix and Tucson areas.

The enthusiastic extraction of water for irrigation during the early to mid-1900s, coupled with the rapidly growing population in these areas has led to a significant decline of water levels within the aquifer system. As fluid is withdrawn from the aquifer structure and the water level decreases, the hydrostatic pressure supplied by the fluid to the granular structure is also reduced. If enough ground water is withdrawn, the aquifer begins to collapse slowly under its own weight. Excessive pumping of groundwater has resulted in compaction of the aquifers, and this subsurface compaction is resulting in surface elevation change, or subsidence.

To address this growing hazard, the Phoenix valley is again turning to technology — the Arizona Department of Water Resources (ADWR), under a NASA grant, has subcontracted Vexcel Corporation (Boulder, Colo.) to apply its satellite radar remote sensing technologies to monitor the extent and rate of the subsidence problem. Using interferometric synthetic aperture radar (InSAR), Vexcel is providing measurements that are highly accurate, spatially dense, and yet significantly less expensive than traditional surveying techniques such as GPS, extensometers, and leveling surveys.

InSAR WE TRUST

The foundation of InSAR is synthetic aperture radar (SAR), a mature technology first demonstrated during the 1960s. Functionally, the approach consists of a radar system aboard a moving platform — in this case, a spaceborne satellite — emitting radar pulses and listening for the returned echoes as they are reflected from the ground. Through signal processing, the received pulses are combined to produce a high-resolution image.

The InSAR technique takes advantage of the coherent nature of SAR to extract very accurate measurements of changes on the Earth’s surface. In the InSAR implementation, the satellite collects data over the region of interest at different times. The phases of the resulting data sets may be compared at each sample area on the ground to detect changes. Surface displacement measurements of less than a centimeter over an area of several square kilometers have been routinely demonstrated in subsidence applications similar to the one in Phoenix using InSAR techniques. Through more advanced applications of the same technology, displacement rates on the order of a few millimeters per year have been demonstrated.

In addition to the accuracy and wide-area continuous coverage made possible by InSAR, the technology has offered better cost efficiency than traditional surveying techniques. A standard InSAR frame covers an area of approximately 10,000 km2 at a pixel resolution of about 50 meters — or 4,000,000 discrete point measurements within the 100 km by 100 km frame. The cost to perform static GPS survey with this same vertical precision but at 1/1000th the resolution would cost conservatively $500,000 for the two surveys required to measure change. The cost per point measurement to produce an InSAR change map using currently available satellite data is less by many orders of magnitude than with conventional technologies.

PRAISING ARIZONA

An important goal in the Phoenix project is to provide time-dependent measurements to the water resource community. To meet this challenge, an entire archive of satellite imagery captured over the Phoenix valley from 1992 to 2000 was acquired. This wealth of data — roughly 80 frames of a 3600 square mile area — allows Vexcel to track the evolution of the subsidence occurring during this particular period of time for which satellite imagery exists. Applying their SAR processing technologies to this data, Vexcel is producing a time series of subsidence maps that accentuate significant ground deformation in Phoenix and the surrounding area over an eight-year period. A subset of these maps is shown in Figure 3.

The ADWR and local water resource community can now compare these data to aquifer pumping, water table levels, weather patterns, urban development and other activities that occurred within this same time frame to understand better what factors are contributing to this subsidence. From this information, water management policies and city planning decisions are being developed to address the problem. The subsidence measurements are assisting the ADWR in its efforts to educate the public and local government agencies on the reality and severity of the land subsidence hazard. For example:

Figure 3 >> Time sequence of ground deformation in Scottsdale, Ariz. area derived interferometrically from ERS 1/2 SAR data. Each cycle of color corresponds to 9 cm of ground motion.
 
  1. The visual impact of the maps is believed to have influenced residents of the city of Peoria who recently voted for an increase in water rates allowing the city to move away from reliance on groundwater pumping and toward reliance on renewable water supplies. Additionally, the city is now in the planning phase of building a $25 million wastewater treatment facility. Initial plans called for a gravity drain line to run through an area that has been identified to be subsiding. This information allows the city either to account for the deformation in their engineering design or possibly to re-route the line and avoid the impacted area.
  2. The Central Arizona Water Conservation District (CAWCD) is using a number of maps to define the extent of a small subsidence feature that impacted a 2.5 km segment of the canal that runs through Scottsdale, Arizona. The center of the feature had subsided approximately 0.5 meters with respect to its edges and was restricting the maximum volume of water movement through the system. A more critical issue arose when a local geologist, using a geo-referenced subsidence map, identified a previously undiscovered earth fissure at the northwestern edge of this same subsidence feature. From the map and geophysical models, he was able to determine where earth fissures were likely to occur and found one within steps of the predicted location. Had the fissure gone unnoticed, approximately 75% of canal deliveries would have been interrupted.
  3. The Flood Control District of Maricopa County is using the subsidence maps to define dam areas with high risk for failure due to fissure formation. The enhanced capability, through InSAR, to estimate potential areas of earth fissuring is resulting in improved dam safety and increased public safety by reducing the risk that an earth fissure will go undetected.

The integration of InSAR displacement maps into the decision support process for a variety of users in Arizona commercial and government organizations demonstrates the value of these tools for hazard mitigation and characterization.

Ground deformation measurements using satellite remote sensing technology are being used in several locations across the U.S. and around the world. The results are providing geotechnical specialists and water resource managers with critical measurements of changes in land surface elevation due especially to depletion of aquifers.

While relatively mature, the InSAR technology continues to evolve to provide increased sensitivity and broad applicability. New radar satellites and burgeoning data processing techniques assure that this technology will continue to lead the way in monitoring changes that indicate more serious water resource issues posing threats to infrastructure.



 
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