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– 1 – PROTOCOL ON POLLUTANT RELEASE AND TRANSFER REGISTERS The Parties to this Protocol, Recalling article 5, paragraph 9, and article 10, paragraph 2, of the Convention on Access to. Google's free service instantly translates words, phrases, and web pages between English and over other languages. Measuring Land Subsidence from Space US Department of the Interior US Geological Survey Fact Sheet April A pdf of the report is available (MB).

The principal causes of subsidence are aquifer-system compaction, drainage of organic soils, underground mining, hydrocompaction, natural compaction, sinkholes, and thawing permafrost National Research Council, A powerful new mapping tool InSAR is a critical element in the assessment and mitigation of subsidence.

InSAR is capable of remotely sensing small changes in land surface elevation at an unprecedented level of spatial detail. The new displacement maps enhance our capabilities to monitor and manage subsidence caused by the compaction of susceptible aquifer systems, and reveal new insights into the controlling physical processes.

Geophysical applications of radar interferometry take advantage of the phase component of reflected radar signals to measure apparent changes in the range distance of the land surface Gabriel and others, ; Massonnet and Feigl, Ordinary radar on a typical Earth-orbiting satellite has a very poor ground resolution of about 3 to 4 miles because of the restricted size of the antenna on the satellite.

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Synthetic Aperture Radar SAR takes advantage of the motion of the spacecraft along its orbital track to mathematically reconstruct synthesize an operationally larger antenna and yield high-spatial-resolution imaging capability on the order of hundreds of feet. The size of a picture element pixel on a typical SAR image made from satellite-borne radar may be as small as tens of thousands of square feet, depending how the image is processed.

For landscapes with more or less stable radar reflectors such as buildings or other engineered structures, or undisturbed rocks and ground surfaces over a period of time, it is possible to make highprecision measurements of the change in the position of the reflectors by subtracting or "interfering" two radar scans made of the same area at different times.

This is the principle behind InSAR.

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Radar is an active sensor, transmitting a signal of electromagnetic energy. Satelliteborne radar using one antenna transmits a pulsed train of microwaves. The waves reflect off the ground surface, and echoes are received by the moving antenna, producing a recorded image of the scanned ground that is continuous along the track of the satellite and about 60 miles wide.

The restricted size of the satellite antenna limits the spatial resolution to 3 to 4 miles on the ground. Synthetic Aperture Radar SAR imaging "synthesizes" an effectively larger antenna about 3 miles long with a spatial resolution on the order of 16 feet by pulsing the microwaves every 16 feet of satellite travel. The 3- to 4-mile-wide footprints overlapped at foot intervals along the ground track are processed through a technique similar to medical x-ray imaging. Averaging of the signal is done to improve signal coherence, and the actual spatial resolution is typically on the order of feet or better.

Interferograms are made by differencing successive SAR images taken from the same orbital position but at different times. Under favorable radiometric conditions 0. Under ideal conditions, it is possible to resolve changes in elevation on the order of 0.

Interferograms, formed from patterns of interference between the phase components of two radar scans made from nearly the same antenna position viewing angle but at different times, have demonstrated dramatic potential for high-density spatial mapping of ground-surface displacements associated with tectonic Massonnet and others, ; Zebker and others, and volcanic strains Massonnet and others, ; Rosen and others, ; Wicks and others, InSAR has also recently been used to map localized crustal deformation and land subsidence associated with geothermal fields in Imperial Valley, California Massonnet and others, , Long Valley, California W.

Values are given in metric units to conform with standard geodetic guidelines. One inch is equal to In the Antelope Valley, Mojave Desert, California, an interferogram for the period October 20, to December 22, revealed up to 2 inches of additional subsidence in areas previously affected by as much as 6 feet of subsidence between Galloway and others, The regions of maximum subsidence detected during the month period correlated well with declining ground-water levels.

In another part of Antelope Valley formerly affected by ground-water depletion and subsidence, about 1 inch of subsidence was detected on the interferogram despite the fact that ground-water levels had been recovering-since This suggests residual time-delayed compaction due to the presence of thick aquitards. An independent computer simulation of aquifer-system compaction for the same period as the interferogram showed that though the general patterns of subsidence detected by the interferogram could be explained by the simulation, the detailed spatial variations in the interferogram could not.

These results highlight the potential use of spatially detailed InSAR measurements to provide better constraints for computer simulations of land subsidence. Interferograms for Las Vegas Valley demonstrate the intimate connection between ground-water withdrawal, faults and subsidence.

Measuring Land Subsidence from Space

An interferogram for the period April 21, to December 5, delineates two main features--a subsidence bowl in the northwest and an elongated subsiding zone in the central part of the valley. The northwest subsidence bowl includes the area of maximum subsidence of nearly 7.

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  • Its southeastern boundary is aligned with the Eglington fault, one of several Quaternary faults cutting the valley-floor alluvium.

    Little subsidence is detected immediately southeast of the fault. Similarly, the shape of the central subsidence zone follows the general trends of several mapped faults.

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    The map suggests that the spatial distribution of land subsidence in Las Vegas Valley is controlled by Quaternary faults, to a much greater degree than previously suspected. The faults may juxtapose sediments of differing time-consolidation properties, or they may act as barriers to ground-water flow, impeding the horizontal propagation of fluid-pressure-changes and creating ground-waterlevel differences across the faults.

    In Santa Clara Valley, California, subsidence has been arrested by importation of surface water and careful management of the aquifer system.

    However, the potential for renewed subsidence is a concern for the Santa Clara Valley Water District. Seasonal and longer-term elevation changes were measured from successive satellite radar passes during This uplift is correlated to the recovery of ground-water levels that has been occurring for several years as a result of reduced pumpage and increased recharge. In contrast, the seasonal 8 month interferogram reveals a large region in San Jose undergoing elastic deformation caused by seasonal groundwater-level fluctuations.

    The eastern extent of this deformation appears to be aligned with a Quaternary fault, the Silver Creek fault, several miles west of and roughly parallel to the tectonically active Hayward Fault. The InSAR derived displacements are consistent with historical patterns of subsidence and vertical displacements measured in deepseated borehole extensometers in Sunnyvale and San Jose.

    Catchings, USGS, written communication, Radar data used to produce the interferograms shown in this Fact Sheet were obtained from the European Space Agency, distributed through Eurimage Corporation and Radarsat International for purposes of research and development.

    Amelung, F. Fielding, E. Gabriel, A. Galloway, D. Ikehara, M. Massonnet, D. Rosen, P. Vadon, H. Wicks, C.

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    Zebker, H. Interferogram of Santa Clara Valley shows patterns of subsidence and some uplift occurring over 7 months in , from January 4 to August 2.

    A central subsidence zone, red the result of seasonal ground-water pumping, is fully recoverable. San Jose and surrounding communities sprawl across Santa Clara Valley "Silicon Valley" where a maximum detected subsidence of 14 feet occurred in downtown San Jose between and Since the late s subsidence has been arrested due largely to the importation of surface water.

    Urban settings are good candidates for radar interferometry because the structures make good radar reflectors.