Scientific Investigations Report 2006–5218

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

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Introduction

Land subsidence in Las Vegas Valley (fig. 1) was first recognized by comparing leveling surveys of benchmarks made between 1915 and 1935 (Maxey and Jameson, 1948). Subsidence continues to be measured with benchmark surveys, but the collection and continuity of these data have become jeopardized by rapid development destroying many of the benchmarks. Even with the addition of new benchmarks and Global Positioning System (GPS) technology, estimating subsidence rates and delineating valleywide subsidence patterns is difficult because of limited temporal data and a sparsely distributed benchmark network.

Satellite interferometric synthetic aperture radar (InSAR) data are capable of mapping spatially dense (20–90 m) deformation at temporal resolutions ranging from daily to monthly. A pair of synthetic aperture radar (SAR) images or scenes is used to develop an interferogram, which maps phase differences of radar waves reflected from the same location at different times. Phase differences can result from vertical and horizontal displacement associated with the deformation of the land surface and other conditions unrelated to deformation. During the processing of the interferograms described in this report, phase differences unrelated to deformation were minimized or eliminated, and all remaining phase differences were assumed to be the result of vertical displacement. InSAR technology and interferograms provide an effective and relatively inexpensive method for detecting and estimating sub-centimeter vertical deformation of the land surface at unprecedented spatial detail over large areas.

As part of a cooperative agreement with the Nevada Department of Conservation and Natural Resources—Division of Water Resources and the Las Vegas Valley Water District, the U.S. Geological Survey (USGS) has provided support for processing and analyzing interferograms and evaluated InSAR data for monitoring land subsidence and uplift in Las Vegas Valley. Personnel from Stanford University processed interferograms for Las Vegas Valley and personnel from the USGS, Stanford University, University of Hawaii, and Nevada Bureau of Mines and Geology analyzed them. The 44 individual interferograms presented in this report are for periods that range from 1.2 to 34.6 months between September 8, 1992, and December 10, 1999 (table 1). The stacked interferogram is a composite of 8 time-sequential interferograms that span the same range of dates as the 44 individual interferograms. The analyses indicate that subsidence rates generally are lower than rates measured before 1991 (Bell and others, 2000; 2002), geologic faults influence patterns of land subsidence (Amelung and others, 1999; Bell and others, 2000), and seasonal uplift occurs in many areas across the valley (Hoffmann and others, 2001; Hoffmann, 2003).

Mechanics of Land Subsidence

Land subsidence in Las Vegas Valley results from compaction of fine-grained sediments (silt and clay) within the basin-fill aquifer system (Maxey and Jameson, 1948; Malmberg, 1965; Bell, 1981). This compaction results from an increase in effective stress within the aquifer system resulting from lower water levels caused by pumping ground water (Terzaghi, 1925; 1943). In addition to compaction resulting from ground-water pumpage, seasonal water-level fluctuations associated with natural processes and ground-water-use practices have resulted in corresponding fluctuations in vertical deformation of the aquifer-system (Pavelko, 2000). Detailed and technical discussions about the relations between water-level fluctuations and aquifer-system stress and deformation are in Tolman and Poland (1940), Poland (1960), and Riley (1969).

Land Subsidence in Las Vegas Valley

Land subsidence has been a concern in Las Vegas Valley since Maxey and Jameson (1948) first recognized its occurrence. Surveys of various regional and local networks indicated that subsidence occurred at a relatively slow, steady rate throughout large regions of the valley into the mid-1960s. Subsidence rates increased throughout the late 1960s to the 1980s, forming localized subsidence bowls within a larger regional area of subsidence (fig. 1). The deepest localized bowl subsided more than 1.7 m as of 2000, and the other bowls subsided more than 0.6 m (Bell, 1981; Bell and Price, 1993; Bell and others, 2000; 2002).

Land subsidence has resulted in damage to wells, homes, roads, and water lines in Las Vegas Valley (Mindling, 1971; Bell, 1981, 1997; Bell and Price, 1993). Since the 1990s, land-subsidence rates have decreased (Bell and others, 2000) as water levels across the valley have remained stable or have risen. Artificial ground-water recharge programs, implemented by the Las Vegas Valley Water District and the City of North Las Vegas, have contributed to stable and rising water levels; also, land uplift has occurred in some areas (Bell and others, 2000). Increasing ground-water demands from a growing population in Las Vegas Valley could result in lower water levels and, ultimately, an increase in land-subsidence rates throughout the valley.

Purpose and Scope

This report allows for viewing and downloading interferograms and Landsat images for Las Vegas Valley, Nev. The interferograms demonstrate the value of InSAR technology to delineate land-subsidence and uplift patterns and trends. Thus, interferograms could assist water and land mangers in mitigating land subsidence and associated damage. The general approach used to develop the interferograms and selected observations pertinent to water-use and land-deformation studies are described herein.

SAR data were processed into individual interferograms where, for purposes of visualization, displacements are scaled differently using a multicolor and a three-color sequence. The multicolor scaling accentuates smaller-scale displacement and the three-color scaling accentuates areas of displacement. The interferograms depict deformation for 44 time intervals ranging from 1.2 to 34.6 months between September 8, 1992, and December 10, 1999 (table 1). A stacked interferogram created from 8 time-sequential interferograms spans the same dates as the individual interferograms. The European Space Agency’s Earth Remote Sensing satellites (ERS-1 and ERS-2), which began collecting SAR data for the Las Vegas area in 1992, acquired the SAR data used to create the interferograms. National Aeronautics and Space Administration’s Landsat 5 and 7 satellites acquired the Landsat images in 1993 and 2000, respectively.

The interferograms and the Landsat images are viewable in appendix A using an interactive map designed for side-by-side comparisons, and are available for download for use with Geographic Information Systems (GIS) software. The three-color individual and stacked interferograms available for download are in a grid format and the multicolor interferograms and Landsat images are georeferenced tiff images. The Landsat images provide a frame of reference for the interferograms. The extent of figures 2–7 and all appendix A; images in the interactive map is shown in figure 1; the extent of the images available for download is larger than those in the report and appendix A.

Previous Investigations

Amelung and others (1999) measured land subsidence and uplift in Las Vegas Valley using interferometry and noted that faults and the thickness of clay units, in part, control the areal distribution of land subsidence. Bell and others (2000; 2002) examined InSAR, GPS, and past benchmark-survey data and noted that subsidence in Las Vegas Valley is localized more than was believed previously and that land-subsidence rates generally have decreased since 1991. Based on this information, previously constructed subsidence contours for 1963–87 (Bell and Price, 1993) were modified and updated (Bell and others, 2000; 2002). Hoffmann and others (2001) used interferometry to measure seasonal land subsidence and uplift, and ground-water level data at selected monitor-well sites to estimate aquifer-system elastic storage coefficients. Hoffmann (2003) investigated the application of interferometric techniques to the measurement and interpretation of vertical deformation over pumped aquifers, including a detailed description of methods and procedures and a Las Vegas Valley case study.

Other studies that used InSAR to map subsidence and (or) uplift attributed to ground-water level changes in areas outside of Las Vegas Valley, Nev., include investigations in Albuquerque Basin, New Mex. (Heywood and others, 2002), Antelope Valley, Calif. (Galloway and others, 1998; Hoffmann and others, 2003); San Luis Obispo County, Calif. (Valentine and others, 2001); Coachella Valley, Calif. (Sneed and others, 2001; 2002); Houston–Galveston Bay area, Tex. (Stork and Sneed, 2002; Buckley and others, 2003); Santa Ana Basin, Calif. (Bawden and others, 2001), Santa Clara Valley, Calif. (Ikehara and others, 1998; Schmidt and Bürgmann, 2003), and Yucca Flat, Nev. (Laczniak and others, 2003; Halford and others, 2005). Galloway and others (2000) provide a general overview of measuring land subsidence using interferometric techniques.

Interferometry has been used for high-density spatial mapping of ground-surface deformation associated with tectonic (Massonnet and others, 1993; Zebker and others, 1994; Bawden and others, 2001) and volcanic strains (Massonnet and others, 1995; Rosen and others, 1996; Wicks and others, 1998). InSAR has been used to map localized crustal deformation and land subsidence associated with geothermal fields in Imperial Valley, California (Massonnet and others, 1997), Long Valley, Calif. (Thatcher and Massonet, 1997), and Iceland (Vadon and Sigmundsson, 1997), and with oil and gas fields in the Central Valley, Calif. (Fielding and others, 1998).

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