FS_110-02.fmPage1Friday,December20,20021:01PM In cooperation with the U.S. Fish and Wildlife Service Houston-Galveston Bay Area, Texas, From Space— A New Tool for Mapping Land Subsidence Interferometric Synthetic Aperture Radar (InSAR) is a powerful new tool that uses radar signals to measure displacement (subsidence and uplift) of the Earth's crust at an unprecedented level of spatial detail and high degree of measurement resolution. The Houston-Galveston Bay area, possibly more than any other metropolitan area in the United States, has been adversely affected by land subsidence. Extensive TEXAS subsidence, caused mainly by ground-water pumping but also by oil and gas extraction, has increased the frequency of flooding, caused extensive damage to industrial and transportation infrastructure, motivated Lake Livingston A N D S N U P L major investments in levees, reservoirs, and surface- L T A S water distribution facilities, and caused substantial loss A O of wetland habitat. Ongoing patterns of subsidence in C T Subsidence study area R I N the Houston area have been carefully monitored using I T Y S D borehole extensometers, Global Positioning System N AR (GPS) and conventional spirit-leveling surveys, and L I Lake Houston V W E O R more recently, an emerging technology—Interferomet- L ric Synthetic Aperture Radar (InSAR)—which enables development of spatially-detailed maps of land-surface Houston L displacement over broad areas. A T Galveston S Bay This report, prepared by the U.S. Geological Survey A O Texas City Galveston (USGS) in cooperation with the U.S. Fish and Wildlife C Service, briefly summarizes the history of subsidence Extent of two in the area and the local consequences of subsidence satellite scenes used in the study GULF OF MEXICO and describes the use of InSAR as one of several tools 0 20 MILES in an integrated subsidence-monitoring program in the 0 20 KILOMETERS area. Figure 1. Subsidence study area and extent of two satellite scenes, Houston-Galveston Bay area, Texas. Land subsidence in the Houston-Galveston Bay area (fig. 1) terns of subsurface fluid extraction. Subsidence caused by oil and first occurred in the early 1900s in areas where ground water, oil, gas extraction typically is restricted locally to the field of produc- and gas were extracted. Subsidence continued throughout the 20th tion such as at Goose Creek oil field, in contrast to the regional- century, primarily owing to compaction of the aquifer systems scale subsidence typically caused by ground-water pumpage caused by ground-water pumpage and the associated ground- (Coplin and Galloway, 1999). water-level declines. More than 10 feet (ft) of subsidence was measured near Pasadena during 1906–95, and almost 3,200 square Until 1942, essentially all water demand in Houston was miles (mi2) had subsided more than 1 ft (fig. 2) The patterns of supplied by local ground water. By 1943, subsidence had begun subsidence in the area closely follow the temporal and spatial pat- to affect a large part of the Houston area, although the amounts U.S. Department of the Interior USGS Fact Sheet 110–02 U.S. Geological Survey December 2002 in the coastal area was stabilizing, subsidence inland was continu- T 45 R ing, especially in the rapidly growing areas north and west of I N I Houston that mostly are still dependent on ground water (fig. 3). Harris-Galveston T N Y Coastal1 Subsidence District R In this region ground-water levels in the Evangeline aquifer I 2 3 4 V E declined more than 100 ft during 1977–97, and 2.3 ft of subsid- 5 R 6 San Jacinto ence was measured near Addicks (fig. 3) between 1978 and 1995 Monument Houston Ship Channel 10 (Coplin and others, 2001). During the 1989 legislative session, the 10 Fort Bend Subsidence District (fig. 2) was created to manage and Houston 6 9 Baytown Pasadena control subsidence in Fort Bend County. 7 9 Fort Bend 8 Goose Creek 7 oil field Subsidence District 6 Consequences of Land Subsidence Galveston 5 4 Bay Land subsidence in the Houston-Galveston Bay area has R E 3 V I 4 increased the frequency and severity of flooding (fig. 4). In this R Texas City S 2 low-lying coastal environment, more than 10 ft of subsidence O 6 Z A (fig. 2) has shifted the position of the coastline and changed the R B 1 Galveston distribution of wetlands and aquatic vegetation. In fact, the San Lines of equal subsidence (interval 1 foot) Jacinto Battleground State Historical Park, site of the battle that 0 20 MILES Galveston 0 20 KILOMETERS Island GULF OF MEXICO won Texas independence and commemorated with the San Jacinto Monument (fig. 2), is now partly submerged. At many localities in Figure 2. Extent of subsidence in the Houston-Galveston Bay area the Houston area, ground-water pumpage and subsidence also during 1906–95. have induced fault movement. Since the late 1930s, 86 active faults in the Houston area have offset the land surface and dam- generally were less than 1 ft. By the mid-1970s, 6 ft or more of aged buildings and highways in the metropolitan area (Holzer and subsidence had occurred throughout an area along the Houston Gabrysch, 1987; Coplin and Galloway, 1999). Ship Channel between Baytown and Houston as a result of Near the coast, the result of land subsidence is an apparent declining ground-water levels caused by increased ground-water increase in sea level, or a relative sea-level rise—the net effect pumpage associated with rapid industrial expansion in the area. of global sea-level rise and regional land subsidence in the coastal Growing awareness of subsidence- zone. The sea level is in fact rising because related problems on the part of community of regional and global processes. The Annual 0.2 combined effects of the eustatic sea-level and business leaders prompted the 1975 pumpage rise and natural consolidation of the sedi- Texas Legislature to create the Harris- (millions of Galveston Coastal Subsidence District acre-feet) ments along the Texas Gulf Coast yield a (fig. 2), “. for the purpose of ending sub- 0 relative sea-level rise from natural causes sidence which contributes to, or precipi- 1976 1981 1986 1991 1996 that locally might exceed 0.08 inch (in.) tates, flooding, inundation, and overflow of Pumpage in northwestern zones per year (Paine, 1993). Although global any area within the District . .” (State of of the subsidence district warming is contributing to the present- Texas, 1975, p. 672). This unprecedented day sea-level rise and might result in a sea- subsidence district was authorized to issue Harris-Galveston level increase of nearly 4 in. by 2050 (Titus Coastal Subsidence and Narayanan, 1995), human-induced (or refuse) well permits, promote water 1 District conservation and education, and promote 2 subsidence, by far, has been the dominant 3 conversion from ground-water supply to Houston Ship Channel cause of relative sea-level rise along the surface-water supply, but the district was Addicks Baytown Texas Gulf Coast during the 20th century, forbidden to own property such as water- Houston exceeding 1 in. per year throughout much supply and conveyance facilities. Pasadena of the affected area (Coplin and Galloway, 1999). The initial Subsidence District reg- ulatory plan (1976) was designed to have Texas City Galveston Bay is one of the most “. an immediate impact in the area where Line of equal land important bay ecosystems in the United most of the subsidence had taken place subsidence, 1978–95 States. The estuary is Texas’ leading bay (Interval 0.5 foot) . .” (Harris-Galveston Coastal Subsid- 0 20 MILES fishery, and it supports vibrant recreation ence District, 1999, p. 3). The plan gener- 0 20 KILOMETERS and tourism industries. More than 60 per- cent of the bay’s 232 miles (mi) of shore- ally has succeeded in its primary objective: Pumpage in southeastern Since the late 1970s, subsidence largely zones of the subsidence line is composed of highly productive has been abated along the ship channel district wetlands; because of subsidence, however, and in the Baytown and Pasadena areas more than 26,000 acres of emergent wet- in the coastal lowlands south and east of Annual 0.2 lands have been converted to open water Houston owing to a reduction in ground- pumpage and barren flats (White and others, 1993). (millions of Subsidence also has contributed to an water pumpage (fig. 3). Surface water acre-feet) from Lake Livingston on the Trinity River 0 appreciable loss of submerged aquatic (fig. 1) was used as a source of water for 1976 1981 1986 1991 1996 vegetation (mostly seagrass) since the 1950s. Some bay shorelines have become industrial use in the area of the Houston Source: Harris-Galveston Coastal Subsidence District Ship Channel (fig. 2). The additional water more susceptible to erosion by wave supplied by Lake Livingston was sufficient Figure 3. During 1978–95, subsidence in areas action because of loss of wetlands. As sea southeast of Houston essentially ceased as level rises, marshes along the shoreline are to appreciably reduce ground-water use and pumpage there decreased; and subsidence ultimately led to a recovery of water levels continued in areas northwest as pumpage there drowned. When development is located over a large area. However, as subsidence increased. near the shoreline, the potential for the 2 maps of relative surface displacement constructed from InSAR data—have demonstrated strong potential for high-density spatial mapping of ground-surface displacement (Galloway and others, 2000). Areas that include stable reflectors may appear as coherent or correlated colors or groups of colors in the interferogram.