Measuring Land Subsidence
In 1983, Brownwood (in the Houston/Galveston Bay area) was flooded after hurricane Alicia produced a storm surge up to 11 feet. When this subdivision was built (beginning in 1938), the area was about 10 feet or less above sea level; by 1978, more than 8 feet of subsidence had occurred. The neighborhood was abandoned, and now is a swampy marsh. Photo by Harris-Galveston Subsidence District.
Measurements of elevations, aquifer-system compaction, and water levels are used to improve our understanding of the processes responsible for land-surface elevation changes. Elevation or elevation-change measurements are fundamental to monitoring land subsidence, and have been measured by using interferometric synthetic aperture radar (InSAR), continuous GPS (CGPS) measurements, campaign global positioning system (GPS) surveying, and spirit-leveling surveying. Aquifer-system compaction is measured by using extensometers; these measurements have the added benefit of being depth-specific because extensometers are anchored at specific depths of interest. So, while each extensometer measures some fraction of total subsidence, the measurements can help us better understand the depths at which compaction is occurring. The most precise measurements tend to be made using spirit-leveling surveys and extensometers. The least precise measurements tend to be made by using GPS surveying, with CGPS and InSAR measurements falling somewhere in the middle with regard to preciseness.
Spirit leveling, GPS, and extensometer measurements tend to be spatially sparse because these measurements are taken at only a few locations. InSAR measurements are spatially dense. Measurement frequency is dependent on study objectives, measurement methods, and subsidence rates. For example, if subsidence rates are slow, extended periods between GPS measurements may be necessary to permit a greater signal-to-noise ratio (enough subsidence needs to occur between measurements to exceed the expected measurement error), or a more accurate method such as spirit leveling may be necessary. Ancillary measurements depend on the process responsible for the subsidence; and, a variety of ancillary measurements might be needed to determine the process responsible. Ancillary measurements might include water levels, consolidation tests, carbon gains/losses in a system.
Interferometric Synthetic Aperture Radar (InSAR)
The European Space Agency's (ESA) ENVISAT satellite was used to map and measure displacement. The satellite is side-looking, orbits the Earth at an altitude of approximately 500 miles (800 kilometers), and has 35-day repeat cycles. Illustration by the European Space Agency.
Interferometric Synthetic Aperture Radar (InSAR) is an effective way to measure changes in land surface altitude. InSAR makes high-density measurements over large areas by using radar signals from Earth-orbiting satellites to measure changes in land-surface altitude at high degrees of measurement resolution and spatial detail (Galloway and others, 2000).
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 ground-surface displacement (range change) between the two time periods.
Advantages of InSAR
InSAR is ideally suited to measure the spatial extent and magnitude of surface deformation associated with fluid extraction and natural hazards (earthquakes, volcanoes, landslides). It is often less expensive than obtaining sparse point measurements from labor-intensive spirit-leveling and global positioning system (GPS) surveys, and can provide millions of data points in a region about 10,000 square kilometers. By identifying specific areas of deformation within broader regions of interest, InSAR imagery can also be used to better position specialized instrumentation (such as extensometers, GPS networks, and leveling lines) designed to precisely measure and monitor surface deformation over limited areas.
Satellites are an integral part of InSAR. In March 2002, the European Space Agency (ESA) launched Envisat, an advanced polar-orbiting Earth observation satellite which provides measurements of the atmosphere, ocean, land, and ice. The Envisat satellite has an ambitious and innovative payload that will ensure the continuity of the data measurements of its predecessor, the ESA European Remote Sensing (ERS) satellites. Envisat data supports earth science research and allows monitoring of the evolution of environmental and climatic changes. Furthermore, the data will facilitate the development of operational and commercial applications. Description and illustration courtesy of the European Space Agency.
Interferograms are maps of relative ground-surface change that are constructed from InSAR data to help scientists understand how tectonic or human activities, such as groundwater pumping and hydrocarbon production, cause the land surface to uplift or subside. Interferograms require 2 images taken at intervals in time to determine if there has been any shift in land surface levels. 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. The direction of displacement - subsidence or uplift - is indicated by sequence of the color progression of the fringe(s) toward the center of a deforming feature.
Reading an interferogram isn't as complicated as it might seem. The process can be broken down into a few steps:
Step 1: Mapping InSAR displacement. In this illustration, two InSAR fringes are equal to 56 mm of deformation.
Count the number of InSAR fringes between two points on the interferogram, where one fringe is one complete color cycle (i.e. red, orange, yellow, green, blue, purple).
The figure illustrates how land-surface displacement, in this example it's subsidence, is represented on an interferogram. In this example, each fringe, or color cycle, represents 28 mm of range change
Step 2: Processing interferograms. In this example, two InSAR fringes are equal to 56 mm of subsidence.
Multiply the number of fringes by 28 mm (1.1 in.).
Because there are 2 fringes, the maximum displacement (at the bottom of the bowl), is 56 mm. Depending on processing, one fringe could represent a different magnitude of displacement. Generally, about 1/3 of a fringe (two colors) is discernible displacement; in this example, 10 mm of displacement. If we process the data such that 14 mm of displacement is represented by 1 fringe, then about 5 mm of displacement is discernable.
Step 3: Determining deformation. An increase in range means subsidence is depicted. A decrease in range denotes uplift.
Determine if the ground moved closer (uplift) or farther away (subsidence) from the satellite by matching how the colors change between the two points with the InSAR scale bar. An increase in range (i.e. red, orange, yellow, green, blue, purple) signifies subsidence, and a decrease in range indicates uplift.
Continuous Global Positioning System (CGPS) Stations
CGPS Station P303 in Los Banos, CA.
A CGPS station continuously measures the three-dimensional (3D) position of a point on, or more specifically, near the earth's surface. There are more than 1,000 Continuous Global Positioning System Stations operating in Western North America, and hundreds of them in California alone; many of them are managed by the Plate Boundary Observatory/UNAVCO and by Scripps Orbit and Permanent Arrary Center (SOPAC), but other groups such as Caltrans, also operate some of them as part of their Central Valley Spatial Reference Network. They generally have been constructed to monitor motions caused by plate tectonics, but are widely used for other applications, including subsidence monitoring.
These GPS stations generally collect position information every 15 seconds which are then processed to produce a daily position. These daily positions are then concatenated to produce a daily time series, which allow us to track the 3D position of the station.
USGS survey party spirit leveling near Colusa, Sacramento Valley, circa 1905. Photo by John Ryan, donated courtesy of Thomas E. Ryan.
Spirit leveling, the oldest method of measuring subsidence and uplift, derives its name from the primary tool utilized in the process — the spirit level. Spirit leveling is a precise way to obtain data for smaller land areas, and is commonly used along road, railroad tracks, aqueducts, and canals.
Spirit leveling was once a common method of determining elevation. Before the advent of the satellite-based Global Positioning System (GPS) in the 1980s, the most common means of conducting land surveys involved either the theodolite or, since the 1950s, the geodimeter (an electronic distance-measuring device, or EDM). When only vertical position is sought, the spirit level has been the instrument of choice. The technique of differential leveling allows the surveyor to carry an elevation from a known reference point to other points by use of a precisely leveled telescope and graduated vertical rods. Despite its simplicity, this method can be very accurate. When surveying to meet the standards set for even the lower orders of accuracy in geodetic leveling, 0.05-foot changes in elevation can be routinely measured over distances of miles. At large scales, leveling and EDM measurement errors increase. When the scale of the survey is small (on the order of 5 miles or less) and the desired spatial density is high, spirit leveling is still commonly used because it is accurate and relatively inexpensive. Large regional networks warrant use of the more efficient Global Positioning System (GPS) surveying for differential surveys.
The image below illustrates the precise nature of the spirit-leveling method of measurement. The leveling data was collected by the California Department of Water Resources along the Central Valley's Delta-Mendota canal. The inset graph displays historical subsidence data calculated from leveling measurements for a portion of the Canal. These data are from the Delta-Mendota Water Authority and the Central California Irrigation District.
An extensometer measures the one-dimensional (1D) change in thickness of a specified depth interval. In other words, it measures the compaction and expansion of the aquifer system to a specific depth. More than two dozen extensometers in the Central Valley were constructed in the 1950s, 1960s, and 1970s by the U.S. Geological Survey in cooperation with the California Department of Water Resources (Ireland and others, 1984), the early group of which represent the first extensometers ever built in the United States. Additionally, several extensometers have been constructed in the San Joaquin Valley more recently. All of the extensometers are constructed as cable or pipe borehole extensometers (see Figure 1).
Figure 1. Recording-extensometer installations. A, cable extensometer, and B, free-standing-pipe extensometer. The depth interval being measured in these generic cases is land surface to reference point. Figure from Poland and others, 1984.
Piezometers and Groundwater Levels
When extensometer or CGPS data are paired with groundwater-level data from a nearby well, some storage properties of the affected aquifer system can be calculated. The CGPS data and water-level data are used for stress-strain analysis. If water levels fluctuate in the elastic range of stress, the elastic skeletal storage coefficient will be computed. The elastic skeletal storage coefficient is a standard measure of aquifer storage that is directly related to the compressibility of the aquifer-system skeleton and largely governs the recoverable (reversible) deformation of the aquifer system. If water levels continue to decline beyond historically low levels (the inelastic range of stress), it may be possible to compute the inelastic storage coefficient that governs the permanent compaction of the aquifer system. If water levels are fluctuating in both ranges of stress (fluctuating seasonally and declining annually), both the elastic and inelastic storage coefficients can be computed.
Groundwater Sustainability Indicators
Groundwater Availability of the Central Valley Aquifer, California
USGS Professional Paper 1766
Guidebook to studies of land subsidence due to ground-water withdrawal
Prepared for the International Hydrological Programme, Working Group 8.4
Land Subsidence along the Delta-Mendota Canal in the northern part of the San Joaquin Valley, California, 2003-10
USGS Scientific Investigations Report 2013-5142
Land Subsidence from Groundwater Use in California
Report of Findings, 2014
Land subsidence in the San Joaquin Valley, California, USA, 2007-2014
Proceedings of the International Association of Hydrological Sciences
Land Subsidence in the United States
USGS Circular 1182
Land Subsidence in the United States
USGS Fact Sheet-165-00
Measuring Land Subsidence from Space
USGS Fact Sheet-051-00