Approximate location of maximum subsidence in the U.S., identified by research efforts of Dr. Joseph F. Poland (pictured). Signs on pole show approximate altitude of land surface in 1925, 1955, and 1977. The site is in the San Joaquin Valley southwest of Mendota, CA.
What is land subsidence?
Land subsidence is a gradual settling or sudden sinking of the Earth's surface owing to subsurface movement of earth materials. The principal causes are aquifer-system compaction, drainage of organic soils, underground mining, hydrocompaction, natural compaction, sinkholes, and thawing permafrost (National Research Council, 1991). Three distinct processes account for most of the water-related subsidence--compaction of aquifer systems, drainage and subsequent oxidation of organic soils, and dissolution and collapse of susceptible rocks.
How is land subsidence measured?
Measurements of elevations, aquifer-system compaction, and water levels are presented, interpreted, and integrated to improve understanding of the processes responsible for land-surface elevation changes. Elevations, and elevation changes, have been measured using Interferometric Synthetic Aperture Radar (InSAR), Continuous GPS (CGPS) measurements, campaign Global Positioning System (GPS) surveying, and spirit-leveling surveying.
Importance of Monitoring Subsidence
Continued groundwater-level and land-subsidence monitoring in the San Joaquin Valley is warranted because groundwater levels are poised to decline when surface-water deliveries do not meet demand, which may result in additional land subsidence. Even in precipitation record-setting years such as 2010-11, water deliveries fell short of requests in the Central Valley. Therefore, it is likely that groundwater levels will decline in the future. Integrating subsidence, deformation, and water-level measurements-particularly continuous measurements-permits analysis of aquifer-system response, which enables identification of the preconsolidation head and calculation of aquifer-system storage properties. This information could be used to improve numerical models of groundwater flow and aquifer-system compaction, to refine estimates of governing parameters, and to predict potential aquifer-system compaction which could be used to manage water resources while considering land subsidence.
A subsidence monitoring network in the San Joaquin Valley consisting of 31 extensometers was
developed and maintained in the 1960s to help identify the extent of the subsidence that was discovered in the 1950s. By the 1980s, the monitoring network's spatial and temporal measurements had deteriorated.
To identify existing and future subsidence, a new monitoring network is
currently being developed. This includes refurbishing some of the
extensometers and piezometers from the old network, and augmenting these ground-based measurements with remotely-sensed measurements from continuous Global Positioning System (CGPS) stations and Interferometric Synthetic Aperture Radar (InSAR). Preliminary results from the monitoring network indicate that subsidence is occurring in locations of known historical subsidence.
31 extensometers operating at 21 sites
Extensive spirit-leveling networks
26 extensometers operating at 18 sites
Reduced spirit-leveling networks
Frequency of measurements severely reduced
6 extensometers operating at 5 sites
21 discontinued/unknown status
4 extensometers refurbished (2011-12)
new reference tables, shelters, and instrumentation
measurement frequency increased
Spirit-leveling and campaign GPS networks generally
maintained on major water-conveyance canals and highways only
13 Continuous GPS sites (maintained by various
agencies/groups) on the valley floor
Conventional and Persistent Scatterer InSAR techniques are
applied to the western portion of the San Joaquin Valley from
the city of Tracy on the north to the intersection of
Interstate 5 and California Highway 41 on the south
As the monitoring network becomes fully developed, the
integration of remotely-sensed measurements from InSAR and
GPS with ground-based measurements from extensometers and
spirit-leveling will enable construction of spatially and
temporally dense timeseries of aquifer-system compaction and
land subsidence. The high frequency measurements of
compaction, land subsidence, and groundwater levels will
enable inclusion of shorter-term elastic deformation in
simulations that currently is not well-constrained in most of
the San Joaquin Valley.
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.
Interferometric Synthetic Aperture Radar (InSAR) is an effective way to measure changes in land surface altitude. InSAR makes high-density measurements over large areas 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 line-of-sight ground-surface displacement (range change) between the two time periods. 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 the color progression of the fringe(s) toward the center of a deforming feature.
Digital Elevation Maps
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 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 the ESA 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 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 groundwater pumping, hydrocarbon production, or other human activities 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.
Reading an interferogram isn't as complicated as it might seem. The process can be broken down into a few steps:
Count the number of InSAR fringes between two points on the interferogram, where one fringe is one complete color cycle (i.e. yellow, red, blue, green, yellow).
The figure to the left illustrates how land-surface displacement, in this example it's uplift, is represented on an interferogram. In this example, each fringe, or color cycle, represents 28 mm of range change.
Multiply the number of fringes by 28.3mm (1.1 in.).
Because there are 3 fringes, the maximum displacement (at the top of the pimple), is 85 mm. Depending on processing, one fringe could represent a different magnitude of displacement. Generally, about 1/3 of a fringe (two colors) is discernable 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 5mm of displacement is discernable.
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. yellow, red, blue, green) signifies subsidence, and a decrease in range indicates uplift.
Continuous Global Positioning System (CGPS) Stations
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 timeseries, which allow us to track the 3D position of the
USGS survey party spirit levelling 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. Sprit leveling is a precise way to obtain data for smaller land areas, and is commonly used along road, railroad tracks, aqueducts and canals, etc.
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). If only
vertical position were 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.
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 some depth. More than 2 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), and represent the first extensometers ever built in the United States. Additionally, a couple of 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).
Some of the older extensometers are now in various stages of disrepair, but some are in surprisingly good shape. As part of this study, the older extensometers were assessed to determine if and how we might refurbish them to improve data accuracy and increase the number of measurements (some have not been measured for more than a decade). We have selected 4 extensometers to refurbish: 12S/12E-16H2, 18S/16E-33A1, 20S/18E-6D1, and 14S/13E-11D6 (see map).
The blue dots on the map represent the extensometers that were refurbished by the USGS for the Delta-Mendota Canal study.
The refurbishment is 2-pronged: we will construct a more reliable reference table by drilling 3 shallow auger holes at each site which will greatly reduce any vertical motion related to surface processes such as the swelling and contracting of clays near the earth's surface in response to rain events, and we will install digital instruments to measure continuous aquifer-system deformation and analog instruments to control the quality of the digital measurements and also as a backup measurement in case of digital equipment failure. The refurbishment will improve the data quality by focusing the measurements on the deeper aquifer-system processes we are interested in, and will greatly improve the data quantity as we will set the digital instruments to make hourly measurements.
Before Refurbishment (Click image to enlarge)
After Refurbishment (Click image to enlarge)
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 continuous GPS data and water-level data is 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 owing 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 could be estimated.
Water level and CGPS data for Delta-Mendota Canal CGPS site P304.