Land Subsidence

In this Story Map we will show how Synthetic Aperture Radar (SAR) images can be used to detect and map land subsidence. The capabilities presented here are focused on L-band radars such as NISAR (NASA-ISRO SAR Mission).

Land subsidence, the sinking of the ground, is a global issue commonly caused by groundwater pumping.  The rock surrounding the aquifer compacts due to the absence of subsurface groundwater support, leading to costly damage and sometimes permanent subsidence. Extreme subsidence can also lead to a reduction in the aquifer size, where less groundwater can be stored. Examples of extreme subsidence can be seen in the above figures and relate to the agricultural industry in California. Other notable areas of significant land subsidence include Mexico City, where buildings, highways, and the water supply have all experienced damage. Other causes of land subsidence include aquifer-system compaction, drainage of organic soils, underground mining, hydrocompaction, natural compaction, sinkholes, and thawing permafrost.

Since this phenomenon occurs below ground, the spatial extent of subsidence is not always obvious, especially as aquifers traverse political boundaries. Tracking subsidence features with radar remote sensing can help supplement field measurements of subsidence to monitor known features and warn decision-makers of potential subsidence before it progresses to the point of impacting infrastructure (See NISAR White Paper).

To detect land subsidence with SAR, scientists use interferometric SAR or InSAR. InSAR is a technique that measures the difference between two SAR images of the same area, collected at different times.

Interferograms are maps of land surface change produced from InSAR data. To create interferograms, two SAR images are compared to determine if the ground shifted upwards towards the sensor (uplift) or downwards away from the sensor (subsidence) between the two acquisitions. These ground shifts are reflected in the radar's signal and can be used to create a map of displacement, or interferogram.

Refer to the technology section at the end of this StoryMap to learn more about interpreting interferograms and fringes!

California's Central Valley is a highly productive agricultural region that heavily depends on groundwater for irrigation. Particularly with recent droughts in California, high demand for water has led to over-extraction of the groundwater. As this groundwater is pumped, the soil compacts and causes sinking or subsidence of the ground surface.

The California Aqueduct is a water conveyance system consisting of canals, tunnels, and pipelines that pass through the Central Valley to transport water from Northern to Southern California. Excessive groundwater pumping and the resulting land subsidence pose a threat to the aqueduct's infrastructure, increasing the risk of flooding events and potentially reducing water carrying capacity.

SAR's ability to detect and map land subsidence can assist in identifying and monitoring areas with high subsidence rates. SAR can help inform adjustments to groundwater pumping rates or where to prioritize fortification of aqueduct infrastructure.

Miller et al. 2020 and Jones et al. 2020 used UAVSAR data to detect subsidence adjacent to California's aqueduct. The figure below shows a "subsidence bowl" along the aqueduct between June 2013 and November 2017. For these figures, the interferogram’s fringes were converted to subsidence rates and displayed in a different color scale to show subsidence and uplift in inches. In parts of the subsidence bowl, the land sank by over 40 inches between 2013 and 2017.

SAR can be used to track how subsidence features progress over time. The time series animation below shows the fluctuations in vertical change over the same subsidence bowl between June 2013 to 2016.

Radar remote sensing of land subsidence can offer decision makers a more efficient and less manually intensive method of monitoring vulnerable infrastructure. Given the availability of free and open SAR data from operational satellite-based instruments, SAR-derived subsidence maps are much less costly than traditional field measurements and global positioning system (GPS) surveys, collected point by point. SAR data has centimeter-level accuracy, and can also be used to improve upon established field monitoring (e.g. extensometers, leveling lines, GPS networks etc.), which are still required as tie-points to convert the measured phase shifts to actual ground displacement values. LiDAR, Light Detection and Ranging, can also be employed for high resolution products, but is more expensive to collect and is often accompanied by large data volumes.

Radar sensors emit microwave pulses and measure the reflected signal. The signal has an amplitude and a phase. Amplitude is the strength of the reflected signal determined by the signal’s interaction with surface properties on the ground. The phase is a specific point on the signal's wavelength cycle. The phase can be used to determine the distance from the radar sensor to the ground.

A technique called Interferometric Synthetic Aperture Radar or InSAR uses differences in reflected signal between two or more radar images to map surface changes down to the centimeter level. InSAR can be used to map surface deformation such as from land subsidence, earthquakes, and volcanic eruptions.

NISAR will use Repeat-Pass InSAR where observations are collected from the same location in space but at different times. When the ground moves between acquisitions, the distance changes between the ground and the radar, and a different fraction of the wavelength cycle, or phase shift, is reflected back to the radar.

An interferogram is generated by "interfering" or differencing phase values from two or more radar images to map ground displacement relative to the sensor. Using known radar wavelengths, interferograms detect change down to fractions of a wavelength or phase, which can then be converted to centimeters.

Interferograms are often depicted with a repeating color scale that shows the magnitude of the displacement. The direction of the displacement is indicated by the sequence of "fringes" or color progressions that repeat more frequently towards the center of the subsiding or uplifting areas. Each fringe cycle represents a certain range of change and the number of fringe cycles can be counted to determine displacement in centimeters. This USGS webpage details how to interpret interferograms:  https://www.usgs.gov/centers/land-subsidence-in-california/science/interferometric-synthetic-aperture-radar-insar 

The interferogram below shows land subsidence within the Belridge oil fields in California.

Sensors with longer wavelengths like NISAR will also provide better estimates of land subsidence, especially in vegetated areas. NISAR will collect data in the L-Band wavelength, or 23.8 cm.

Check out NISAR's webpage on interferometry for more information:  https://nisar.jpl.nasa.gov/mission/get-to-know-sar/interferometry/  

Explore the NISAR Sample Data Product Suite, or planned NISAR data products and their product specifications, here:  https://nisar.jpl.nasa.gov/data/sample-data/ 

InSAR infographics:  https://www.unavco.org/education/outreach/infographics/lib/images/insar-infographic.pdf  

“Interferometric Synthetic Aperture Radar (InSAR) .” Interferometric Synthetic Aperture Radar (InSAR) | U.S. Geological Survey, https://www.usgs.gov/centers/land-subsidence-in-california/science/interferometric-synthetic-aperture-radar-insar. 

Jones, C.E., Farr, T.G., Liu, Z. & Miller, M.M. (2020). Measuring Subsidence in California and Its Impact on Water Conveyance Infrastructure. Springer Remote Sensing/Photogrammetry 211–226. DOI: 10.1007/978-3-030-59109-0_9

“Land Subsidence .” Land Subsidence | U.S. Geological Survey, https://www.usgs.gov/special-topics/water-science-school/science/land-subsidence. 

Miller, M.M., Jones, C.E., Sangha, S.S. & Bekaert, D.P. (2020). Rapid drought-induced land subsidence and its impact on the California aqueduct. Remote Sensing of Environment 251, 112063. DOI: 10.1016/j.rse.2020.112063

NISAR: The NASA-ISRO SAR Mission. (2017). Drought and the Rapidly Changing Landscape [White paper]. NASA.   https://nisar.jpl.nasa.gov/documents/20/NISAR_Applications_Drought_and_Groundwater_Withdrawal1.pdf  

NISAR: The NASA-ISRO SAR Mission. (2017). Subsidence and a Sinking Landscape [White paper]. NASA.   https://nisar.jpl.nasa.gov/documents/23/NISAR_Applications_Subsidence.pdf  

UAVSAR data courtesy NASA/JPL-Caltech

 https://uavsar.jpl.nasa.gov/cgi-bin/data.pl