March 7, 2014
NASA Radar Data Could Help Forecast Sinkholes
redOrbit Staff & Wire Reports – Your Universe Online
For well over a year now, sinkholes have been in the news, as the geological features have been responsible for incidents such as the February 2013 death of a Florida man whose entire bedroom was swallowed up by one and the more recent damaging of eight historic automobiles at the National Corvette Museum in Kentucky.
Sinkholes, which are depressions in the ground formed when Earth surface layers collapse into the caverns beneath them, entered the public consciousness following the February 28, 2013 incident in which 37-year-old Seffner, Florida resident Jeff Bush was killed after a 20-foot wide sinkhole opened up beneath his bedroom sometime around 11pm Eastern time.
Rescue workers spent two days trying to find Bush, who was swallowed up by the sinkhole along with all of his bedroom furniture, but were ultimately forced to call off the search before the area around the depression grew too unstable. Experts said that the depression occurred because the ground beneath the house was known as “karst terrain,” meaning that soluble rocks such as limestone are dissolved by the circulation of acidic groundwater.
The incident at the National Corvette Museum occurred during the early hours of February 12, 2013. According to the Los Angeles Times, a 25- to 30-foot deep and 40-foot wide sinkhole opened up and swallowed eight of the classic Chevrolet vehicles stored at the Bowling Green facility. No one was hurt, and to date four of the automobiles have been recovered, though they were obviously damaged.
The less publicized Bayou Corne sinkhole predated both of those incidents, first appearing on August 3, 2012 following weeks of minor earthquakes and multiple community reports of natural gas bubbles. It was caused when the sidewall of an underground storage cavity connected to a nearby well operated by Texas Brine Company and owned by Occidental Petroleum collapsed, the US space agency said in a statement Thursday.
“On-site investigation revealed the storage cavity, located more than 3,000 feet (914 meters) underground, had been mined closer to the edge of the subterranean Napoleonville salt dome than thought,” NASA added. “The sinkhole, which filled with slurry – a fluid mixture of water and pulverized solids – has gradually expanded and now measures about 25 acres (10.1 hectares) and is at least 750 feet (229 meters) deep. It is still growing.”
However, the Bayou Corne sinkhole occurred in a region monitored as part of an ongoing NASA campaign to keep track of the sinking of the ground along the Louisiana Gulf Coast. Interferometric synthetic aperture radar (InSAR) imagery collected during Uninhabited Airborne Vehicle Synthetic Aperture Radar (UAVSAR) occurring as part of that operation was analyzed by NASA officials and found indications that could have predicted the sinkhole.
Cathleen Jones and Ron Blom of NASA's Jet Propulsion Laboratory (JPL) studied the data and discovered that the ground surface layer was “significantly” deformed at least one month prior to the actual collapse, moving primarily horizontally more than 10 inches towards the location of the eventual sinkhole.
Those movements covered a larger area (roughly 1,640 feet by 1,640 feet) than the initial sinkhole (approximately two acres), the researchers discovered. Jones and Blom report their findings in the February issue of the journal Geology.
“While horizontal surface deformations had not previously been considered a signature of sinkholes, the new study shows they can precede sinkhole formation well in advance,” Jones explained. “This kind of movement may be more common than previously thought, particularly in areas with loose soil near the surface.”
“Our work shows radar remote sensing could offer a monitoring technique for identifying at least some sinkholes before their surface collapse, and could be of particular use to the petroleum industry for monitoring operations in salt domes,” added Blom. “Salt domes are dome-shaped structures in sedimentary rocks that form where large masses of salt are forced upward. By measuring strain on Earth's surface, this capability can reduce risks and provide quantitative information that can be used to predict a sinkhole's size and growth rate.”
Sinkholes can vary greatly in size, and they can be found throughout the US, with the highest amounts of sinkhole-related damage being reported in the states of Florida, Missouri, Texas, Alabama, Kentucky, Tennessee and Pennsylvania. The Bayou Corne sinkhole was a human-produced one that occurred in a region not typically prone to such geological events, though observations show that it is continuing to grow.
In fact, according to Jones and Blom, UAVSAR monitoring of the area as recently as October 2013 has shown “a widening area of deformation” that could impact other nearby storage cavities. Its continued growth could threaten the people and the infrastructure in that area, so there is a need for reliable estimates as to how quickly it could expand and how large it could ultimately become – information that InSAR could provide.
“Every year, unexpected ground motions from sinkholes, landslides and levee failures cost millions of dollars and many lives. When there is small movement prior to a catastrophic collapse, such subtle precursory clues can be detected by InSAR,” Jones said.
However, Blom said that there is currently no plan to fly UAVSAR over sinkhole-prone areas.
“You could spend a lot of time flying and processing data without capturing a sinkhole,” he added. “Our discovery at Bayou Corne was really serendipitous. But it does demonstrate one of the expected benefits of an InSAR satellite that would image wide areas frequently.”
Image 2 (below): Analyses by NASA's UAVSAR after the Bayou Corne, La., sinkhole formed show it detected precursory ground movement of up to 10.2 inches (260 millimeters) more than a month before the sinkhole collapsed. Colors represent surface displacement (one full color wrap equals 4.7 inches (120 millimeters). Credit: NASA/JPL-Caltech