Radar could be used to predict the appearance of sinkholes, according to new analyses of Nasa airborne radar data.
The data was collected by the agency’s Uninhabited Airborne Vehicle Synthetic Aperture Radar (UAVSAR), which uses Gulfstream III jet to carry an L-band radar antenna that produces polarimetric (PolSAR) and interferometric synthetic aperture radar (InSAR) imagery.
Data collected in 2012 revealed the radar detected indications of a huge sinkhole before it collapsed and forced evacuations near Bayou Corne, Lousianna, that year, suggesting that if such radar data was collected routinely from airborne systems or satellites it could sometimes foresee sinkholes before they happen.
"You could spend a lot of time flying and processing data without capturing a sinkhole," said Ron Blom of Nasa's Jet Propulsion Laboratory in Pasadena, California.
"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, such as the planned Nasa-Indian Space Research Organisation Synthetic Aperture Radar mission."
Sinkholes are depressions in the ground formed when Earth surface layers collapse into caverns below and they usually form without warning. The data was collected as part of an ongoing Nasa campaign to monitor sinking of the ground along the Louisiana Gulf Coast.
Working with colleague Cathleen Jones, Blom analysed InSAR imagery of the area, detects and measures very subtle deformations in Earth's surface.
Their analyses showed the ground surface layer deformed significantly at least a month before the collapse, moving mostly horizontally up to 260 millimetres toward where the sinkhole would later form.
These precursory surface movements covered a much larger area – about 500 metres squared – than that of the initial sinkhole, which measured about one hectare. Results of the study are published 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," said Jones. "This kind of movement may be more common than previously thought, particularly in areas with loose soil near the surface."
The Bayou Corne sinkhole continues to grow, threatening the community and Highway 70, so there is a pressing need for reliable estimates of how fast it may expand and how big it may eventually get.
"This kind of data could be of great value in determining the direction in which the sinkhole is likely to expand," said Jones. "At Bayou Corne, it appears that material is continuing to flow into the huge cavern that is undergoing collapse."
The researchers said there are no immediate plans to fly UAVSAR over sinkhole-prone areas, but the discovery is an important discovery nonetheless.
"Every year, unexpected ground motions from sinkholes, landslides and levee failures cost millions of dollars and many lives," said Jones. "When there is small movement prior to a catastrophic collapse, such subtle precursory clues can be detected by InSAR."