During natural disasters like high-magnitude earthquakes, areas most in need of assistance must be located as fast as possible. Researchers, led by Sang-Ho Yun at NASA's Jet Propulsion Laboratory, Pasadena, California, have developed a way to make maps of damage using the remote sensing technology of satellites. The method works even if the satellite images are taken at night or in cloudy skies.

NASA used the approach to study the impacts of Nepal's magnitude 7.8 Gorkha earthquake.

"Our mapping system shows great potential, especially for isolated remote areas where there is no communication and the roads are blocked. Those are the communities in desperate need of help, and our maps could help responders provide efficient assistance," said Yun.

Using software developed at JPL, the researchers produced damage proxy maps, with data from the Italian Space Agency's (ASI) COSMO-SkyMed system and the Japan Aerospace Exploration Agency's (JAXA) ALOS-2 satellite. For each data set, the team examined the similarities between two radar images: two archival images from before the earthquake and one taken after.

The software allowed researchers to generate a distribution of colored pixels on a transparent background, which they overlaid on top of maps from Google Earth. The colors are on a scale from yellow to red, with red representing the areas of greatest potential damage. At 100-foot (30-meter) resolution, it is possible to see damage to major buildings, as well as large landslides.

The researchers then compared their damage proxy maps to maps that were made from human inspection of high-resolution optical satellite imagery, which came from the National Geospatial-Intelligence Agency (NGA) and the United Nations Operational Satellite Applications Programme.

Both satellite systems used in the study make use of "synthetic aperture radar," or SAR, a technology that allows for detailed radar imaging from space without an extremely large antenna. Like a real-aperture radar system, a SAR satellite transmits microwave signals that are reflected by Earth's surface, and those reflections produce radar images.

Though the maps sometimes overestimated the breadth of damaged areas, they helped the United States Agency International Development team in Nepal be better prepared with other types of information to coordinate a relief response, said Indra Sharan K.C., geographic information systems specialist for USAID in Kathmandu.

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Also: Read about FINDER, a NASA device that detects individuals during a disaster.