Scientists working with satellite imagery have developed a new technique for determining the likelihood that a given swath of forest canopy is concealing the ruins of an ancient civilization: stress on the vegetation caused by the decay of man-made structures that lie beneath.

As these structures decay, the limestone from which they were built leaches into the soil, according to Tom Sever, a senior scientist with NASA’s Marshall Space Flight Center in Huntsville, Ala. The effect this limestone has on the health of local plants shows up in certain types of satellite imagery, he said.

Archaeologists have been using satellite and airborne imagery for years to look for ruins and other evidence of ancient civilizations in remote corners of the world. Radar sensors, for example, can detect structures underneath jungle canopies or even buried underneath the soil. Because they cover such wide swaths of territory, satellites and airborne sensors can help archaeologists limit expensive on-site excavations to only the most promising sites.

NASA has been assisting archaeologists at the University of New Hampshire in an effort to find Mayan ruins in Central America since 2003. The agency is contributing satellite imagery and scientists to the project, which is funded at about $100,000 per year and is slated to run until 2009.

Another goal of the project is to determine how deforestation by ancient Mayan civilizations affected the local climate, Sever said. To do this, NASA and university scientists are relying in part on modern day climate-change models, he said.

The NASA-assisted survey already has led to the discovery of ruins that previously were not known to exist, Sever said in a telephone interview. “We see major sites that no one has ever been to that we know of; well, we’re sure the looters have been there,” Sever said. “This helps us in the protection and preservation of those areas.”

Sever said researchers basically stumbled upon the method of using vegetation stress to determine whether ruins might be present.

NASA was providing imagery from the U.S. government Landsat and the commercially operated Ikonos and QuickBird satellites to support the survey effort. By 2004, William Saturno, an archaeologist with the University of New Hampshire, noticed that the images of areas where ruins had been discovered had a distinct color shade, or signature, Sever said.

When Saturno brought this to the attention of his NASA partners, the team decided to investigate. They scoured their imagery archives and picked out 40 more locations where the same signature was visible. They then sent teams to do ground surveys of the 40 sites, and all were found to have ruins, Sever said.

The team concluded that the signature was associated with stress on the vegetation caused by the residue of limestone and lime plasters — the building blocks of Mayan structures — in the local soil, Sever said. The team has since revised its imagery-processing techniques to zero in on areas that have the telltale signature, he said.

A future goal for Sever and his team is to use the vegetation signature model to investigate archaeological sites in Southeast Asia. The group will meet with other archaeologists to determine which areas and civilizations to study in late spring or early summer of this year, he said. The research team also wants to extend the area in Guatemala it is investigating to the entire 36,000-square-kilometer Peten region, and beyond that into Mexico, he said.

Meanwhile, the team is drawing on modern climate-modeling techniques to determine how Mayan civilizations affected the local environment, Sever said. Climate change is normally associated with modern industrialization, but ancient peoples also had an impact — albeit a more localized one — through activities such as deforestation.

Sever estimates that for every square meter of lime plaster the Mayans produced, they had to burn some 20 trees to get at the limestone in the ground. He noted that in secondary settlements, the limestone-based building blocks tend to be thinner because there was less raw material to work with — another indication of deforestation.

Deforestation also tends to reduce local rainfall and increase temperatures, Sever said.

Karl Benedict, a senior research scientist at the University of New Mexico’s Earth Data Analysis Center in Albuquerque, said climate models, many of which are fed by satellite data, can help scientists understand the relationship between climate and civilization in prehistoric times.

Just as satellite imagery showing vegetation density can be correlated with rainfall levels today, information gleaned from on-site surveys can provide clues about the local environment in ancient times, Benedict said. For example, he said, scientists can estimate prehistoric rainfall levels based on the space between the rings in both dead and preserved trees. This data can in turn give scientists some idea about the local vegetation density back then, he said.

“So you can essentially look at landscape dynamics through time,” Benedict said.