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THIS is not your average pet cemetery. The elephants, zebras and buffalo buried in a Canadian safari park are notable for more than their size: they are also yielding new methods for detecting mass graves from the air. The technique, which searches for signs of chemical changes in the vegetation growing on grave sites, could ultimately help police and human rights investigators locate human remains years after the bodies have been disposed of.

Known as hyperspectral imaging, the technique analyses a range of visible and infrared wavelengths as its scans terrain from the air. Cameras mounted on a light aircraft or helicopter detect variations in the intensity of light of various wavelengths reflected by vegetation on the ground. The precise pattern of intensities has been found to reflect changes caused by nutrients released into the soil as bodies decompose.

When searching for clandestine graves, investigators traditionally look for signs of disturbance on the ground, or dig small test trenches to identify the most likely area. “From personal experience, I know it’s possible to miss remains by a few centimetres, then realise it later and have to come back,” says of McGill University in Montreal. Costopoulos is a member of a team of forensic archaeologists who have been putting hyperspectral imaging to the test in a search for animal carcasses buried at in nearby Hemmingford, Quebec. “Even quite substantial remains within an acre can be hard to find,” Costopoulos says.

The team’s technique could prove useful to investigators looking for victims of war or genocide who have been buried in mass graves. They often do not know where to start their search, and so have to depend on possibly unreliable tip-offs from local people. Satellite imagery and pictures from reconnaissance aircraft have been used to show soil disturbance, vegetation being cleared – even bodies on the ground awaiting burial – but such images are not always available.

The McGill team was originally called in by Parc Safari to help hunt for the remains of an elephant, which the park wanted to exhume and piece together for an educational exhibit. “We dug a test pit where the park’s owner told us he had buried the elephant, but the elephant wasn’t there,” says Costopoulos. It took several weeks of digging to locate it.

“We dug a test pit where the park’s owner told us he had buried the elephant, but the elephant wasn’t there”

Margaret Kalacska, another member of the McGill team, was already investigating how plants are affected by the composition of the soil they grow on. “Plants are living systems, and any changes in water content or the soil chemistry are going to affect how they reflect light,” she says.

To see whether the chemicals released by decomposing corpses had an effect, the team took samples of soil and vegetation from known grave sites and from random points across Parc Safari’s half-hectare burial field. They found differences in the chlorophyll content of leaves, and identified the spectroscopic signatures in light reflected by the leaves and soil that might be used to identify similar differences from the air.

They then flew over the safari cemetery, recording images of the ground using two sensors which between them could scan all the visible wavelengths and short-wavelength infrared. These images showed clear differences between areas that were known to contain animal remains – some of them decades old – and those that did not. It also picked up sites that appeared to contain buried animals, but whose location had not previously been recorded.

Kalacska presented the team’s findings in February at the (AAFS) annual meeting in Seattle. The team hopes to go back to the site to confirm whether what appear to be previously unrecorded grave sites really do contain animal remains.

The technique has great potential, says , a forensic archaeologist at the University of Bournemouth, UK, who has investigated mass graves in Iraq and Bosnia. “Some of these animals were buried around 20 years ago, so you could take new imagery over areas where bodies were buried 20, 30, 40 years ago and discover things that no one has ever been able to find before.” This could be particularly useful in detecting older mass graves, he says.

A related method that is currently being developed by the FBI detects living humans, and recently dead bodies lying on the ground, by recognising the chemical signature of human skin. It could be used when trying to locate and rescue people who are lost or missing, and to track down fugitives.

Kerri Moloughney of the FBI in Quantico, Virginia, and her colleagues fitted a helicopter with a hyperspectral camera covering visible and infrared wavelengths in the range 400 to 2350 nanometres to see whether skin signatures could be spotted from the air. They then flew it over a specially prepared site where human and animal remains at various stages of decomposition had been scattered on the ground, and where there were also a number of live human volunteers. The signals it picked up showed a clear distinction between living human skin and the skin of long-dead humans and animals.

Moloughney says the technique could be combined with visual aerial searches and thermal imaging to pinpoint individuals in a landscape. “We hope it will enable us to find so much more than we can currently,” says Moloughney, who also presented her results at the AAFS.