Scientists have long suspected that microwave radar from satellites could "see" below the surface of very dry ground. Many were startled when images from a shuttle mission in the 1980s revealed what appeared to be ancient river drainage patterns below the eastern Sahara desert. Since then there have been other intriguing finds, including ring structures buried under Antarctic ice that look like meteorite craters or the remains of subglacial volcanic eruptions.
But until now there has been no proof that these images really do show underground objects. Researchers were unsure how radar is affected by underground features, and no one had ever used radar sensing to detect objects they knew were there. So although the images looked convincing, it was possible that they simply showed varying soil properties or surface slopes.
Dan Blumberg and Julian Daniels of the Ben Gurion University of the Negev in Israel decided to test the idea. They buried flat squares of aluminium at different depths under the sand in the Negev desert, then flew an aircraft over the area to carry out radar sensing of the layers beneath the surface.
By comparing the radar results with the squares' known positions, the researchers showed that the patterns detected by the radar really did show the buried pieces of metal. "Now we have systematic proof," Blumberg told New Scientist. "Buried objects can be detected from airborne systems."
That suggests the satellite images show real structures, too. Blumberg says the result confirms that ancient river routes do lie hidden beneath centuries of Saharan sand. Their location ties in with that of the oases and temporary lakes around which desert peoples build their communities. "Mapping river channels buried in sandy areas can improve our understanding of the geological and climatic history of the region," says Daniels.
So far, the researchers have only looked for objects buried up to 40 centimetres deep. But now that they have proved the technique works, they are planning studies with different types of object, buried deeper.
For their experiments, they use microwaves of the longest possible wavelength, called P-band (see Graphic). At the moment, satellites generally use microwaves with shorter wavelengths because the resolution is better. But P-band radiation can penetrate farther underground, so Blumberg hopes that adding it to satellite sensors will allow them to probe deeper, perhaps up to 9 metres down.
"Using the P-band is quite new," says Andrew Wilson, a remote-sensing expert for Britain's Natural Environment Research Council. "It can penetrate the ground farther, so it would be good at revealing archaeological objects."
Blumberg hopes that as well as archaeological remains, the method will in time be used to find fossils and geological structures. There could also be military or humanitarian applications. The resolution would probably be too low to pick up individual landmines, but it could show underground buildings or pipes, or perhaps even mass graves.
The main snag is that radar can only penetrate the ground in very dry areas, because liquid water tends to absorb the radiation. But Blumberg says that 15 per cent of the Earth's surface is dry enough for the method to work, including the Antarctic and deserts. The surfaces of some planets and moons fit the bill too, he points out. He hopes P-band radar might be used to reveal structures such as water channels below the dry, frozen surface of Mars.
Hazel Morris
UK CONTACT - Claire Bowles, New Scientist Press Office, London:
Tel: 44-207-331-2751 or email claire.bowles@rbi.co.uk
US CONTACT - Michelle Soucy, New Scientist Boston Office:
Tel: 617-558-4939 or email michelle.soucy@newscientist.com
New Scientist issue: 12th July 2003
PLEASE MENTION NEW SCIENTIST AS THE SOURCE OF THIS STORY AND, IF PUBLISHING ONLINE, PLEASE CARRY A HYPERLINK TO: http://www.newscientist.com
"These articles are posted on this site to give advance access to other authorised media who may wish to quote extracts as part of fair dealing with this copyrighted material. Full attribution is required, and if publishing online a link to www.newscientist.com is also required. Advance permission is required before any and every reproduction of each article in full - please contact celia.thomas@rbi.co.uk. Please note that all material is copyright of Reed Business Information Limited and we reserve the right to take such action as we consider appropriate to protect such copyright."