They used to look to historical writings and rely on shovels and brushes to carry out archaeological excavations, which have remained their primary investigative methods for centuries.

But things have changed greatly in the last couple of years, with the introduction of remote sensing technologies into archaeological investigations in China, greatly broadening archaeologists' scope.

They are now able to reach places once considered inaccessible, carry out exploration without needing to be at the site, and search vast areas for cultural relics that might have been impossible to find using conventional methods.

"The introduction of remote sensing technology in archaeological exploration is revolutionary," said Zhao Dianzeng, a senior researcher at the Institute of Cultural Heritage and Archaeology of Sichuan Province.

Zhao said the technology could be particularly useful when searching large sites of cultural relics, such as the Sanxingdui Relics in Sichuan, which encompass an area of over 1.2 million square meters. "It would take over 100 years to explore this site using conventional methods," he said.

He likened the technology to the carbon dating technology that was invented by physicists, but has now become a standard method to accurately date cultural relics.

Once introduced, this method was embraced by many Chinese archaeologists, who have used remote sensing technology in a number of large-scale investigations in order to crack many historical enigmas.

Earlier this month, a joint exploration of the mausoleum of the first emperor of Qin Dynasty (221-206 BC), Ying Zheng, started in Northwest China's Shaanxi Province. A variety of remote sensing technologies were used to detect whether artificial mercury rivers and the many fabled treasures lay underground.

Previous efforts had been frustrated because field excavation was banned by the authorities in order to protect the mausoleum.

Remote sensing technologies allow archaeologists to detect the underground conditions without digging up the mausoleum, says Guan Haiyan, an expert who participated in the exploration.

Another project intended to identify the sites of the mausoleums of the emperors of Western Zhou Dynasty has also been carried out in Shaanxi, using aerial photographing, one of the methods most often used in remote sensing.

The technology was also used in a number of underwater explorations of cultural relics which took place this year in Southwest China's Yunnan Province and in the South China Sea.

This trend has become more evident since the first State Lab of Remote Sensing & Archaeology was established last year as a collaboration between the archaeologists and remote sensing experts.

Six centers under the lab have been set up across the country and another six will be in place soon, says lab researcher Guo Huadong. "Archaeologists have shown real interest in the technology when they found it could give them a new perspective, a 'third eye'," he said.

The lab has conducted exploration in northwestern China using aerial photographing and discovered a deserted ancient city in the north of Shaanxi Province, Guo told a symposium held last week.

Apart from their work, a three-year-long project has yielded results, with over 5,000 tombs around the Taihu Lake area in Eastern China having been identified using aerial photographing. Some of them dated back 2,000 years and were located on the mountain ridges.

"They could not have been discovered if we had not used aerial photographing," said Fang Qidong, a researcher with the project.

Fang said these regions were overgrown with bushes, and little human activity had taken place there since the tombs were built.

Despite the many advantages of remote sensing technology, experts said many issues remain unresolved.

The thorniest issue is how to accurately distinguish the cultural relics from other objects such as mounds and water shades in paddy fields on the photos, especially when they are not large enough.

"We still do not have much experience in this work," said Lei Shenlin, a researcher at the Center for Aerial Archaeology under the Chinese Museum of History.

Lei and his colleagues conducted aerial photographing over cultural relics in the deserts and grassland of North China's Inner Mongolia Autonomous Region during 1997 and 2001, and were constantly plagued by this problem.

There is no universally accepted principle in how to "distinguish ancient relics from modern ones or natural entities." "Getting more experience is therefore crucial," he said, "It takes time and patience."

(China Daily December 26, 2002)