Not a Spadeful of Soil Gets Disturbed in This British Dig

Schoolchildren in England will soon be able to switch on a computer and "walk through" a buried Roman city nearly 2000 years old.

Using remote-sensing techniques and virtual-reality technology, archaeologists at Wroxeter in Shropshire are well ahead with the three-dimensional mapping of nearby Viroconium - Britain's largest surviving ancient city.

A technique known as "time slicing" will enable the city, Britain's fourth largest ancient Roman site, to be recreated without disturbing the soil above it, says archaeologist Andrew David. The data will later be converted into 3-D form and put on CD-ROM.

A team from Birmingham University has already produced a 3-D image of a stone-built church using the new techniques. "It is nearly 100 feet long, and seems likely to be one of the oldest Christian churches ever discovered in Britain," Dr. David says.

In former times, British archaeologists seeking details of the settlements established after Julius Caesar invaded in the 1st century BC had to seize their spades and start digging.

"The disadvantage of that," says Vincent Gaffney, leader of the Viroconium team, "was that a lot of damage was done. Now we can get a clear picture of a site without disturbing the terrain."

To uncover the 400-year history of Viroconium, the team is using three related virtual-dig methods.

* One is resistivity, in which electrodes are sunk in the ground and an electric current is passed between them. Resistance varies, with, for example, walls registering high and ditches offering less resistance.

*Another approach uses magnetometry, which measures the distortion caused by different buried remains in the earth's magnetic field. With a device known as a fluxgrade gradiometer, archaeologists measure distortions at half-meter intervals and download the data to a computer. Both magnetometry and resistivity have a drawback: They record only the upper levels of buried remains.

*The latest technique to be used at Viroconium is ground-penetrating radar, a recent technology that enables the team to explore the site at various levels. Pulses of electromagnetic radiation are directed into the soil from a transmitter, then bounce back to a receiver that registers whatever they hit. The resulting data is fed into a computer.

The transmitter-receiver array is pulled on a wheeled platform across the ground, and produces a radar profile. Dr. Gaffney says the radar profiles produce successively deeper horizontal images, which are used to estimate the depth and physical condition of buried structures. This allows a 3-D picture to be created.

So far, only a tiny fraction of the 140-acre site has been explored using time-slicing techniques, but the team has already traced part of a Roman forum and a huge baths complex.

Neil Linford, a scientist at English Heritage, a London-based group that supports research on the country's past, says a computer package including data discovered at Viroconium will be available to British schools in a year or two.

Gaffney says Viroconium is a perfect outdoor laboratory for testing the latest methods of electronic archaeology. But he is cautious in assessing what can be achieved with time-slicing and does not see it replacing excavation completely. "It enables us to map a site in detail, and target ... a significant feature," he says. "If we identify something of huge importance, then obviously we have to switch to the time-honored method of using a spade."