杏吧原创

YOU might think the holy grail of archaeology would be, well, the Holy Grail.

For Neil Linford, an archaeologist with English Heritage based in Portsmouth, UK, it is something altogether more down to earth. Linford鈥檚 goal is to speed up the laborious process of tracking down artefacts buried at archaeological sites.

For decades archaeologists鈥� only option has been to walk backwards and forwards, carefully scanning the ground with a variety of hand-held devices to pick up the telltale magnetic, electrical and density anomalies caused by buried objects. It鈥檚 a process that can take weeks.

Now techniques borrowed from physics are adding a new range of tools to archaeologists鈥� kit, helping them build detailed maps with unprecedented ease. Unlike traditional instruments, these devices can be strapped to the back of vehicles to produce a picture of the subterranean scene in a fraction of the time. 鈥淲e鈥檙e seeing things we鈥檝e never seen before, without excavating,鈥� says Linford.

One such instrument is the superconducting quantum interference device, or SQUID. These sensors use superconductors to pick up weak magnetic signals, and are often used in medicine to measure magnetic fluctuations in electrical activity of the brain. Their extreme sensitivity means they can pick up magnetic fields more than 200 times as weak as those detectable by conventional magnetometers, which has understandably attracted the interest of archaeologists hoping to search for magnetic signals from artefacts buried in the soil.

Existing devices are not ideal for fieldwork because they only operate at -270 掳C and must be cooled by liquid helium, but researchers have managed to develop a portable version that can be towed behind an off-road vehicle. Although this dramatically speeds up surveys, it also creates its own problems. The trouble is that SQUIDs are sensitive to everything metallic, says quantum physicist Andreas Chwala, who has developed the archaeological SQUID magnetometer with his colleagues at the Jena Institute for Physical High Technology in Germany. They found that metal nails, the vehicle and even the Earth鈥檚 magnetic field all swamped signals from archaeological artefacts.

To get round this problem, the team used two sensors, one near the ground, which records signals from artefacts plus the background magnetic noise, while a second sensor placed higher up just picks up the noise. Subtracting one signal from the other gives you the reading from the artefacts.

鈥淪QUIDs are brilliant for locating small items made from wood or leather,鈥� says Linford. So far, Chwala鈥檚 team has detected the signals not only from wooden artefacts 鈥� created by bacteria that chomp on the decaying wood, turning weakly magnetic minerals into more magnetic minerals 鈥� but even from holes that once contained wooden posts and now only contain traces of wood. They have also recently used the SQUID to search the area near the giant drawings etched in the ground in Nazca, Peru, for signs of the ancient civilisation that created them. 鈥淚t鈥檚 very exciting, and we think we have found signs of housing,鈥� says Chwala. The team is waiting to see if excavations being carried out at the site will confirm their readings.

The SQUID magnetometer is best suited to shallow field searches between 1 and 5 metres below the ground. The tool cannot be used to spot deeply buried objects because the signals become too faint, so to delve further archaeologists are looking at a technique that measures the electrical resistance of soil.

It stemmed from the idea that differences in the amount of water held within archaeological features change their electrical resistance compared with the surrounding soil. For example, non-porous stone has a higher resistance than soil, while organic matter retains more water than the soil so it will conduct electricity more easily.

Traditionally, geophysicists measure electrical resistance by sticking two electrodes into the ground, applying a voltage and measuring the current between them. 鈥淭he detectors are built like Zimmer frames, with electrodes as the front legs,鈥� says Chris Leech at Geomatrix, a company that sells geophysical equipment. The further apart the electrodes are spaced, the greater depth they can reach, so the Zimmer-frame design places a limit on how deep you can measure, he says.

However, geophysicists from Moscow State University working in Russia came up against a more basic problem: inserting the electrodes in the first place. 鈥淚magine trying to stick electrodes in frozen Russian ground,鈥� says Leech. When they gave up attempting to bury the electrodes and left the equipment lying on the ground, they were surprised to find that they still picked up a faint signal. They realised the system was acting as a capacitor, he says.

A capacitor is simply a device that stores charge on two conducting plates separated by insulating material. In this case the electrodes鈥� wires and the Earth acted as 鈥減lates鈥�, while the plastic sheaths surrounding the wires provided the insulation. When the oppositely charged electrodes were placed on the ground on the insulating plastic, they caused charged particles in the ground to move towards the plates to cancel out this charge difference, creating a current. Differences in the ease with which this current is conducted through the soil indicate the presence of any artefacts. Because the capacitor plates lie on the ground, it is far easier to separate them, allowing them to probe deeper into the soil.

Geomatrix recently launched a device called the OhmMapper which uses capacitors and can be towed across the site behind a motorcycle. Because it can scan to depths of more than 10 metres, it is ideal for reconnaissance surveys before digging begins, and was used at the site of a Roman amphitheatre in Richborough, UK, for example.

鈥淲ithout even excavating we鈥檙e seeing things that we have never seen before鈥�

Creating sensors that can be towed or mounted on vehicles is certainly speeding up surveys, but geologist Ian Hill at the University of Leicester, UK, thinks that is only half the problem. 鈥淭hese surveys still aren鈥檛 used as much as they could be because it costs so much time and money to collect data,鈥� he says.

To rectify this, Hill and his colleagues have created the Geophysical Equipment Exploration Platform (GEEP), which can be fitted with multiple sensors, such as SQUIDS, to carry out a comprehensive survey in one sweep. The platform is also equipped with a processor to analyse all the data in real time and wirelessly transmit it to a geophysicist on site.

Sticking five or six different sensors onto the same vehicle may seem like a simple idea, but in practice it has been extremely difficult to achieve. If the sensors are too close together they can interfere with each other, and any metal in the vehicle can disrupt the readings.

So the GEEP team opted for a fibreglass sledge designed to be flexible enough to handle rough terrain as it is towed behind a tractor. The sledge can be fitted with long booms to place sensors as much as 40 metres apart. What鈥檚 more, the archaeologist using the GEEP can keep track of its position using GPS, which means there is no need to mark out a grid beforehand for the team to follow.

Hill tested the GEEP by mapping part of a Roman city in Wroxeter in Shropshire, UK, which Linford was also surveying using conventional techniques (Reports on Progress in Physics, vol 69, p 2205). 鈥淚t takes the conventional surveyors a matter of weeks to cover the whole region with various sensors and then analyse the data,鈥� says Hill. 鈥淚t took the GEEP two hours.鈥�

Combining sensors in this way gives archaeologists the chance to obtain much more information on objects without having to carry out multiple surveys, says Hill. 鈥淏efore, we were constrained to studying one feature at a time 鈥� like only being able to describe the objects in a dark room in terms of their shape, size or smell,鈥� he says. 鈥淣ow, it鈥檚 as though we can turn on the lights and see the contents clearly.鈥�