Waiting for the earth to move: Tom Wilkie explains how a small group of British scientists can monitor secret nuclear testing from a north London flat

LAST TUESDAY China exploded a nuclear warhead deep under the desert at its remote Lop Nor nuclear testing site in Xinjiang province, some 2,300km (1,500 miles) west of Peking. The first the outside world knew of this event was when, in the bedroom of a small flat off the Seven Sisters Road in north London, Philip McNab's personal computer woke him up with an ear-splitting siren.

While Mr McNab slept, his computer had been sampling data collected by a network of 71 seismographs and collated at the US Geological Survey's database in Golden, Colorado. The machine recognised the provenance of a 5.5 magnitude 'earthquake', triggered at 2am GMT, and alerted Mr McNab. It was spectacular confirmation of a prediction made two weeks earlier by Vertic, the Verification Technology Information Centre, for which Mr McNab works. On 22 September, Vertic's analysts had declared that a Chinese nuclear test was imminent.

Contrary to popular belief, commercially available satellite images are too coarse to pick out fine details - such as trucks or even large buildings. It took years of developing computer programs to squeeze the maximum information out of these images before Vipin Gupta, a research student at Imperial College, was able to produce the first detailed maps publicly available in the West of the Lop Nor testing site. Mr Gupta, who has devoted the three years of his PhD research to the site, knows it so well that he was able to pick out the preparations for a nuclear test and his analysis was so reliable that Vertic was able to make the advance warning public.

Last Tuesday China initially refused to confirm that it had conducted a nuclear test, confusing the matter by referring to natural seismic activity in the region. But when, on Vertic's behalf, Dr Roger Clark, a geophysicist at the University of Leeds, analysed records from seismographs at Harrogate, Durham, Richmond, Whitby, Huddersfield and Market Rasen, the traces were clearly characteristic of a nuclear explosion rather than a natural earthquake. It had taken just 9 1/2 minutes for the seismic waves from the explosion, estimated at between 20 and 40 kilotons, to arrive in the UK.

Vertic is a small group of independent scientists and engineers who have no access to the classified intelligence reports or high-quality surveillance satellite images available to governments. This was by no means the first time, although perhaps the most spectacular, that its expertise has been demonstrated. But the news that its prediction had come true was received with mixed feelings at Vertic.

The Chinese explosion may have destroyed the momentum that had been growing among the nuclear weapons states to reach agreement on a treaty banning all nuclear weapons tests. A Comprehensive Test Ban (CTB) treaty has been a long-term aim of international diplomacy since the Partial Test Ban, which outlawed test explosions in the air, outer space or underwater but which permits underground tests, came into force in 1963.

Negotiations to achieve a CTB open in Geneva in January next year. A comprehensive test ban is seen as an essential part of a wider strategy to prevent the spread of nuclear weapons to more nations. The issue is pressing, because the Treaty on the Non-Proliferation of Nuclear Weapons comes up for renewal in 1995, and non-nuclear states have been pressing for a CTB as evidence of good faith on the part of the superpowers.

One of the stumbling blocks on the road to a comprehensive nuclear test ban has been worry about the possibility of some nations cheating and going undetected. If the discussions at the Conference on Disarmament in Geneva do bear fruit in the shape of a comprehensive ban, then it will be due in no small measure to the efforts of Vertic in reassuring the negotiators that detection is possible.

The sheer quality of Vertic's technical work goes some way to explaining how this tiny organisation has become a force to be reckoned with, consulted by governments. But it combines with technical expertise a commodity not usually associated with scientists and engineers: political sensitivity and an understanding of diplomacy. In no small measure, the success of the organisation rests with its director, Patricia Lewis.

Dr Lewis is unusual in a number of respects. Female physicists are comparatively rare; still rarer are those with a PhD in nuclear structure physics. After a year off in India, she spent several years lecturing in physics at Auckland and Canberra universities, studying high-spin states of atomic nuclei, calcium-42 in particular. However, as she now puts it, 'I had fallen out of love with physics.' At the end of her three-year appointment, 'I didn't want to carry on. I wanted to do science journalism.'

She had her first brush with the military side of the atom through the agency of Sana - Scientists Against Nuclear Arms - when nuclear weapons in the South Pacific were big issues in New Zealand. She returned to the UK just at the time that Jeremy Leggett, a young geologist at the Royal School of Mines, realised that his science was being abused by the military establishments of the nuclear weapons powers.

Dr Leggett knew that anyone who tried to explode a nuclear weapon underground could be detected. Both the scientific understanding and the seismic technology existed. The problem was that the military had a monopoly of the science, its interpretation, and of the channels by which information was communicated to government.

Vertic was set up to break this monopoly. At first, Dr Leggett combined his work at Imperial College with Vertic. Dr Lewis became its first employee and then became director in 1989, when Dr Leggett was made Greenpeace's first director of science.

Vertic's analysis of Chinese nuclear testing is perhaps the most vivid illustration of what its scientists can do, even when they are confined to using publicly available data. The organisation's remit stretches much wider than nuclear weapons. It has been involved in verification measures for conventional force reductions in Europe, and in extending verification to cover environmental issues, such as how one might monitor the production of greenhouse gases. 'We are expanding into biodiversity, endangered species and desertification,' Dr Lewis says.

Much of her time is occupied with finding funds, mainly from charities, for Vertic. 'We never take money from governments for the baseline - salaries and rent,' she explains. Vertic does accept commissions for specific projects, but these are additional. 'We never have to need the money, so they have no hold over us.'

The most immediate item on Vertic's agenda is to update its study on verifying a comprehensive nuclear test ban, in time to be of use to the Conference on Disarmament when it revisits the topic next January. But Dr Lewis believes verification can build confidence between states on a regional as well as global scale. She would like to start a project that would bring military experts from India and Pakistan to Europe to look at the verification of troop reductions and to attend on-site inspections. In this way it might be possible for the two countries on the Indian sub-continent to reach their own agreements, so rebuilding confidence between them.

It is a far cry from the nuclear structure of calcium, but perhaps no further than from the deserts of Xinjiang to the Seven Sisters Road.

(Photograph omitted)