Plumbing the depths of Bath's rocky past

Scientists have discovered a lost landscape that may explain Bath's hot springs. Steve Farrar reports.

Buried beneath the Somerset countryside is a remarkable piece of plumbing. Every day, it supplies 1.25 million litres of water at a temperature of 47C to the hot springs at Bath.

No one has ever seen this geological wonder and there has long been speculation about its nature. Now a team of geophysicists has probed through several kilometres of rock and caught a glimpse of what could be the solution to an age-old mystery.

The Romans constructed temples and bathhouses on the site. The Georgians made it the centrepiece of their most fashionable resort.

Bath and North East Somerset Council recently embarked on a scheme to construct a new spa in the city. To protect the springs, it decided that an ambitious geological investigation was required.

The project would ultimately cost £500,000 - money raised from the council, the Millennium Commission and local quarry firms. To spearhead the work, the authority brought in Geoffrey Kellaway, their consulting geologist, who has more than 50 years experience of the thermal springs.

Kellaway had some information about local geology that indicated the city sat just above a great plateau of Carboniferous limestone. Furthermore, he knew the spring water carried a distinctive chemical signature that indicated it had travelled within the limestone for some time.

He could also surmise that the unique phenomenon probably had a unique cause, something that enabled the water to rise rapidly from depths of at least 2km below the surface.

Kellaway put together an expert team to chart the Carboniferous limestone in and around the city. This included Clive and David McCann, brothers and professors of geophysics at Reading University, and Andrew Mann, of IMC Geophysics, a Midlands-based firm specialising in such survey work.

In March 1999, the first Vibroseis trucks arrived in Bath. They looked like a cross between a lunar rover and the skeletal remains of a dustcart. Each one thumped the ground with a large vibrating plate as they made their way in a straight line across open parkland.

The seismic vibrations they produced penetrated deep into the earth. Each time a pulse of sound reached an abrupt shift in the seismic properties of the rock, a part of it was reflected back to the surface and recorded by an array of sensitive geophones. By measuring the time taken for each pulse to return, a detailed two-dimensional slice of the earth could be reconstructed.

As Carboniferous limestone is denser than the rocks known to lie on top of it, the team was hopeful its upper surface would stand out.

It did so beautifully. The full survey involved six separate lines of up to 7km in length, crisscrossing the city and surrounding land. The Vibroseis trucks became a familiar sight negotiating Bath's Georgian crescents and the Somerset countryside.

The geophysicists then turned to gravity data collected in the area some decades earlier. These measure subtle variations in the gravitational field caused by differences in the total mass of the underlying rock. Modern computer analysis techniques allowed the team to reinterpret this data and combine it with the seismic survey.

What emerged was a contour map of a lost landscape. Bath's Carboniferous limestone was formed 350 million years ago from the remains of marine organisms. It was buried, squeezed and twisted deep within the earth.

Now it was laid bare for the first time. The scientists' eyes were drawn 2km to the southwest of Bath. Here, the surface of the limestone plunged sharply, dropping from 400m below the surface to 1.35km within 1.8km. In every other direction, it fell no more than 300m.

"Nobody knew this incredible structure was here," McCann says. "This is steep. This is very dramatic." In addition, cutting down the slope was a great fault, with rock 200m lower on one side than on the other.

There are hints that this great fault could be slowly opening. This could create gaps: perhaps big enough gaps for water to flow through.

In a special publication of the Geological Society of London, to be published in February 2002, the scientists note: "It can be speculated that the steeply dipping limestone could provide a path for the rapid transport of the hot water into the region of the springs," - so long, they state, as there is a large enough crack associated with it.

Finding the precise location of the channel could be near impossible, McCann admits. The quantity of water involved could be handled by a 10cm diameter pipe.

Nevertheless, the geophysical work suggests a plausible theory: the water fell as rain close to Bath and somehow reached the Carboniferous limestone. It pushed through cracks on the steeply sloping surface of the limestone to reach the surface.

The alternatives are not so compelling. The team's work casts doubt on the traditional view that the water comes from rain falling on the Carboniferous limestone of the Mendip Hills, 20km south of Bath. The limestone under the hills and that under the city are not directly linked. A suggestion that the water was heated by radioactivity in a nearby granite deposit has also been dismissed. The rock could not be found.

Kellaway has his own theory involving convection in a fracture zone running from the Avon to the Solent. Forthcoming geological surveying planned for next year will probe this suggestion further. The data so far indicate a possible pathway through the Carboniferous limestone for the hot water. The final stage of the project will combine these with geological and geochemical research in 2002 to trace how the rainwater gets into the deep limestone basin in the first place.