Image caption The wells that produce oil and gas may end up being carbon storage sites

Carbon capture and storage (CCS) is a proposed route to mitigate fossil-fuel greenhouse gases' impact on Earth's atmosphere, oceans and climate.

In CCS, CO2 is captured from fuel burning at the power station, and pumped back down into the very rocks that maybe once yielded that fuel.

Likely storage sites for CO2 will be depleted gas and oil fields one to two km down, with many candidates in the old oil and gas fields of the North Sea.

A few test sites around the world suggest that CO2 can indeed be returned to the deep Earth successfully, but attention has now turned to monitoring its behaviour at depth.

Checking that it stays locked within the porous rocks of any CCS repository is an important step in validating the method.

Prof Jon Gluyas of Durham University stressed, at the British Science Festival in Newcastle, the importance of monitoring CO2 once buried, to ensure that it stays within the storage site and to chart any changes with time, and simply to ensure that the carbon capture is permanent.

"It will be captured from power stations and other industrial plants, compressed into liquid form, and injected deep beneath the Earth. The methods we have for monitoring at the moment are expensive and just a snapshot. It costs around £1m a go," he said.

The current cost of monitoring CCS repositories is a major hurdle to adopting the method. Existing technology uses expensive geophysical seismic imaging techniques, which would need to be continued and repeated long into the future.

Prof Gluyas is working with Dr Lee Thompson at Sheffield University and colleagues at Bath University and Nasa to develop new "passive" methods for cheap monitoring of deep carbon repositories.

Seeing through rock

Their method exploits natural radiation to see through kilometres of rock, in the same sort of way as medical X-rays are used see inside a patient's body.

The particles used for imaging occur naturally in Earth's upper atmosphere when cosmic rays from distant dying stars hit oxygen and nitrogen atoms to create new sub-atomic particles called muons.

Image caption The Boulby Mine is being used to test certain principles

Muons are charged particles a bit like electrons, but are 200 times heavier, and there is a constant shower of muons hurtling to Earth from cosmic bombardment.

The group is deploying particle detectors deep underground that are sensitive to the rain of muons. "We can build up a picture of the storage site in the same way as you have a CT scan or X-ray in hospital," Prof Gluyas explained.

"Muon tomography has already been used to test for cavities in Pyramids and magma chambers in volcanoes, where you can put detectors around the periphery of the volcano."

The same approach has recently been proposed as a method to see inside the damaged nuclear reactors at Fukushima, with scientists from the US Los Alamos laboratory carrying out some test measurements there earlier this summer.

At a CCS repository, muons could provide an image of where the carbon dioxide is, and whether it is staying put.

Deep physics

Scientists have already built a physics lab at the base of the Boulby deep mine in Cleveland near Whitby, 1km beneath the surface. The mine has galleries that lead out 7km beneath the North Sea.

Image caption Muons are produced when cosmic rays strike the atmosphere

They are testing their method by monitoring muons that have travelled through the North Sea and then through the 1km of rock. As the tide changes, the amount of water that the muons travel through changes, and these variations can be picked up by the detector system.

This proof of concept experiment is now being extended to designing a system that could be used as a probe. Prof Gluyas continued: "We have drilled a number of geothermal wells in the North East as test grounds.

"The programme should take us to the point, in mid 2015, to be ready to deploy in that setting. If this works out, by 2020 we should have some working schemes."

The energy penalty of capturing carbon dioxide from power stations is about 30%, which is considered a prohibitive expense. Most of that is associated with capturing the carbon from burning fossil fuels in the first place, but reducing the cost of monitoring will help reduce the long-term costs or carbon storage, which may yet breath new life into the old reservoir rocks of the North Sea fields.