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New technology for underground probes

Birmingham gravity imager cropped

As a trained physicist, Trevor Cross’ day job involves sending imagery back from comets beyond Mars. 

So it might seem odd that he is also speaking to New Civil Engineer about sinkholes.

But quantum technology is now allowing humans to see far underground, as well as deep into space. The implications could have a major impact on geotechnics, improving the speed and quality of ground investigation work.

With the University of Birmingham’s Physics Department and consultant RSK, Cross’ company E2V has devised a gravity sensor that can measure the density of material at a certain depth below the feet of the user.

Screen shot cropped

Screen shot cropped

Ground probe output

“We have a box, it’s about the size of a small refrigerator, it’s on wheels. After settling in one spot, in 20 to 30 minutes you can get a measurement,” says Cross, group chief technology officer at E2V.

The team hopes that by mid-next year the device will be able to provide a detailed profile of how density changes, with depth.

The following step within the next three years will be to add more data points to the cross section, which provides a 3D view of underground density. It would pick up different soil and rock types, utilities, tunnels and even backfilled mineshafts.

It would cut by a factor of 100 the time it takes to survey a certain space

Trevor Cross, e2v

Cross says that even if this data had to be taken away and analysed offsite, this would be more efficient than current methods.

“It would cut by a factor of 100 the time it takes to survey a certain space,” says Cross.

“Of the 4M holes dug in the road every year, a huge amount are in the wrong place, because the maps are just not good enough to tell you where things are.”

The gravity sensor’s core ability derives from a concept called laser cooling, where atoms are cooled down to a few milli-Kelvin above absolute zero, using laser light.

There’s an astonishing amount of stuff underground that people don’t know about

Trevor Cross, e2v

Cooling a cloud of atoms makes them easier to trap. The cloud is then released and falls, with precisely-timed laser pulses used to control it as it goes. The atoms behave as waves, which interfere with one another like ripples crossing the surface of water. The pattern made by the interference depends on what is influencing them as they go; if the influence is simply gravity, it allows for very precise gravity sensing.

By detecting fluctuations in gravity across the Earth’s surface, one can infer what lies beneath.

The process is altogether too fragile for a construction site. But after the device is proven to work, Cross says over the next few years the electronics could be miniaturised.

Off the shelf

“So far it’s constructed out of ancillary ‘off the shelf’ components, but it’s getting smaller and smaller. We’ve got the core part reduced down to a 1l milk bottle.

“I can’t imagine ever attempting to hold one still enough to take a measurement, but I can imagine it being small enough to pick up with one hand.”

Citing the example of silicone based digital electronics, which have rapidly shrunk over the years, Cross says quantum tech will do the same.

“Who would have thought that with the first transistor invented in 1947, you can have 500 billion billion devices on a consumer product [flash drive] you can buy for a few dollars?

Table top device

“Five years ago if you wanted to prepare a single atom and use it as a specialist sensor, to measure inertia or gravity, you needed a laboratory. Last year we concentrated this down to a table-top device.”

There are already gravity sensors in existence, but Cross variously dismisses these as: prone to vibrations, more time consuming, or prone to confusing features at the surface. Quantum sensors, theoretically at least, would go faster, deeper, and be more precise.

Getting the device to market would be the next challenge, but it seems there is no shortage of demand. Defence forces could locate underground structures. Bodies could be found faster after natural disasters. More than 5M UK homes are in high risk zones for sinkholes and subsidence. UK rail infrastructure needs continuous monitoring for water accumulation leading to landslips.

“If you look at the development of brownfield sites, there’s an astonishing amount of stuff underground that people don’t know about.”

High speed rail

Cross says he has been approached by the mining industry, and there could be applications in high speed rail. “You have to dig or understand solid foundations down to 7m for a high-speed rail line, and it’s only about 2m for a normal rail line. And High Speed 2 is going to go over an area where there is a lot of historic mines.”

In another application, gravity sensors – if they became precise and mobile enough – could be attached to a drone or satellite, resulting in the production of wider “gravity maps”.

“If you take it into a spacecraft, you would be able to look at the change in water tables in individual countries – that’s something you just can’t do today.”

Government development programme

To build the market, the UK government has announced a national quantum technologies programme and has earmarked £330M for national quantum technology hubs.

But this is not all poured into gravity sensing.

The Quantum Age will have huge impactson, among other things, imaging and measuring. Perhaps the most-hyped consequence would be a quantum supercomputer, which would be 100M times faster than regular computers. It would reduce from years to seconds the time taken to answer complex problems or to model complex scenarios.

“If we look at the impact on a macro scale, quantum technology represents a new platform of technologies,” says Cross. “It will be a billions-of-pounds industry in the next 15 to 20 years as it adds to, augments and erodes the current world of electronic devices.”

Commercial products are already available within quantum clocks and communication. Other quantum technology is set to follow.

Quantum research in the UK

The UK currently ranks third in the world on progress for quantum science and technology, behind US first and China second, based on spending, research publications and patent applications.

Many more governments around the world have invested heavily within the past five years, including those in Germany, Japan, Canada and Australia.

Cross says the UK is doing well, with a highly advertised and successful national programme: “The UK government put money in for innovation at the beginning, it’s got a decent five year plan for the science base.

“Credit must go to the research councils who flagged six to seven years ago that they were going to invest well into this area, which attracted a number of people.”

E2V only became aware of advanced quantum technology about five years ago.

It has dealt in precision atomic clocks, gravity, magnetic and inertial sensors – all useful for spacecraft.

E2V’s imagers most recently were responsible for producing pictures from the European Space Agency’s Rosetta craft, which flew by Mars to collect data on a comet.

“Space is such fun. It’s great for bringing young people into the business, you can say, ‘see that thing on TV, that was us’.”

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