Dr Giles Hammond discusses wee-g, a radical new MEMS-based gravimeter which could enable gravity surveys to be performed by drone within a matter of months
Gravity surveys are an expensive business. Requiring multiple readings spread across hundreds of miles, they are labour-intensive for geophysicists. For larger operations, in addition to the cost of using offshore vessels or aircraft, surveys also require gravimetry equipment – used to measure the local acceleration of gravity, and which can infer mineral and hydrocarbon deposits deep underground. They can also weigh hundreds of kilos and cost hundreds of thousands of dollars. To put that in perspective, the industry-leading LaCoste FG5 – an absolute gravimeter – costs around US$200,000 and weighs about 200kg. A more portable, relative gravimeter like the SCINTREX CG-5 will go for around US$75,000 and weighs a more manageable 10kg, but requires many hours of manual readings, as well as GPS co-ordinates and elevation recordings to record an accurate survey. Making equipment more cheaply and the process of measurement easier would not only offer a vital lifeline to an industry struggling with low prices, but could change how the industry undertakes resource surveys altogether. That may now be possible, thanks to a tiny gravimeter developed by a team at the University of Glasgow. Based on cheap and mass-producible micro-electromechanical systems (MEMS), their innovation combines the server-sized technology of geophysical surveys with the everyday portable accelerometers found in a smartphone. But while the so-called “wee-g” – a nod to the city of its birth – may be small in scale, the implications of the technology are not. These were qualified in an early 2016 paper published in Nature, in which the device’s creators successfully measured the Earth tides – the elastic deformation of the Earth’s crust caused by the pull of the Sun and Moon – by calculating the changes in the planet’s magnetic field from the basement of the University. The device owes a lot to the recent advances of nanotechnology. Developed at the University’s James Watt Nanofabrication Centre, wee-g is the result of a cross-disciplinary effort between the School of Physics and Astronomy (Institute for Gravitational Research) and the School of Engineering (Electrical & Nanoscale). InnovOil spoke with Dr Giles Hammond of the first department to find out more.
Resonating thought “I realised three years ago that we could build small form-factor silicon-based resonators – micro-electromechanical sensors (MEMS) – but make them a lot more sensitive than current devices,” Hammond explained. Indeed, the use of silicon is key to the accuracy of the device. “When you build MEMS devices, the thing that defines how much deflection you get on the proof mass – what happens when gravity changes – tends to be quite stiff.” As a result, accelerometers in the average phone can measure deviation from the standard gravity (1G), but little more beyond that. But by developing a manufacturing process which could produce very soft, low frequency resonators, the accuracy of a very similar system could be improved by many orders of magnitude. Using a highly sensitive silicon spring “ten times thinner than a human hair,” a 12mm sensor in the wee-g can detect the minute changes in gravity needed for survey applications. Hammond quantified this more specifically, explaining: “Our MEMS devices oscillate up and down at about 2Hz, whereas the ones in a smartphone may be around a few kHz.” Hammond worked with Engineering Professor Douglas Paul and PhD student Richard Middlemiss to develop the gravimeter and to publish the aforementioned paper. In addition to sensitivity, the team was able to show a notable achievement in engineering stability. He explained: “We can now monitor the position of our proof mass in the device to about 2 nanometres over five to six days… To do that, we had to control the temperature of the device in many places, because it will change the thermodynamic properties of the spring.” According to Hammond, that perceived lack of stability is what has held back MEMS technology thus far. “That’s why MEMS devices haven’t pushed into the market. They’re quite standard for seismic surveys but for seismometer surveys you need short-term stability over 5-10 seconds – whereas we are showing good stability over 5-10 days.” In doing so, the research pushes MEMS technology into new realms.
Array of options The playing field between the wee-g and existing equipment is not yet an even one. Although thousands of times more sensitive than the average accelerometer, it is about ten times less sensitive than the best devices available. Nevertheless, according to the team, it is still capable of sensing a tunnel of 2m2 at a depth of 2m, and could be used to find an oil deposit of 50 cubic metres at depths of up to 150m. Its real benefits stem from the improvements that could be made to surveys themselves. “Because our devices are MEMS devices, potentially they could be a lot cheaper than US$75,000 as they’ll be developed out of standard processing,” he said. Indeed, the team has previously estimated that they could be in the region of US$100 a unit. That cost base, he continued, “would allow us to have a sensor array, which opens up a new way of sensing the gravitational field, more like a seismic survey.” He envisions a system with a number of sensors set down across an area which can take measurements over a defined period before being moved on, allowing surveys to be conducted far more quickly than is currently manageable with a relative gravimeter. The wee-g device is now being developed into a prototype which would support an autonomous system, logging data automatically to an SD card. The final iteration of this system is some way off, but the team has already embarked on a project with geosciences firm Bridgeporth to undertake “comparative, side-by-side field surveys” which will help qualify the resolution and accuracy of the wee-g against the current state of the art.
Defying gravity With the concept proven and a paper published in March, most of this year has been spent working the wee-g prototype into something more commercial and field-ready. “We started off with a lab-based system – everything running on mains power and rack-mounted electronics. What we’ve done in the last 6 months is taken all our electronics and miniaturised it to something less than half the size of a piece of A4 paper,” Hammond said. The wee-g now consists of a card of electronics run by a digital signal processor, and powered by standard 12V batteries. “We’ve also miniaturised the vacuum system, so now we have something that weighs about 3kg and fits in your hand,” he continued. Looking ahead, this is only set to get more ‘wee.’ “The final device will be put into a standard MEMS package, typically about 15mm x 15mm x 15 mm – a little cube with a self-contained vacuum and the device will sit in there,” he added. This paves the way for the next stage of development, slated to be completed by mid-2017. Once this occurs, one begins to see the truly disruptive effects the technology could have. “We’re really thinking about taking the current devices of about 10kg and shrinking them down to about 100g. That’s exciting, because when you make them that light, you can mount them on drones,” Hammond enthused. This as Hammond has said, opens up new ways of sensing. “The thought is that we can get away from using these traditional aircraft and get over to a drone-based survey. If you make something light and cheap, then in principle we’re looking to work with end users to do some surveys on drones.” A few high-end drones, equipped at a total cost of a few thousand dollars each, could then perform a survey which may currently cost well into the hundreds of thousands. As the autonomous flight capabilities of drones increase further, even larger areas could be covered in greater detail, without the need for clear weather and a charter aircraft. From there, the landscape of gravity surveying begins to look very different indeed. While the latter may be a few years off, the team is confident that next year will see the next stage of the project take off. “By mid-2017 we would like to have this packaged into its small form factor… By then we’d really like to have a device that we can give out to end users. Within a year I’d say we should be at the point at which we could put this thing on a drone.” Unsurprisingly, the proposition has generated a lot of interest, Hammond said, with “tens of companies” getting in touch to request access to the technology. In addition to the trials being run with Bridgeporth, the group has an ongoing optical sensing project with Schlumberger and is working with nearby Clyde Space to include the gravimeter as a sensor package within one of the company’s mini CubeSat satellites. For now, the team has plenty to get on with to meet their 2017 deadline. Beyond that, InnovOil suspects that it may not be that long before the wee-g starts to make big waves in the geoscience sector.