Gravity along Earth's surface is anything but constant. The variation from location to location may not be enough to handicap basketball games, but it is large enough to matter. A gravity map of Earth then winds up looking more like the malformed wad below rather than a smooth uniform sphere.
The biggest factors in these gravitational distortions are pretty obvious: elevation and latitude. Elevation changes the distance from a point to Earth's gravitational center (thus changing its local gravity), while latitude can have the same effect for the simple reason that Earth is really more of a smooshed ball than a perfect sphere. The maximum gravitational variation on Earth's surface winds up being around 0.7 percent.
In addition to elevation and latitude, there are other reasons for gravitational variance, and these are more difficult to detect. For instance, a subterranean tunnel or oil reservoir can yield relatively tiny changes in local gravity. It's these slight disturbances that a low-cost gravimeter recently developed at the University of Glasgow, and described in the current issue of Nature, is able to register. While an existing gravimeter of comparable sensitivity might cost over $100,000, the Glasgow instrument is built onto a 15-millimeter-square piece of silicon using the same fabrication methods used to build smart-phone accelerometers.
The utilities of such an instrument are clear. Hydrocarbon hunting is one thing, but tiny gravitational fluctuations could reveal magma movements prior to a volcanic eruption in addition to other geological information useful to engineers. Local gravitational variations can also yield information about tidal forces that result from varying configurations of the Sun, Moon, and Earth. Note that in the case of tidal forces, the relative strength of gravity changes with time instead of location.
"Gravimeters are now used on ships and aircraft, on land, on the seabed and even in boreholes to produce maps of the relative value or of the vertical gradient of gravity," explains Hazel Rhymer, a geologist at the Open University, in a separate Nature commentary. "These maps can be interpreted in terms of subsurface mass anomalies in applications such as oil prospecting. When changes in gravity through time are measured, applications extend to cavity detection beneath structures such as railway tracks and even to the monitoring of magma and fluid movement beneath active volcanoes."
Despite some 50 years of evolution, the smallest current gravimeters in use today are still around the size and weight of a car battery. Through that time, the basic idea has remained constant—hang some mass on a spring and you can determine gravitational changes by observing the spring extend and retract. It's a simple—but still cumbersome and expensive—system.
The gravimeter-on-a-chip system developed at the University of Glasgow isn't exactly a smartphone app, but it's a major step in that direction. It's built on basically the same idea as the micro-electromechanical system (MEMS) accelerometer in your phone except super-charged to the point of being about 1,000 times more sensitive while also maintaining the stability required to make such fine measurements. An accelerometer, after all, is already a conceptual mimic of the aforementioned spring-mass gravimeter only at micro- or nano-scales. Here, the spring is replaced with just a tiny bit of sealed gas surrounding a similarly tiny mass affixed to a cantilever beam.
Put somewhat differently, the Glasgow group's gravimeter is really an accelerometer capable of registering displacements at very low frequencies. To verify this low frequency capability, the researchers used their gravimeter to successfully monitor tidal forces for several days at a time.
"This MEMS gravimeter could be flown in drones by oil and gas exploration companies, reducing the need for dangerous low-altitude aeroplane flights, it could be used to locate subterranean tunnels, and it could be used by building contractors to find underground utilities," the Glasgow team writes. "Networks of sensors could be operated in areas unsafe for humans, to monitor natural and man-made hazards, for example, on volcanoes or unstable slopes to measure the spatial and temporal resolution of subsurface density changes and improve hazard forecasting."