Australasian Science: Australia's authority on science since 1938

Gravity Mapped Precisely

By Stephen Luntz

Satellite measurements have been used to map the Earth’s gravity to an unprecedented resolution.

The Huascaran Mountain in the Andes could have a future as a weight loss resort, with the benefits guaranteed. Unfortunately, visitors will put all the weight back on when they return home to higher gravity.

Physics students are taught to use a figure of 9.8 m/s2 for gravitational acceleration on Earth in their calculations. However, some variation exists due to un­evenness in the shape of the planet and the presence of particularly dense or light geological structures.

Dr Christian Hirt of Curtin University has been part of a team using satellite measurements to map the Earth’s gravity to an unprecedented resolution. “Our research team calculated freefall gravity at three billion points – that’s one every 200 metres – to create these highest-resolution gravity maps. They show the subtle changes in gravity over most land areas of Earth.”

Prior to Hirt’s work it was thought the variation across the Earth’s surface was 0.05 m/s2. However, Hirt’s team found that the true value is 0.07 m/s2.

“The standard reference relies on a simplified model,” Hirt says. “Our modelling takes into account gravity anomalies as measured by the GOCE gravity satellite, and estimates the expected local variations in gravity from topographic data.”

As anticipated, gravity is strongest at the North Pole, a consequence of the slight polar flattening of the Earth caused by its rapid spin. The combination of the equatorial bulge and its height above sea level gives the Peruvian peak the distinction of having the lowest gravity on the surface of the planet.

Hirt has previously produced a detailed map of lunar gravity anomalies (AS, June 2012, p.8). While that was done without clear applications in mind, the terrestrial project will be useful for mining and civil engineering.

“The gravity maps can be helpful in mine surveying because in most surveying techniques the anomalous gravity field leads to ‘distortions’, so must be corrected in high-precision applications,” Hirt says. “The maps might be helpful to reduce (i.e. remove) most of the gravity signal from local gravity surveys, without further time-consuming calculations. This then reveals localised anomalies, which might tell you about the underground features.

“A local gravity survey is still needed. Our maps cannot substitute this part of exploration, but may save time in extracting the right information from the survey.”

Similarly, Hirt says that gravity anomalies must be corrected for in calculations made for tunnel boring.