Gravity: More than meets the eye

Ever opted to take the stairs over the lift for the sake of additional exercise? You’ll be pleased to know that on reaching the top, you’ll appear to weigh an average 100 mg less than what you did at the bottom!

Think that’s impressive? Try 2.4 grams less if you first weighed yourself in South East Cape Tasmania, and then in Cape York, Far North Queensland – fact! Why? According to Geoscience Australia’s Geophysicist Richard Lane, it all comes down to gravity.

“Gravity decreases as you get further away from the centre of the Earth, hence you will feel lighter when you climb some stairs or travel towards the equator where there is a bulge in the radius of the Earth,” says Richard. “For us at Geoscience Australia, we’re particularly interested in gravity because it also varies depending on the type of rocks and the amount of water that’s underground.”

“When it comes to working out what’s going on underground, we combine gravity information with data from other geophysical tests or surveys such as those measuring changes in the Earth’s magnetic field, and the electrical conductivity of rocks and groundwater. Together, the different measurements tell us what energy, mineral and groundwater resources are beneath the surface,” says Richard.

Data from gravity surveys is used in conjunction with that from other geophysical tests to help a variety of individuals from a whole range of sectors.

Once the measurements have been taken, data crunched, integrated with other results and reports written, the information is ready for distribution to the public. As to who uses this information, Richard explains “…it’s a variety of individuals from a whole range of sectors, including:

  • mineral and energy exploration companies deciding where to explore
  • geodesists improving the accuracy of their surveying methods
  • land managers obtaining a broad knowledge of groundwater stores, and
  • engineers identifying major natural hazards.”

Looking back, gravity surveys have come a long ways since the first measurements were taken in Australia in the 1920s. Richard says “…at that stage a complete laboratory would have been set up at each site, and measurements taken several days to acquire.”

“By the 1960s, mobile gravimeters allowed ground-based measurements to be collected more quickly and efficiently. The introduction of GPS navigation in the 1990s made positioning both faster and more accurate. The final piece of the puzzle was the advent of systems that could be carried by aircraft, allowing us to gather more information more quickly across all types of terrain.”

“At Geoscience Australia, we’ve been developing our expertise in performing gravity surveys over the whole period, working with State and Territory agencies along the way to achieve this, and other common goals,” says Richard.

An example of one such opportunity is the Western Australian Airborne Gravity Program, which commenced in 2013 and involved Geoscience Australia and the Geological Survey of Western Australia (GSWA). The program set an ambitious target; to carry out a series of airborne gravity surveys over large parts of Western Australia (WA).

In a collaborative effort, Geoscience Australia and the Geological Survey of Western Australia worked together to deliver Australia’s largest ever airborne gravity survey.

The acquisition took several field seasons and by its conclusion in early 2019, covered an area just under 1 million square kilometres, making it Australia’s largest ever government-funded gravity survey.

Speaking of the program, Richard says “It was a difficult decision to employ airborne methods over conventional ground measurements, as the technology was untested for use in large scale regional surveys but it was also the only way to cover these large areas.”

Left. Survey blocks flown with airborne gravity and airborne gravity gradiometry as part of the Western Australian Airborne Gravity Program. Right. Bouguer gravity image showing the variations in anomalous gravity with highs in red and lows in blue. The textures and patterns are indicative of different geological provinces, while breaks and linear features indicate faults and structures.

But by working together, Geoscience Australia and GSWA pulled off the seemingly impossible, with GSWA’s Dr Ian Tyler saying “The program could absolutely not have been able to be accomplished in the time concerned by either agency alone, without contributions of the other.”

Dr Tyler later described the program as a “…perfect example of the synergy that can be had from successful collaboration between State and Commonwealth Government agencies.”

Data from the program is currently being processed, and is expected to be available in early 2020.