Gravity

Last updated:7 October 2020

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Gravity is the force that attracts masses towards each other. In the absence of friction and other forces, it is the rate at which objects will accelerate towards each other. At the surface of the Earth, gravity is approximately 9.8 m.s-2.

We are interested in gravity for geoscience applications primarily because gravity varies over different rocks and at different distances from the centre of the Earth. Gravity is a little stronger over a heavy rock type like gabbro, and it is a little weaker over less dense rocks like sandstone or granite. Gravity is also a little stronger in valleys than it is on hilltops because you are closer to the centre of the Earth in a valley than you are on a hilltop. These differences are too small to be felt but they can be measured using sensitive instruments. Instruments that we use for typical geoscience applications can measure the force of gravity to a precision of one part in a million or better.

Geoscience Australia uses gravity for two primary applications; geological mapping and geodesy.

Gravity data, in combination with surface geological mapping, airborne magnetic data and many other data sets are used for geological mapping of both the surface and subsurface. In a closely related application, gravity data are used for mineral and energy exploration.

In geodesy, gravity data are used to determine the shape of the Earth. This helps to relate measurements made with Global Navigation Satellite Systems (GNSS) to physical measurements made with traditional surveying methods. Further information regarding the use of gravity in geodesy applications can be found on the Australian Height Datum and Geoid Models webpage.

There are many different types of gravity instruments, but the ones that are relevant to us are absolute gravimeters, relative gravimeters, gravity gradiometers, and radar altimeters.

An absolute gravimeter can measure gravity at a single location. At present, they are restricted to ground-based acquisition.

A relative gravimeter measures the difference in gravity between two locations. Relative gravimeters are much more common than absolute gravimeters, and can be used in ground, airborne and satellite-based acquisition. To be truly effective, surveys carried out with relative gravimeters must include acquisition at one site or more where absolute gravity is known.

Gravity gradiometers measure the difference in gravity between two points very close together. These gradient measurements may be made in several different directions simultaneously. Gravity gradiometers are currently used in airborne and satellite acquisition.

Satellites fitted with very accurate radar altimeters can measure the slope of the ocean from which the force of gravity can be derived. There have been several different satellite missions that have provided useful gravity data over the majority of the world’s oceans.

Geoscience Australia, in collaboration with the State and Territory governments, have acquired ground and airborne gravity data over the Australian continent and its Remote Offshore Territories.  Periodically, Geoscience Australia produces a collection of grids of different gravity quantities to support geological mapping applications. The latest edition is the 2019 Australian National Gravity Grids.  This is a 400m cell size grid that has incorporated 1,430,447 ground gravity observations with 451,000 line kilometres of airborne gravity and gravity gradiometry data and with satellite data covering the offshore regions around Australia.

Geoscience Australia maintains the Australian National Gravity Database (ANGD), which contains 1,850,295 ground gravity stations collected throughout Australia and its Remote Offshore Territories. These data have been sourced from Geoscience Australia, State and Territory governments and agencies, mineral and petroleum exploration companies, universities and overseas organisations.

The ANGD is underpinned by the Australian Fundamental Gravity Network (AFGN) which provides the datum for gravity surveys carried out throughout Australia. The AFGN is a network of permanently marked locations where absolute gravity is known very accurately. By taking a measurement at an AFGN benchmark, a gravity survey can be “tied” to the gravity datum. This allows the data from the survey to be joined with data from other surveys to form a coherent data set over the combined area of the surveys.

Video presentations from the “Airborne Gravity 2016” workshop

Below are links to video recordings of the presentations given at a one-day workshop “Airborne Gravity 2016 - Advances in airborne gravity and airborne gravity gradiometry” that was part of the 2016 ASEG conference. The aims of this workshop were to review developments in airborne gravity (including airborne gravity gradiometry) that had occurred since the previous workshop held in 2010 and to anticipate expected developments in the next 5 to 10 years. These developments include the introduction of new airborne systems, improvements to related technologies (e.g., GPS and terrain mapping), and developments in data processing and interpretation.

Session one

Speakers in this session were;

  • Mark Dransfield - Welcome and introduction
  • Tom Meyer - Recent advances in Lockheed Martin's gravity gradiometer technology
  • Chris van Galder - Recent developments with Falcon AGG
  • Brian Main - First data from the HD-AGG instrument
  • Frank van Kann - VK1 - A next-generation airborne gravity gradiometer
  • Questions for all session speakers

Session two

Speakers in this session were;

  • James Brewster - Reducing noise by transforming and combining gravity gradient components
  • Mark Dransfield - Equivalent source and Fourier transform techniques in Falcon AGG data processing
  • John Paine - An investigation of the effects of filtering in the analysis of airborne gravity gradient data
  • Questions for all session speakers
  • Richard Lane - Open questions for Australia's geoscience organizations regarding the future of airborne gravity and airborne gravity gradiometer surveys

Session three

Speakers in this session were;

  • Yaoguo Li - Inversion of gravity gradiometry data: Evaluation of information content through quantitative interpretations
  • Helen Gibson - Litho-Constrained Stochastic Inversion of Gravity Gradients
  • Ian Macleod - The VOXI approach to modelling 3D density and magnetic properties of the R. J. Smith Test Range
  • Anthony Christensen - Mapping gravity over inhospitable terrain
  • Questions for all session speakers

Session four

Speakers in this session were;

  • Louise Sander - Airborne gravity case histories
  • Kit Campbell - Recent applications of airborne gravity gradiometry in mineral and petroleum exploration; examples and lessons learned
  • Tony Rudge - Applications of gravity gradient data for hydrocarbon exploration in the Canning Basin
  • Mike Enright - The value of airborne gravity gradiometry to exploration in the Pilbara
  • Questions for all session speakers and open discussion

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