The Earth consists of many rock types which have different physical characteristics such as density, mineral composition and micro and macro structure. These physical properties emit or generate various signals which can be measured remotely using special instruments. Earth’s mass creates a gravity field which attracts everything to the Earth with an acceleration of approximately 9.8 metres per second per second (m/s/s). This gravity field is not uniform, with denser rocks producing a stronger gravity field and less dense rocks producing a weaker gravity field. These differences are very small, are not felt and don’t affect normal, everyday life, but can be measured using a very precise and sensitive instrument called a gravimeter. This instrument can measure the force of gravity to a precision of one part in one million.
There are two types of gravimeter, relative and absolute. Relative gravimeters use a calibrated spring to measure the relative difference in gravity from one place to another while absolute gravimeters directly measure the acceleration of an object dropping in a vacuum. Absolute gravimeters are very high precision instruments and are generally used in a stationary mode at particular sites. Relative gravimeters are used on land by hand and in vehicles, aircraft, surface ships and submersible vessels. Gravity also can be measured using satellites.
The ability to measure these small changes in gravity due to changes in the density of the rocks which make up the Earth provides a method which makes it possible to remotely map the distribution of different rock types and the structure of the rocks. Gravity data are part of the suite of data which is used to map the 3D structures of the subsurface rocks to determine where energy and mineral deposits may be located. Other applications of gravity data include finding caves in urban engineering studies, investigating archaeological sites and in the detection of buried waste drums as well as in Geodesy to determine the shape of the Earth and to underpin the Global Positioning System (GPS) navigation.
Geoscience Australia maintains the Australian National Gravity Database (ANGD), which contains data from gravity surveys conducted throughout Australia and its Remote Offshore Territories. These data have been sourced from within 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 conducted throughout Australia. The AFGN is a network of permanently marked and documented gravity base stations which allow gravity surveys to tie their surveys to a consistent datum.
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