Rocks are made up of different minerals and the magnetic properties of a rock depend on the amount and type of iron rich minerals it contains and the 3D distribution of the rock formation within the crust. Earth's magnetic field interacts with these iron rich minerals to generate variations in the magnetic field. These variations can be large and can affect compass directions.
Measuring and mapping these variations allows remotely mapping of the distribution and patterns of magnetic rocks and, as a result, map the subsurface geology. The instrument used to measure the magnetic field is a magnetometer. There are number of different types of magnetometer and can be hand held or easily deployable in vehicles, aircraft, surface and submarine vessels and satellites.
The magnetic method is a non-invasive geophysical method which ultimately measures the magnetic field associated with magnetic minerals in crustal rocks. This allows sub-surface geology to be mapped to reveal geological structure and mineral potential, paving the way for planning more detailed surveys for exploration and environmental management.
The magnetic method measures variations in the intensity of the Earth's magnetic field caused by the contrasting content of rock-forming magnetic minerals in the Earth's crust. The content of magnetic minerals ranges from negligible in most sedimentary rocks to appreciable amounts in igneous and metamorphic rocks. The magnetic method is a valuable technique for mapping the boundaries of extensive rock units which contain only a small percentage of magnetic minerals, but with enough contrast in magnetic mineral content to distinguish them from neighbouring rock units.
In marine magnetic surveys the magnetometer is towed at a distance behind a ship where the magnetic effect of the ship is very small. These surveys can be carried out in conjunction with other marine geophysical methods such as seismic and gravity surveys. Geoscience Australia holds extensive marine magnetic data sets recorded by Australian and foreign vessels in Australian waters.
An airborne magnetic survey is flown either by airplane or helicopter to which a magnetometer is attached and are a very fast and efficient method for mapping large areas of geology. The magnetometer measures the total intensity of the Earth's magnetic field along continuous flight lines a fixed distance apart. The aircraft is flown at a constant terrain clearance along straight flight lines a fixed distance apart, with modern surveys flown at distance of less than 100 metres above ground level.
Since 1951 the Australian, State and mainland Territory Governments have conducted ongoing programs of acquisition of airborne magnetic data. Private companies also use airborne surveys as part of their mineral and hydrocarbon exploration programs.
The airborne geophysical data collected by government surveys are held in Geoscience Australia's National Airborne Geophysical Database (NAGD). By May 2007, the NAGD contained more than 1000 surveys conducted by or for the Australian Government and the State and Northern Territory Geological Surveys. The NAGD contains more than 27 million line kilometres of total magnetic intensity data.
Prior to 1990 most of the surveys had a flight line spacing of 1500 metres or more. Since 1990, surveys have usually been conducted with flight line spacings of 400 metres or less.
The data collected are available for free download through the Geophysical Archive Data Delivery System (GADDS) .