Magnetics

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, the distribution and orientation of these minerals 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.

The magnetic method is a non-invasive geophysical method which ultimately measures the magnetic field associated with magnetic minerals in crustal rocks. Measuring and mapping these variations allows mapping of the distribution and patterns of magnetic rocks. From maps of magnetic variation, inferences about the subsurface distribution of different rock types can be made. This can inform on the depth and orientation of rocks with mineral potential, paving the way for planning more detailed surveys for exploration and environmental management.

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.

The instrument used to measure the magnetic field is a magnetometer. There are number of different types of magnetometer which can be hand held, or easily deployable in vehicles, aircraft, surface and submarine vessels and satellites.

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 is a rapid and efficient method for geological mapping over large areas. 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, with modern surveys flown at a 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.

All magnetic surveys collected by government surveys are available from the geophysical archive data delivery system (GADDS 2.0), which includes data from over 1200 surveys. 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 latest editions of magnetic grid products are the 2019 national magnetic grids, including Total Magnetic Intensity (TMI) at 40 and 80 m cell size and First Vertical Derivative, and Total Magnetic Intensity with Variable Reduction to Pole. These grids are produced from 33 500 000 line kilometres of survey data. In addition, Intrepid Geophysics have created an enhanced products package that enhances the interpretability of the original data and allows additional and novel insight into the regions covered. Magnetic data is available for free download through the Geophysical Archive Data Delivery System (GADDS).