Magnetic Surveys

One of the most important tools in modern geological mapping is the use of aeromagnetic surveys. They are fast, provide a great deal of information for the cost and can provide information about the distribution of rocks occurring under thin layers of sedimentary rocks, useful when trying to locate orebodies.

Aeromagnetic surveys are taken from a moving plane. A magnetometer is the instrument used to measure the intensity of the magnetic field at a particular place.

Magnetism in Rocks

The Earth has a natural magnetic field caused by the motion of materials deep within the Earth's core. This magnetic field has been known and used by navigators since the Vikings used "lodestones" as simple compasses to find north. Today, detailed mapping of the Earth's magnetic field is continually updated to assist navigation.

When the Earth's magnetic field interacts with a magnetic mineral contained in a rock, the rock becomes magnetic. This is called induced magnetism. The same thing occurs if you place an iron nail near a magnet, the magnet's magnetic field will induce a magnetic field in the nail. This induced field is often strong enough that you can pick up another smaller nail with the first nail. However, move the magnet away from the nail and the induced magnetic field ceases.

A rock itself will be magnetic if at least one of the minerals it is composed of is magnetic. The strength of the rock's magnetism is related not only to the amount of magnetic minerals they contain but also to the physical properties, such as grain size, of those minerals. The main magnetic mineral is magnetite (Fe3O4) - a common mineral found disseminated through most rocks in differing concentrations.

Once the aeromagnetic data has been collected, this data is then processed to remove the Earth's natural magnetic field and any diurnal field changes (night to day variations) to reveal the variations in magnetisation due to the underlying geology.

Individual magnetic anomalies - magnetic signatures different from the background- consist of a high and a low (dipole) compared to the average field. In the Southern Hemisphere the high is located to the north and the low to the south of the magnetic body. The position and size of the anomaly depend on the position and size of the magnetic body. A change in latitude will also affect the positioning of anomalies over the magnetic body.

Magnetic readings

This allows the geoscientists to interpret the position of the body which has caused the anomalous reading. Often however the reading is complicated because of the position of the body in relation to other rocks, its size, and what happens to the body at depth.

The data for a survey can be plotted as a contour map using lines which join points of equal "magnetic" value.

From these maps geoscientists can locate magnetic bodies (even if they are not outcropping at the surface), interpret the nature of geological boundaries at depth, find faults etc.

Contoured map of magnetic area

Like all contoured maps, when the lines are close together they represent a steep gradient or change in values. When lines are widely spaced they represent shallow gradient or slow change in value.

A modern technique is to plot the magnetic data as a colour image (red=high, blue=low and all the shades in between representing the values in between). This gives an image which is easy to read - especially when using 3-D glasses.

When interpreting the aeromagnetic image it is useful to know that magnetite is found in greater concentrations in igneous and metamorphic rocks. In some localities it is in such high concentrations it is mined as an iron ore. Magnetite can also be weathered or leached from rocks and redeposited in other locations, such as faults.

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