Aeromagnetic survey
An aeromagnetic survey is a common type of geophysical survey carried out using a magnetometer aboard or towed behind an aircraft. The principle is similar to a magnetic survey carried out with a hand-held magnetometer, but allows much larger areas of the Earth's surface to be covered quickly for regional reconnaissance. The aircraft typically flies in a grid-like pattern with height and line spacing determining the resolution of the data (and cost of the survey per unit area).
As the aircraft flies, the magnetometer records tiny variations in the intensity of the ambient magnetic field due to the temporal effects of the constantly varying solar wind and spatial variations in the Earth's magnetic field, the latter being due both to the regional magnetic field, and the local effect of magnetic minerals in the Earth's crust. By subtracting the solar and regional effects, the resulting aeromagnetic map shows the spatial distribution and relative abundance of magnetic minerals (most commonly the iron oxide mineral magnetite) in the upper levels of the crust. Because different rock types differ in their content of magnetic minerals, the magnetic map allows a visualization of the geological structure of the upper crust in the subsurface, particularly the spatial geometry of bodies of rock and the presence of faults and folds. This is particularly useful where bedrock is obscured by surface sand, soil or water. Aeromagnetic data was once presented as contour plots, but now is more commonly expressed as colored and shaded computer generated pseudo-topography images. The apparent hills, ridges and valleys are referred to as aeromagnetic anomalies. A geophysicist can use mathematical modeling to infer the shape, depth and properties of the rock bodies responsible for the anomalies.
UAVs such as the InView Unmanned Aircraft System are also starting to be used in aeromagnetic survey work.
Aeromagnetic surveys are widely used to aid in the production of geological maps and are also commonly used during mineral exploration. Some mineral deposits are associated with an increase in the abundance of magnetic minerals, and occasionally the sought after commodity may itself be magnetic (e.g. iron ore deposits), but often the elucidation of the subsurface structure of the upper crust is the most valuable contribution of the aeromagnetic data.
Aeromagnetic surveys are now used to perform reconnaissance mapping of Unexploded ordnance (UXO). The aircraft is typically a helicopter, as the sensors must be close to the ground (relative to mineral exploration) to be effective. Electromagnetic methods are also used for this purpose.
References
- Burger RH, Sheehan AF, Jones CH (2006) Introduction to Applied Geophysics. Published by W. W. Norton, 600 p., ISBN 0-393-92637-0 / ISBN 978-0-393-92637-8.
See also
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