An aeromagnetic survey is a common type of geophysical survey carried out using a

magnetometer A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, ...

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).

Method

As the aircraft flies, the magnetometer measures and records the total intensity of the

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

at the sensor, which is a combination of the desired magnetic field generated in the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

as well as tiny variations due to the temporal effects of the constantly varying

solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...

and the magnetic field of the survey aircraft. By subtracting the solar, regional, and aircraft effects, the resulting aeromagnetic map shows the spatial distribution and relative abundance of magnetic minerals (most commonly the

iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of wh ...

mineral

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With ...

) in the upper levels of the

Earth's crust Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...

. Because different rock types differ in their content of magnetic minerals, the magnetic map allows a visualization of the

geological structure Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover informatio ...

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 thematic (colored) and shaded computer generated pseudo-topography images. The apparent hills, ridges and valleys are referred to as aeromagnetic anomalies. A

geophysicist Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' som ...

can use mathematical modeling to infer the shape, depth and properties of the rock bodies responsible for the anomalies. Airplanes are normally used for high-level reconnaissance surveys in gentle terrain, and helicopters are used in mountainous terrain or where more detail is required.

History

Aeromagnetic surveys were first performed in

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...

to detect submarines using a

Magnetic Anomaly Detector A magnetic anomaly detector (MAD) is an instrument used to detect minute variations in the Earth's magnetic field. The term refers specifically to magnetometers used by military forces to detect submarines (a mass of ferromagnetic material c ...

attached to an aircraft. This method is still widely used by military maritime patrol aircraft.

Uses

Aeromagnetic surveys are widely used to aid in the production of geological maps and are also commonly used during

mineral exploration Mining in the engineering discipline is the extraction of minerals from underneath, open pit, above or on the ground. Mining engineering is associated with many other disciplines, such as mineral processing, exploration, excavation, geology, and ...

and

petroleum exploration Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth using petroleum geology. Exploration methods Vis ...

. Some mineral deposits are associated with an increase or decrease in the abundance of magnetic minerals, and occasionally the sought after commodity may itself be magnetic (e.g.

iron ore Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the ...

deposits), but often the elucidation of the subsurface structure of the upper crust is the most valuable contribution of the aeromagnetic data. It has also been used to find buried fault zones that are prone to damaging earthquakes.

Unexploded ordnance

Aeromagnetic surveys are also used to perform reconnaissance mapping of unexploded ordnance. The aircraft is typically a helicopter, as the sensors must be close to the ground (relative to mineral exploration) to be effective.

Electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...

methods are also used for this purpose.

UAV aeromagnetic survey

Recent developments in aeromagnetic surveying include the use of drones. The market of unmanned aerial systems is exponential development, so the arrival of these technologies in some niches was inevitable including geophysical surveys. UAVs have proven to be especially useful for mineral exploration, detection and identification. It is also possible to detect Unexploded Ordnance objects using a drone-mounted magnetometer. Very interesting and useful research (2018–2019) was conducted by the

Binghamton University The State University of New York at Binghamton (Binghamton University or SUNY Binghamton) is a public research university with campuses in Binghamton, Vestal, and Johnson City, New York. It is one of the four university centers in the Stat ...

and aviation production compan
UMT
Namely, they were successfully able to design a UAV-based aeromagnetic survey system integrating the UMT Cicada hybrid-powered UAV platform and the Geometrics MFAM Micro-fabricated Atomic Magnetometer system. This approach with unmanned aerial vehicles application should make missions faster, cheaper and safer.

References