Magnetic anomalies measured by planes and drones help scientists identify faults, magma intrusions, and possible mineral deposits underground.
A new data processing allowed for a clearer observation of a large underground formation in the Northern Territory, Australia, without requiring excavations, drilling, or another aerial campaign. The result demonstrates how magnetic anomalies can transform extremely small changes in the Earth’s field into clues about rocks located kilometers deep.
The advancement occurred in the analysis of the Australia Magnetic Anomaly, a structure whose shape resembles the outline of the Australian continent. The formation was already known, but modern algorithms removed distortions from previous surveys and revealed details related to volcanic and sedimentary materials deposited more than 1.5 billion years ago.
Magnetic anomalies function as signatures of rocks
Two samples with a similar appearance can produce very different magnetic responses. This happens because some minerals, including magnetite, titanomagnetite, hematite, and pyrrhotite, locally interfere with the field surrounding the planet.
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In Geophysics, the anomaly corresponds to the difference between the predicted value for a region and that actually recorded by the instruments. Although some variations are only a few nanoteslas, sensitive magnetometers can capture them and provide clues about the composition, depth, size, and orientation of buried formations.
Rocks can also retain magnetization acquired when they formed or during subsequent geological events. In this way, the current signal carries information about conditions existing millions or even billions of years ago.
Planes record magnetic anomalies near the surface
In regional surveys, aircraft follow parallel trajectories, usually about 100 meters above the ground. The magnetometer is kept away from the main structure of the plane to reduce interference caused by metals, electrical circuits, and other components.
During the journey, the sensor produces dozens or hundreds of records every second. At the end of the flight, however, scientists still have only a large volume of raw data, mixing the response of the rocks with natural oscillations of the Earth’s field and noise generated by the operation itself.
Processing programs correct navigation variations, compensate for the aircraft’s influence, and remove the main portion of the magnetism produced in the planet’s core. After these steps, the signals associated with the crust become more evident.
Computers transform measurements into underground maps
Mathematical filters help separate responses originating near the ground from those related to deeper rock bodies. Geophysical inversion techniques then calculate which combinations of materials could produce the values found by the sensors.
These calculations allow for the creation of two-dimensional and three-dimensional representations. The maps can highlight faults, dikes, shear zones, ancient magma intrusions, boundaries between different rocks, and areas with possible mineral concentration.
Despite the frequent comparison to an X-ray, the result does not equate to a direct photograph of the Earth’s interior. The same measurement can be explained by structures with different shapes, dimensions, depths, or amounts of magnetic minerals.
Magnetic anomalies need other scientific techniques
To reduce uncertainties, magnetometry is often combined with methods that analyze distinct properties. Gravimetry identifies density differences, while electrical and electromagnetic techniques assess the materials’ ability to conduct or resist the passage of current.
Seismic surveys investigate how waves travel through rocks. Gamma spectrometry measures the natural emission of elements like potassium, uranium, and thorium, helping to differentiate formations that present similar magnetic responses.
The integration of these results guides the locations that deserve more detailed studies and reduces the area designated for drilling. Thus, the technique reduces uncertainties and directs the investigation, although it does not provide all the answers in isolation.
Old data can produce new discoveries
The Australian study shows that collections gathered decades ago may still hold unknown information. More recent software can remove artifacts, enhance resolution, and test computational models that were not available at the time of the measurements.
This reuse avoids the immediate repetition of expensive campaigns and allows for reviewing extensive regions with more advanced tools. Countries that preserve historical geophysical surveys, including Brazil, have databases that can be scientifically explored again.
The current potential of Geophysics, therefore, does not depend solely on new sensors. Some advances may arise from a different interpretation of records that were already stored.
Drones refine areas indicated by magnetic anomalies
Manned aircraft are efficient for investigating large territories. After identifying a relevant area, drones equipped with magnetometers can fly closer to the ground and produce a higher resolution portrait.
These devices detail selected locations before more expensive stages, such as drilling. In mineral prospecting, the method helps locate faults, fractures, and other structures associated with environments where gold, copper, nickel, and iron ore may occur.
The technique does not alone confirm the existence of a deposit. It indicates regions with geological characteristics that justify further analysis.
Brazil has surveys since the 1950s
The first Brazilian aerial surveys date back to the early 1950s, in São João del-Rei, Minas Gerais. Magnetic and radiometric sensors were used to investigate large areas and support the search for radioactive minerals.
Currently, part of the data produced by public programs is available at the Geological Survey of Brazil. The surveys contributed to research in Carajás, the Iron Quadrangle, Tapajós, Rondônia, Goiás, Bahia, and the Southern Rio Grande Shield.
Magnetic records also played an important role in the history of science. The stripes discovered on the ocean floor during the 1950s and 1960s helped to prove seafloor spreading and strengthened the Theory of Plate Tectonics.
Magnetic anomalies are not the AMAS
The variations used to study the subsurface are produced by crustal rocks and have a regional character. They should not be confused with the South Atlantic Magnetic Anomaly, known as AMAS.
The AMAS is linked to processes in the Earth’s outer core and the global configuration of the magnetic field. Its effects include less protection against energetic particles over a vast area of the South Atlantic, a relevant situation for satellites and spacecraft.
On the other hand, crustal magnetic anomalies allow for the reconstruction of geological formations, the selection of areas of mineral interest, and the examination of deep parts of the planet. By combining sensors, physics, geology, and computing, scientists transform invisible variations into maps of the Earth’s structure and history.
With information from Olhar Digital
