Scientists use technology capable of detecting subtle changes in the planet's magnetic field to determine what lies kilometers beneath the Earth's surface without conducting any excavations.
Detecting Geological Structures
Based on these minor fluctuations, researchers can identify geological faults, ancient magma rivers, mineral deposits, and hidden structures in the depths. A recent study in Australia demonstrated this technique by creating a more detailed image of the Australian Magnetic Anomaly—a vast underground structure beneath the Northern Territory whose shape resembles the outline of the Australian continent.
It is important to note that this work did not require new field measurements or flights; instead, a processing algorithm was applied to eliminate noise and distortions from previous surveys, allowing geological features to be revealed with greater clarity. Thus, researchers were able to gain new information from existing data.
The Principle of Magnetometry
This Australian example illustrates a tool that geophysicists worldwide, including those in Brazil, have used for decades. To understand this technology, specialists discussed how rocks leave 'signatures' in the Earth's magnetic field and how this information helps reconstruct the planet's history.
Even if two rocks look almost identical, they can tell completely different stories to a geophysicist. This occurs because each rock type possesses its own physical properties. Some rocks have almost no effect on the surrounding magnetic field, while others containing minerals such as magnetite, titanomagnetite, hematite, or pyrotite cause small local disturbances. These disturbances can be registered by highly sensitive magnetometers, even if the minerals are present in low concentrations, generating differences on the order of a few nanoteslas—magnitudes millions of times smaller than the field created by a standard refrigerator magnet.
Gelvam André Hartmann from Unicamp emphasized that rocks do not alter the global magnetic field of the Earth but produce local disturbances that can be measured by extremely sensitive magnetometers. According to professors from Unipampa, the term 'anomaly' in geophysics describes precisely the difference between the expected magnetic field for a given area and what is actually measured by instruments in the field.
These small deviations reveal not only information about what rocks are under the ground but also contain data about the region's geological past. In addition to the magnetization caused by the current Earth's magnetic field, rocks usually retain residual magnetization that was recorded during their formation or acquired as a result of subsequent geological events. Thus, the rocks themselves preserve a record of the magnetic conditions that existed during their formation, while aeromagnetic measurements record these signals. The signal intensity depends on factors such as the amount of magnetic minerals, depth, size, geometry, and orientation of the rock bodies.
Distinction from Global Phenomena
It is important to make a distinction: the anomalies studied in geological surveys are not related to the South Atlantic Magnetic Anomaly (SAMA). While crustal anomalies are formed by rocks of the Earth's crust, SAMA is a global phenomenon arising in the Earth's outer core at depths of thousands of kilometers. Its effects are linked to reduced protection from energetic particles in space over the South Atlantic, which affects satellites and spacecraft.
Methods of Subsurface Investigation from the Air
Since rocks cause such subtle disturbances in the magnetic field, scientists use aeromagnetic surveys. During these surveys, aircraft equipped with sensors follow parallel trajectories over the area being investigated. To avoid the influence of the metal fuselage of the aircraft itself on the measurements, sensors are installed on an extended structure at the tail of the aircraft.
During flight, the equipment takes dozens or hundreds of measurements per second. Because the planes operate at low altitudes—usually around 100 meters above the ground—and follow regularly spaced lines, a detailed portrait of the magnetic signal can be constructed along the entire route. After landing, the data consists of a raw sequence of measurements. Before interpreting them, they undergo processing stages to remove equipment noise, navigation variations, and natural fluctuations of the planet itself. Then, researchers exclude the contribution of the main geomagnetic field (generated in the core) to isolate only the signal produced by the crustal rocks.
The result of this refinement is maps of magnetic anomalies showing the spatial distribution of changes in the subsurface. Researchers note that interpreting these maps allows for the identification of patterns associated with important geological features, such as faults, shear zones, dikes, magmatic intrusions, contacts between different lithological units, and potentially mineralized bodies. Geophysical inversion methods are also currently employed: computer models divide the subsurface into smaller elements and estimate what distributions of magnetic properties could have caused the measured surface signals, helping to build two-dimensional and three-dimensional models of the subsurface.
Data Processing and Map Generation
Collecting data in the air is only half the job. When the plane lands, the magnetometer readings form a raw dataset that mixes the reaction of the rocks with interference from the aircraft itself and natural fluctuations of the Earth's magnetic field. Careful filtering is required to isolate the rock signal. Professors from Unipampa explain that one of the first steps is magnetic compensation, a mathematical calculation that eliminates interference created by the metallic frame and electrical systems of the aircraft in motion.
Next, algorithms apply frequency filters to 'separate' the information. This separation allows distinguishing signals originating from shallow structures (such as potential mineral deposits) from those occurring in rocks kilometers deep within the crust. It is this computational refinement that transforms millions of measurements into detailed geological maps. Analyzing the Australian study, Gelvam André Hartmann recalled that 'the anomaly itself was already known.' He continued: 'The main breakthrough of the work lies in applying modern processing and interpretation methods, which allowed extracting much more geological information from the existing dataset.'
Limitations of Magnetic Maps
Despite frequent comparisons to an 'X-ray,' these maps are not a direct photograph of the planet's internal structure. Gelvam André Hartmann notes that 'the analogy helps explain the idea to the general public, but it has limitations.' He believes the best analogy is that aeromagnetic surveying works like a medical examination: it reveals patterns invisible to the naked eye, indicates where further detailed study should be conducted, and significantly reduces uncertainty, but rarely answers all questions about the subsurface on its own.
This limitation stems from the fact that the same magnetic response registered on the surface can be caused by various geological configurations in the depth—for example, different combinations of shape, size, depth, or amount of magnetic minerals. Therefore, magnetic data is rarely interpreted in isolation. Other methods used include gravimetry, which measures changes in rock density; electrical and electromagnetic methods, which analyze specific resistance and conductivity; and seismic methods, which investigate the elastic properties of materials. Furthermore, magnetometry is integrated with gamma spectrometry, a technique that measures naturally emitted gamma radiation from elements such as potassium, uranium, and thorium, to distinguish geological units with similar magnetic responses.
Practical Application of Magnetometry
In addition to helping to understand Earth's history, magnetometry is a vital tool in mineral exploration and planning new exploration campaigns. Many mineral deposits are associated with geological structures that cause contrasts in rock magnetic properties. Faults, shear zones, and fracture systems, for instance, can indicate favorable conditions for the concentration of gold, copper, nickel, iron ore, and other economically important resources.
In recent years, drones have also been included in these studies. Equipped with magnetometers, unmanned aerial vehicles (UAVs) can fly closer to the ground and obtain high-resolution data in specific areas. They are usually used after regional aeromagnetic surveys to detail the subsurface and guide subsequent stages of geological exploration, such as drilling.
The application of this technique has also played an important role in the history of science. In the 1950s and 1960s, the detection of bands of magnetic anomalies on the ocean floor provided evidence for seafloor spreading, which helped solidify the Theory of Plate Tectonics.
Geophysics in Brazil
Despite the recent focus on the Australian case, aeromagnetic surveys have been part of routine Brazilian geophysics for decades. Virtually the entire territory of the country exhibits magnetic anomalies related to crustal rocks. The first aerial surveys in Brazil date back to the early 1950s in São João del Rei (Minas Gerais). This project, commissioned by the National Nuclear Energy Commission (CNEN), used magnetic and radiometric sensors aboard aircraft to aid in the exploration of radioactive minerals, such as uranium.