Magnetic anomaly

The Earth has a pull.

This pull changes in some spots. These changes are called magnetic anomalies. They happen because of the rocks deep underground. Some rocks have more pull than others.

Scientists use tools to find these spots. They can use planes or ships to look. They can even use tools in space! This helps us find metal in the ground. It also shows how the ocean floor grows.

It is like a secret map of the Earth.

The Earth has a magnetic field. This is a force that pulls on certain things. Sometimes, this pull changes in small spots. We call these changes magnetic anomalies.

These changes happen because of the rocks underground. Some rocks have different chemistry or magnetism. For example, magnetite is a mineral with a very strong pull.

Scientists use tools called magnetometers to find these spots. They can work on land, in planes, or on ships. Some tools even fly in space on satellites!

On land, people often use a pole to hold the tool. They must stay away from metal objects like keys or cars. This keeps the data clean.

These maps help us in many ways. They help us find minerals in the ground. They also show how the ocean floor grows. Near ridges in the ocean, rocks form in stripes. These stripes show how the seafloor spreads over time. This helps us understand how our planet works.

The Earth has a giant magnetic field that surrounds our whole planet. Most of the time, this field is steady and predictable. However, sometimes the field changes in small, local spots. We call these changes magnetic anomalies.

These anomalies happen because of the rocks deep underground. Some rocks have different chemistry or magnetism than the ones around them. For example, a mineral called magnetite has a very strong pull. These small changes help scientists see structures hidden under the surface.

To find these spots, scientists use special tools called magnetometers. These tools measure the strength of the magnetic field. There are different kinds of magnetometers for different jobs. A fluxgate magnetometer measures the field along a specific axis. A proton precession magnetometer measures the strength but not the direction.

Some very expensive tools use alkali gases like rubidium or caesium. These are called optically pumped magnetometers. They are very sensitive and work well on satellites. They can detect tiny changes as small as 0.001 nanoteslas.

Scientists have used these tools in many ways since the mid-1900s. In 1958, the spacecraft Sputnik 3 was the first to carry a magnetometer into space. Later, in 1979, a NASA and USGS mission called Magsat was launched. It used both caesium and fluxgate magnetometers to study the Earth. Since then, many more satellites have helped us. The Danish satellite Ørsted launched in 1999. The European Space Agency also uses a group of three satellites called the Swarm mission. They were launched in November 2013.

Measuring these anomalies requires very careful work. On land, people walk between stations that are 15 to 60 meters apart. They must avoid metal things like keys, knives, or cars. These objects can create a "spike" that ruins the data.

In the ocean, a sensor is towed behind a ship in a device called a fish. This keeps the sensor about 15 meters deep. In the air, planes fly in straight, parallel lines. They might use a long boom to keep the sensor away from the plane's own magnetic field.

These magnetic maps tell us amazing stories about our world. One big discovery involves the ocean floor. Near mid-ocean ridges, the magnetic field shows a pattern of stripes.

These stripes are made of rocks like basalt and gabbro. As magma cools, it picks up a permanent magnetic direction. Because the Earth's magnetic field reverses every few hundred thousand years, these stripes form a pattern. This pattern proves that the seafloor is spreading apart. It helps us understand how the giant plates of the Earth move.

A magnetic anomaly is a local variation in the Earth's magnetic field. These variations occur because of changes in the chemistry or magnetism of underground rocks. While the Earth has a large, steady magnetic field, these small differences allow scientists to map structures hidden under surface material.

Mapping these anomalies is essential for understanding the composition of the Earth's crust. It helps researchers identify specific minerals and geological formations that would otherwise remain invisible.

To detect these anomalies, scientists use instruments called magnetometers. These tools must be very sensitive because anomalies are only a small fraction of the total magnetic field. The total field strength ranges from 25,000 to 65,000 nanoteslas (nT). Therefore, a magnetometer needs a sensitivity of 10 nT or less to be effective.

There are three main types of magnetometers used in geophysical research today.

First, the fluxgate magnetometer measures the magnetic field component along a specific axis. Because it relies on a particular direction, the sensor must be oriented correctly. On land, it is often placed vertically. In aircraft, ships, or satellites, it is usually oriented in the direction of the field. While it provides continuous measurements, it can drift over time. Scientists correct this by taking repeated measurements at the same location. Second, the proton precession magnetometer measures field strength but not direction. This means it does not require orientation. It is widely used in ground surveys, though not in boreholes. Third, optically pumped magnetometers use alkali gases like rubidium or caesium. These are highly sensitive, detecting changes as small as 0.001 nT. They are expensive and are mostly used on satellites or in aeromagnetic surveys.

Data collection methods vary depending on the environment. In ground-based surveys, researchers move between stations spaced 15 to 60 meters apart. They often use a proton precession magnetometer mounted on a pole. Raising the sensor helps reduce interference from small metal objects left by humans. Surveyors must also avoid carrying metallic items like keys or knives. They also avoid motor vehicles, railway lines, and barbed wire fences. If a metal object is missed, it creates a sharp spike in the data.

These ground surveys are primarily used to search for minerals.

Aeromagnetic and shipborne surveys allow for much larger areas to be studied. In aeromagnetic surveys, aircraft fly parallel flight lines at a constant height. These lines are occasionally crossed by perpendicular tie lines to check for errors. To prevent the plane's own magnetic field from interfering, the sensor is often placed on a non-magnetic boom or towed on a cable. Shipborne surveys use a device called a "fish" to tow a magnetometer behind a ship. The sensor is kept at a constant depth of about 15 meters.

These methods provide a broader view of regional rock structures.

Spacecraft have played a massive role in the history of magnetic research. The Sputnik 3 spacecraft was the first to carry a magnetometer in 1958. In 1979, the Magsat mission was launched by NASA and the USGS. It utilized both a caesium vapor scalar magnetometer and a fluxgate vector magnetometer. Other notable missions include the German CHAMP satellite, which operated from 2001 to 2010. The Danish Ørsted satellite was launched in 1999. More recently, the European Space Agency launched the Swarm mission, a constellation of three satellites, in November 2013.

Raw data must undergo "data reduction" to be useful. First, scientists must remove short-term variations caused by external sources. For example, the solar wind hitting the ionosphere causes diurnal variations that last 24 hours. Magnetic storms can also cause spikes of up to 1000 nT for several days. Second, the main geomagnetic field must be subtracted to isolate the local anomaly. Scientists often use the International Geomagnetic Reference Field for this. This is a mathematical model based on satellite and observatory data.

The study of these anomalies provides deep insight into plate tectonics. Magnetic surveys of the ocean floor show a pattern of stripes parallel to mid-ocean ridges. These stripes consist of positive and negative anomalies that are often symmetric. They are caused by minerals like titanomagnetite in basalt and gabbro. As magma rises and cools at the ridge, it acquires a permanent magnetization. Because the Earth's magnetic field reverses every few hundred thousand years, these stripes record the history of the field. This pattern provides the evidence needed to calculate the velocity of seafloor spreading.