Magnetic anomaly

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.