How Magnets and Gravity are Used in Mineral Exploration

Magnetic and gravity surveys differ from traditional seismic surveys in that they are passive, rather than aggressive. Instead of causing detonations or pressure waves, they measure variations in the magnetic and gravitational fields of the Earth. Although both can be used for minerals exploration, they have different strengths and weaknesses and often should be used together to give a more complete map of a potential mining location.

The two methods use some common equipment. Aircraft can be used to create large-scale maps of both the magnetic and gravitational anomalies in a large region. Teams on the ground can visit points of particular interest to take more precise measurements.



Magnetic Surveys

Magnetic readings are made with special equipment that logs the relative strength of the magnetic field at various points in a computer for analysis. Creating surveys from these readings requires extremely complicated mathematics. The Earth’s magnetic field is affected by many factors, including its rotation and solar radiation. However, by removing all external factors from a reading, geologists can isolate small variations created by mineral deposits.

The most common type of detection is direct detection, where companies identify the magnetic signature of iron ores or mineral crystals. Iron crystals are particularly evident in magnetic surveys, allowing for pinpoint accuracy when planning iron mining excavations.

However, magnetism can also be used indirectly to discover rarer ores. The same geological processes that lead to iron crystallization also cause fault lines in the rocks. This can allow for extremely accurate maps of underground rock formations. By examining these rock formations, geologists can determine the “story” of each rock formation and thus its likelihood of holding rare ores.

Gravity Surveys

Although it seems uniform everywhere, gravity actually varies slightly from place to place. When you drive a couple of kilometers, you are probably gaining or losing weight, albeit an extremely small amount. These variations are caused by particularly light or heavy rock formations in the Earth’s crust.

Like magnetic surveys, gravity surveys use precise instruments to measure the strength of gravity at various places. This allows geologists to create maps of the density of the crust below our feet. This method is particularly effective at accurately spotting oil deposits. However, they also have applications in mining.

Gravity surveys have been used, with great success, to locate diamonds. Diamonds typically are found in characteristic kimberlite pipes that are dense compared to nearby rocks, leading to a spike in the local gravity.

In addition to and oil diamonds, gravity surveys play a part in confirming and mapping potential deposits of most rare minerals.