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How are NMR magnetometers used to detect magnetic-field variations?

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magnetic resonance Figure 1: Precession of a magnetic dipole moment μ in the presence of a constant field H and a rotating field H′ (see text)

NMR magnetometers detect magnetic-field variations by measuring the precession frequency of proton spins within a magnetic field.1

When a particle with a magnetic moment is placed in a magnetic field, it undergoes Larmor precession, describing a cone around the direction of the field. The frequency of this precession is directly related to the strength of the magnetic field. By measuring this frequency with high accuracy, the magnetometer can determine the absolute value of the field.21

In environments with low magnetic fields, such as the Earth’s magnetic field, the NMR signal is typically weak. However, specialized devices can enhance this signal significantly, allowing these magnetometers to achieve an absolute accuracy of approximately one part in 1,000,000 and detect field variations as small as $10^{- 8}$ gauss. This capability is utilized to identify phenomena linked to spatial or temporal magnetic-field variations, including mineral deposits, archaeological remains, and anomalies caused by objects like submarines.1