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JPM-4 Proton Precession Magnetometer and Sensitivity Estimation

A proton precession magnetometer (PPM) is a traditional quantum magnetometer based on the Larmor precession of hydrogen protons in Earth’s magnetic field. PPMs are widely used in various fields, such as magnetic observation and detection of buried objects. A coaxial solenoid is typically used to construct the PPM sensor. However, the Larmor signal slowly weakens when the axial direction of the sensor gradually approaches the direction of Earth’s magnetic field. Thus, a dead zone exists when Earth’s magnetic field is nearly parallel to the axis of the sensor. An omnidirectional sensor with an “8”-type structure is designed in this study. The signal quality is slightly affected by the orientation of the sensor owing to the orthogonal polarized magnetic field components. The measured signal-to-noise ratio (SNR) of the Larmor signal is approximately 31/1, and the decay constant of the free induced decay (FID) signal is 0.95 s. The electrical parameters of the sensor coil are optimized and the polarization power is 8.0 W. Multiple hourly observations of Earth’s magnetic field in noisy and quiet environments indicate the satisfactory consistency of the measurement results of the two PPMs with the proposed sensor. The standard deviations (STDs) of the measured results for a single instrument in noisy and quiet environments are 6.4 and 0.076 nT, respectively, which effectively reflect the environmental noise level. The sensitivity of the instrument is estimated to be 0.04 nT at a 5 s cycling rate for the two synchronized instruments. This is higher than the sensitivity of most commercial magnetometers of 0.1 nT.