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.
The Overhauser magnetometer is a scalar quantum magnetometer based on the dynamic nuclear polarization (DNP) effect in the Earth’s magnetic field. Sensitivity is a key technical specification reflecting the ability of instruments to sense small variations of the Earth’s magnetic field and is closely related to the signal-to-noise ratio (SNR) of the free induction decay (FID) signal. In this study, deuterated 15N TEMPONE radical is used in our sensor to obtain high DNP enhancement. The measured SNR of the FID signal is approximately 63/1, and the transverse relaxation time T2 is 2.68 s. The direct measurement method with a single instrument and the synchronous measurement method with two instruments are discussed for sensitivity estimation in time and frequency domains under different electromagnetic interference (EMI) environments and different time periods. For the first time, the correlation coefficient of the magnetic field measured by the two instruments is used to judge the degree of the influence of the environmental noise on the sensitivity estimation. The sensitivity evaluation in the field environment is successfully realized without electrical and magnetic shields. The direct measurement method is susceptible to EMI and cannot work in general electromagnetic environments, except it is sufficiently quiet. The synchronous measurement method has an excellent ability to remove most natural and artificial EMIs and can be used under noisy environments. Direct and synchronous experimental results show that the estimated sensitivity of the JOM-4S magnetometer is approximately 0.01 nT in time domain and approximately 0.01 nT/Hz in frequency domain at a 3 s cycling time. This study provides a low-cost, simple, and effective sensitivity estimation method, which is especially suitable for developers and users to estimate the performance of the instrument.
In view of the small dynamic range of the three-component high-temperature superconducting quantum interference device (HTc SQUID) magnetometer and it is easy to exceed the measurement range when working in the field without magnetic shielding environment. A dynamic range expansion method based on step compensation is proposed. By setting a magnetic compensation coil on each axis of the three-component HTc SQUIDs magnetometer probe, a current is passed to generate a magnetic field opposite to the direction of the external magnetic field to offset most of the magnetic field to be measured. The measured values are calculated and processed to obtain the magnetic field value to be measured. The test results show that the method can greatly improve the dynamic range without reducing the sensitivity of the magnetometer, which can work stably in a non-magnetic shielding environment and meet the requirements of field geophysical magnetic exploration.
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