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The absolute measurement of geomagnetic parameters can be applied directly not only in magnetic navigation and target detection but also in the correction of relative measurement. The absolute geomagnetic measurement generally adopts a combined measurement mode. Owing to differences in the principles of different magnetometers, it is difficult to comprehensively apply absolute data. To address this problem, we propose a novel automatic measurement model for the absolute geomagnetic total field and directions. Gearing toward field mobile measurement, we developed an integrated vector magnetometer using an improved Overhauser total-field sensor and a miniaturized spherical magnetic field generator. Component test results obtained for an artificial magnetic field generator show that the expanded uncertainty of Overhauser sensor with reduced dimensions was still below 0.10 nT, and the magnetic gradient formed by the applied bias fields had no negative effect on the performance of Overhauser sensor. In overall tests conducted in geomagnetic observatories, between the proposed magnetometer and a standard manually operated declination–inclination magnetometer, the differences of inclination and declination were only 14.40” and 12.96”, respectively. The noises of total field, horizontal component, and declination measured by the proposed magnetometer were 0.02 nT, 0.08 nT, and 0.47”, respectively, which are much lower than those of a state-of-the-art standard variometer. The proposed magnetometer is expected to be applied to unattended monitoring of geomagnetic field for the geomagnetic navigation, exploration of weak magnetic anomaly produced by the deep mineral resources, automatic remote detection of moving magnetic targets, fine-scale characterization of the plate tectonics, etc.

Magnetic field sensing is of great significance for marine geoscience research and industrial engineering. This article proposes a high-precision and dual-mode Overhauser marine magnetometer, which integrates base station monitoring mode and towed mobile survey mode. To accurately extract the frequency of weak free induction decay (FID) signals in complex marine environments, a precision narrowband resonant pickup circuit is adopted to enhance the signal, and a short-time precision frequency measuring method is presented. In addition, in terms of long-range underwater power supply and communication, we propose an interactive communication method for the Overhauser marine magnetometer based on low-voltage power line carrier, which not only provides power supply but also supports reliable duplex communication at 2.4 kbps baud rate within 3 km in the case of twin-core cable. The effectiveness and superiority of the developed Overhauser marine magnetometer were validated through extensive laboratory and field experiments. Results from a standard magnetic metrology laboratory indicate that the proposed magnetometer covers a field range from 20 to $100 \mu \text{T}$ , while the indicated value error is less than 0.09 nT and the noise is less than 0.06 nT. The geomagnetic observation experiment demonstrates the reliability and the advanced performance of the magnetometer under base station monitoring mode. Furthermore, the comparison results of the marine magnetism survey under towed mobile survey mode show that the agreement of the magnetometer and the commercial instrument installed on board is 0.95, which proves the feasibility and effectiveness of the proposed instrument for the marine magnetism survey.

Coil vector magnetometer is an advanced instrument that can perform integrated multielement geomagnetic measurements and has excellent prospects for geoscience research and resource exploration applications. The attitude deviation is one of the main error sources of magnetic direction measurements of the coil vector magnetometer. The existing attitude determination method requires the use of additional auxiliary instruments (e.g., a spirit level). Furthermore, this method cannot be used to perform direct determination of the directional shift of the bias field caused by attitude deviation, and the magnetism of the detection instrument inevitably introduces new measurement errors. Therefore, it is difficult to achieve high-precision attitude deviation correction. To address this issue, we propose a novel electrical method to enable direct, high-precision determination of the attitude deviation and the corresponding correction indicator for the coil vector magnetometer using only a single rotation of the magnetometer and applying bias fields, thereby realizing comprehensive high-precision hard and soft corrections of the attitude deviation via indicator alignment without relying on auxiliary detection instruments. In addition, we developed a dedicated experimental platform and then validated both the practicality and the performance of the proposed method in a geomagnetic observatory. Comparative experimental results for two coil vector magnetometers indicate that when the correction indicator’s alignment inaccuracy is less than 1 nT, the magnetic direction measurement error caused by the attitude deviation can be less than 6 in.

The information on the high-precision and multidirectional magnetic gradient is highly relevant to detecting and locating shallow subsurface unexploded ordnance (UXO). In this article, the design of a 3-D Overhauser magnetic gradiometer is presented. Taking the sensor array interferences and working efficiency into account, a time slice overlapping based operating time sequence is devised. A precision narrowband tuning capacitance network based on a cascaded subdivision strategy is constructed to improve the signal-to-noise ratio (SNR) of free induction decay (FID) signals. Additionally, a frequency measuring approach combining multiphase clock and multichannel is developed to suppress counting errors and improve the frequency measuring accuracy of FID signals. The 3-D Overhauser magnetic gradiometer’s effectiveness was validated through extensive laboratory and field experiments. The laboratory test results show that the magnetic measuring performance of each sensor from the Overhauser magnetic sensor array is consistent. The proposed 3-D Overhauser magnetic gradiometer is able to measure magnetic fields from 20 to $100~\mu \text{T}$ with an indicating error of less than 0.1 nT. The field exploration demonstrates the applicability of the gradiometer for shallow subsurface UXO detection within the urban complex electromagnetic surroundings. Overall, this study can provide support for the accurate detection and localization of shallow subsurface UXO under strong interference surroundings.