Hongpeng Wang

To address the challenges of extracting weak, rapidly decaying, and noise-corrupted free induction decay (FID) signals from Overhauser magnetometers, this study introduces the rank-sequential truncated tensor decomposition (RSTD) method for effective noise suppression and high signal-to-noise ratio (SNR) signal recovery in high-noise environments. The proposed approach involves acquiring multichannel FID signals using an equal-delay strategy and organizing them into a third-order tensor through data segmentation. Subsequently, the tensor undergoes a higher-order singular value decomposition combined with matrix rank decomposition, following a predefined modal decomposition sequence to enhance SNR. Comparative evaluations against conventional techniques, including singular value decomposition (SVD), principal component analysis (PCA) assisted SVD (C-PCASVD), and multilinear SVD (MLSVD), demonstrate that RSTD achieves a signal-to-noise improvement ratio (SNIR) of 40 dB across initial SNRs ranging from -20 dB to 0 dB. The method matches MLSVD in denoising efficacy while outperforming SVD and C-PCASVD by more than 10 dB in SNIR. In particular, the proposed method completes processing within 100 ms, representing more than 50% improvement in computational efficiency over MLSVD. These results confirm that the proposed method substantially improves real-time processing capabilities without compromising noise reduction performance.

The three-axis fluxgate sensor is the most widely used sensor in vector magnetic measurement. This paper proposes a calibration method for the cumulative error of the three-axis fluxgate sensor considering the influence of temperature. Firstly, various errors of the three-axis fluxgate sensor were analyzed and the cumulative error model was established. The parameters of the sensor cumulative error model were calculated based on the Particle Swarm Optimization-Genetic Algorithm (PSO-GA) optimization algorithm. Then, the water bath heating method was used to implement scalar comparison experiments between the three-axis fluxgate sensor and a reference Overhauser magnetometer at different temperatures. Experimental results show that temperature affects the scale factor coefficients in the error calibration parameters of the three-axis fluxgate sensor, and the scale factor coefficients q_x , q_y , q_z of the three axes have similar trends with temperature. Finally, we designed validation experiments for the proposed calibration method. During long-term magnetic field measurement, using the fixed scale factor coefficient matrix Q for calibration still has a large deviation from the trend of the reference field, with the maximum deviation being about 150 nT. However, using the scale factor coefficient matrix Q( T ) that changes with temperature for calibration, the trend of the calibrated magnetic field is basically consistent with the reference field, and the maximum deviation is less than 20 nT. This means that the proposed method has a better calibration effect.

Magnetic field sensing is of great significance for marine geoscience research and industrial engineering. This paper 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 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 was 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 μT to 100 μT, 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 marine magnetism survey.

Magnetic anomaly detection technologies have been widely used for tracking moving targets. In this paper, we present a fast-tracking method for magnetic abnormalities using a distributed Overhauser magnetometer system based on the genetic algorithm. Our proposed framework of the Overhauser magnetometer system employs multiple sensors to eliminate background interference, and the genetic algorithm efficiently solves magnetic anomaly data without requiring the derivation of the objective function. Test platforms were built to evaluate the distributed Overhauser magnetometer system and the genetic algorithm. Results from the natural outdoor magnetism laboratories showed that the noise of our presented magnetometers was below 0.134 nT. The optimal factors for solution precision and effectiveness in the genetic algorithm were obtained from the simulation. Moreover, the outdoor tracking experiments indicated that the proposed method could accurately and quickly detect the moving ferromagnetic object within 6.9% maximum positioning error in 0.55 m, and the tracking precision of the object velocity can get 5.88% maximum error in 4.33 km/h.

The information on the high-precision and multi-directional magnetic gradient is highly relevant to detecting and locating shallow subsurface unexploded ordnance. In this paper, the design of a three-dimensional (3D) 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 narrow-band 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 multi-phase clock and multi-channel is developed to suppress counting errors and improve the frequency measuring accuracy of FID signals. The 3D 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 3D Overhauser magnetic gradiometer is able to measure magnetic fields from 20 μT to 100 μT with an indicating error of less than 0.1 nT. The field exploration demonstrates the applicability of the gradiometer for shallow subsurface unexploded ordnance detection within the urban complex electromagnetic surroundings. Overall, this study can provide support for the accurate detection and localization of shallow subsurface unexploded ordnance under strong interference surroundings.

The free induction decay (FID) signal of Overhauser geomagnetic sensors may suffer from saturation distortion in the mobile magnetic survey, which results in the loss of signal envelope information. Aiming to suppress the negative effects of saturation, this article proposes a recovery method for the FID signal from saturation based on segmented linear regression. The influences of the signal-to-noise ratio (SNR) and the saturation level on the recovery effect of the proposed method were investigated. Numerical simulations show that the proposed method has a good recovery performance under low SNR and high saturation levels. Furthermore, we set up an Overhauser geomagnetic sensor experimental test platform and conducted extensive experiments. The experimental results show that when the FID signal was partially saturated with an SNR of 20.47 dB, the initial amplitude and the relaxation time recovery error were reduced by a factor of 7.30 and 6.76, respectively. Even if the FID signal is fully saturated, the proposed method can still obtain the initial amplitude and the relaxation time accurately.

Fluxgate sensors are currently widely used for weak magnetic field measurement because of their relatively great performance, such as resolution, power consumption, and measurement of vector magnetic fields directly. Since the analog fluxgate sensor has some drawbacks, e.g., it would be influenced by the noise of the analog circuit. Hence, in recent years, the analog circuit is gradually inclined to be realized by digital processing in which the software parameter adjustment is employed to replace the analog components, which can greatly improve the flexibility of the design. This paper proposes a digital single-axis fluxgate sensor according to the cobalt-based amorphous effect. To be specific, the analog signal output by the fluxgate is sampled directly by an analog-to-digital converter to obtain the signal waveform in digital form after amplification. The demodulation, filtering, and integration of the signal are all solved by mathematical algorithms. Based on the working principle of the fluxgate sensor, the selection of the magnetic core material and coil winding method of the fluxgate sensor probe is introduced in detail. The design and function of the excitation circuit and preamplifier circuit, as well as the specific realization of digital signal processing, are described. Finally, the performance test of the digital fluxgate sensor was performed under laboratory conditions, and the magnetic anomaly detection comparison experiment was performed outdoors with commercial fluxgate sensors. To sum up, the linearity of the digital single-axis fluxgate sensor is better than 1 × 10⁻⁵, and the root mean square noise value is below 0.1 nT. At the same time, it has good magnetic field tracking performance and is extremely sensitive to the magnetic field of the measurement area.

An Overhauser geomagnetic sensor is a precise instrument commonly employed for geomagnetic field observation, magnetic surveys, and so on. Currently, the miniaturization of the Overhauser geomagnetic sensor is limited due to the lower signal-to-noise ratio. Thus, how to effectively extract weaker free induction decay (FID) signal from a miniaturized sensor and how to improve the signal quality have become the bottleneck. To address these problems, we came up with an optimal design of the FID signal sensing coil for a miniaturized Overhauser geomagnetic sensor and propose a front-end matching circuit for the sensing coil to inhibit the attenuation of the signal amplitude caused by high impedance, further reducing the overall noise floor of the signal acquisition system. Finally, the field experimental results show that the miniaturized prototype sensor has a smaller volume and mass with an approximate performance compared with the commercial sensor.