Haobin Dong

Overhauser Geomagnetic Sensor Based on the Dynamic Nuclear Polarization Effect for Magnetic Prospecting

Based on the dynamic nuclear polarization (DNP) effect, an alternative design of an Overhauser geomagnetic sensor is presented that enhances the proton polarization and increases the amplitude of the free induction decay (FID) signal. The short-pulse method is adopted to rotate the enhanced proton magnetization into the plane of precession to create an FID signal. To reduce the negative effect of the powerful electromagnetic interference, the design of the anti-interference of the pick-up coil is studied. Furthermore, the radio frequency polarization method based on the capacitive-loaded coaxial cavity is proposed to improve the quality factor of the resonant circuit. In addition, a special test instrument is designed that enables the simultaneous testing of the classical proton precession and the Overhauser sensor. Overall, comparison experiments with and without the free radical of the Overhauser sensors show that the DNP effect does effectively improve the amplitude and quality of the FID signal, and the magnetic sensitivity, resolution and range reach to 10 pT/Hz1/2@1 Hz, 0.0023 nT and 20–100 μT, respectively.

Research on a secondary tuning algorithm based on SVD & STFT for FID signal

The tuning precision of a Proton precession magnetometers sensor is the key to getting the best signal to noise ratio (SNR) of free induction decay (FID) signals. By analyzing the noises of the magnetometers sensor and conditioning circuit, this paper introduces the principle of tuning and proposes a secondary tuning algorithm based on the singular value decomposition (SVD) and short-time Fourier transform (STFT), targeting the current lack of a tuning method. Moreover, the STFT for an FID signals feature analysis is applied for the first time. First, the space matrix is constructed by the acquisition of ADC for the untuned FID signal, and then the SVD is performed to eliminate the noise and obtain the useful signal. Finally, the STFT technique is applied to the denoised signal to extract the time-frequency feature. By theory analysis, simulation modeling and the testing of an actual FID signal, the results show that, compared with general tuning methods such as peak detection and fast Fourier transform (FFT), the proposed algorithm improves the sensors tuning precision, and the time of the tuning process is no more than one second. Furthermore, the problem of mistuning in strong-disturbance environments is solved. Thus, the secondary tuning algorithm based on the SVD and STFT is more practical.

A High-Precision Frequency Measurement Algorithm for FID Signal of Proton Magnetometer

The accuracy of measuring a magnetic field using a proton magnetometer depends on the accuracy of frequency measurement of the free induction decay (FID) signal generated by the proton magnetometer. Increasing the accuracy of frequency measurement could increase the accuracy of measuring magnetic fields. In this paper, we propose a multichannel frequency measurement to improve the accuracy after analyzing the error sources of existing frequency measurements based on the FID signal. In addition, we describe the basic principle of the proposed method and the formula for the calculation of the measured FID signal frequency and error in detail. Moreover, we implement a prototype device using the multichannel frequency measurement and compare the test results before and after using the proposed method. Finally, the indoor and outdoor experimental data in the case of using the prototype device are compared with those in the case of a very high-precision overhauser magnetometer. The comparisons indicate that the proposed method improves the accuracy of the FID signal frequency measurement, and therefore, improves the accuracy of magnetic field measurement.

Noise characterization for the FID signal from proton precession magnetometer

Proton precession magnetometer is a high-precision device for weak magnetostatic field measurement. The measurement accuracy depends on the frequency measurement of free induction decay (FID) signal, while the signal to noise ratio (SNR) is an important factor affecting the results. Many signal processing methods have been proposed to improve the SNR of FID signal. However, the theoretical analysis of different types of noises for FID signal has not be conducted yet. In addition, the relationship between the frequency measurement accuracy and SNR has not been explicitly established and quantified. This paper first proposes a background noise model based on the extracted features from the FID signal. With this model, background noises, such as white noise, narrow-band noise, and phase noise etc., can be calculated and estimated. Secondly, the relationship between the frequency measurement accuracy and SNR is identified. We also built a prototype proton magnetometer for field tests and validation purpose. Experiments were conducted to investigate this relation through simulation. Different values for frequency accuracy were obtained with different SNRs from the acquired FID signals from field tests. The consistence between the measurement and computational results is observed. When SNR is larger than 30 dB, the absolute frequency accuracy becomes constant which is about 0.04 Hz. With the stability taken into account, the accuracy can be better even when the SNR is higher than 30 dB. This study provides a reference to optimize the design of proton precession magnetometer and the frequency calculation for FID signal.

Construction of an Overhauser magnetic gradiometer and the applications in geomagnetic observation and ferromagnetic target localization

The proton precession magnetometer with single sensor is commonly used in geomagnetic observation and magnetic anomaly detection. Due to technological limitations, the measurement accuracy is restricted by several factors such as the sensor performance, frequency measurement precision, instability of polarization module, etc. Aimed to improve the anti-interference ability, an Overhauser magnetic gradiometer with dual sensor structure was designed. An alternative design of a geomagnetic sensor with differential dual-coil structure was presented. A multi-channel frequency measurement algorithm was proposed to increase the measurement accuracy. A silicon oscillator was adopted to resolve the instability of polarization system. This paper briefly discusses the design and development of the gradiometer and compares the data recorded by this instrument with a commonly used commercially Overhauser magnetometer in the world market. The proposed gradiometer records the earth magnetic field in 24 hours with measurement accuracy of ± 0.3 nT and a sampling rate of 3 seconds per sample. The quality of data recorded is excellent and consistent with the commercial instrument. In addition, experiments of ferromagnetic target localization were conducted. This gradiometer shows a strong ability in magnetic anomaly detection and localization. To sum up, it has the advantages of convenient operation, high precision, strong anti-interference, etc., which proves the effectiveness of the dual sensor structure Overhauser magnetic gradiometer.

A high precision proton magnetometer based on a multi-channel frequency measurement

In order to improve the precision and anti-interference ability of traditional proton magnetometer, and provide reference basis for developing Overhauser magnetometer, a high precision proton magnetometer based on a multi-channel frequency measurement was designed. This paper introduces the basic principle of larmor precession effect, makes a detailed description about the signal disposal system and the multi-channel frequency measurement algorithm of the instrument. Finally, the comparison experiments between the proposed instrument and commercial Overhauser magnetometer under outdoor was conducted; the test results show that the performance of the instrument is close to that of the commercial Overhauser magnetometer; the proposed instrument has the advantages of convenient for field operation, high field magnetic measurement precision, strong anti-interference and etc, which proves the effectiveness of the proton magnetometer based on the multi-channel frequency measurement in weak magnetic measurement.

The Design of Embedded Electromagnetic Data Acquisition System Base on Linux

Recently, in the field of geological exploration, electromagnetic instruments have been tremendous growth, but they also have some typical shortcomings, such as big size, large power consumption, complex operation and so on. Therefore, base on the current development of electronic technology, this paper has discussed hardware and software design of the electromagnetic data acquisition system, which makes use of 24-bit Σ-Δ analog to digital converter AD7763, field programmable gate array, ARM9 processor S3C2440 and embedded Linux operating system. The system has high accuracy, small size and low power consumption, can be applied to a number of occasions.

A high-precision and fast-sampling frequency measurement method based on FPGA carry chain for airborne optically pumped cesium magnetometer

Improving the precision and sampling rate of the resonance frequency of cesium atoms is the key to enhancing the same factors of an airborne optically pumped cesium magnetometer (AOPCM). Aiming at the existed problems of AOPCM and characteristics of resonance signal, this paper proposes a high-precision and fast-sampling frequency measurement method based on carry chains of Field-Programmable Gate Array (FPGA). In order to achieve a fast-sampling rate, an improved equal precision frequency measurement method is proposed to measure the standard signal and the resonance signal continuously. Besides, by using the serial full adder to connect FPGA carry chains to a delay line, the delay line is used to compensate the unsynchronized clock edge, so the counting error can be reduced, and the precision of frequency measurement can be improved greatly. Experiments show that the frequency resolution is 0.014 nT and the relative error is lower than 2 × 10-6 when the sampling rate is 500 Hz. The experimental result indicates that the proposed method improves the precision and sampling rate of resonance frequency measurement greatly. Consequently, the precision and sampling rate of AOPCM can be improved.

A virtual instrument for diagnosis to substation grounding grids in harsh electromagnetic environment

To improve the traditional electromagnetic induction methods for grounding grid diagnosis, a virtual instrument is proposed to detect weak signals in the harsh and noisy substation electromagnetic environment. The induced electric signal in the coil of wire, containing multi-source and intense noises, is collected by the NI DAQ device. Then virtual phase intensive detectors and digital filters constitute the lock-in amplifier to suppress noise. In the experimental test, a small model of grounding grid is diagnosed in a man-made break situation. The visualized result shows the feasibility and accuracy of the proposed virtual instrument.

An Automatic Wideband 90° Phase Shifter for Optically Pumped Cesium Magnetometers

An automatic wideband 90° phase shifter for optically pumped cesium magnetometer has been investigated experimentally. As an important part of the self-oscillating optically pumped cesium magnetometer detecting circuit, the band and automation of the phase shifter directly affect the continuous measurement of the magnetic field. At the same time, a phase deviation of 1° will cause 1 Hz change of the resonant frequency. In order to meet the continuous measurement of the geomagnetic field, an automatic wideband 90° phase shifter based on the all-pass filter has been designed. It can be used to precisely make the phase of the detected signal shifted by 90° in a wide range of frequencies from 50 to 350 kHz. Through the actual test and analysis, the phase shift error of the automatic phase shifter is less than 0.1°, and the relative error less than 0.1%. It has been proved the feasibility of the automatic wideband 90° phase shifter applied to the cesium optically pumped magnetometer.