| With the advancement of science and innovation of technology, groundwater detection methods are becoming more and more diversified, but MRS is the only non-invasive detecting method of groundwater exploration at present. Compared with the traditional geophysical method of groundwater exploration, this method has the advantages of high resolution, high efficiency, rich information, unique interpretation and quantitative results. In the past three decades, MRS has been widely promoted and applied in the areas of exploring and evaluating the groundwater, preventing the leakage of the dam, monitoring the landslide, the advance detection of the mine/tunnel disaster etc.Based on the MRS instrument, one of the key technologies for the transmission system is resonance emission technology. This technology which utilizes the LC series resonance principle transmits a high voltage sine wave with the local Larmor frequency to the area to be measured and induces the excitation field which has a certain angle with geomagnetic field. Also, as MRS transition being motivated among the proton in water, macro MRS effect is occurs. The more accurate the larmor frequency is, the more obvious the macro MRS effect is, and the more realistic the exploration results are. There is a quantitative proportional relationship between the local geomagnetic field B0 value and the Larmor frequency f L, therefore, measuring B0 accurately is the precondition to apply the MRS instrument successfully. With the expansion of the application scope and field, especially in complex application environments, such as towns and villages nearby, mine/tunnel where the electromagnetic interference noise is large, traditional proton precession magnetometer can’t get accurate magnetic field value. In order to provide reference of accurate frequency for the transmission system, there is an urgent need to develop a strong anti-interference ability, stable and reliable proton precession magnetometer to measure the geomagnetic field accurately.In view of the disadvantage of the traditional proton precession magnetometer, the anti-interference proton precession magnetometer is designed in this paper. The controller of the hardware circuit is based on ARM and CPLD. Broadband bandpass filter with low squareness factor is added into the signal conditioning circuit, and the 24 bit acquisition card designed by American NI Instrument Corporation is used for signal acquisition and storage. At the same time, signal superposition and Single-frequency measurement are conducted based on PC Lab VIEW software, which overcome the disadvantage of low signal to noise ratio(SNR) of the free induction decay(FID) signal and inaccuracy of the frequency measurement in the areas where the electromagnetic interference is large relatively. The parameters of the instrument can be set according to the actual situation flexibly, the anti-interference proton precession magnetometer realizes the auxiliary magnetic measurement function for the MRS instrument well.The anti-interference proton precession magnetometer in this paper absorbs some effective anti-interference measures, so that good measurement results have been achieved in both indoor and outdoor environments. In the areas where the electromagnetic interference is small, the single measurement precision of the geomagnetic field can reach ±0.01 n T. In the areas where the electromagnetic interference is large relatively, other magnetic measuring instruments can not operate regularly. Through these effective measures, such as broadband bandpass filter with low squareness factor, signal superposition and Single-frequency measurement based on software, the instrument designed in this paper can measure the geomagnetic field accurately, and the precision can reach 1n T. This design is closely combined with the needs of the project, and realizes the stable measurement of the geomagnetic field and the reliability and practicability are verified. The design achieves the desired goals successfully. |
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