| The proton MRS(Magnetic Resonance Sounding) technology is advancing all over the world nowadays and using MRS technology to explore underground water directly. It realized searching groundwater directly by means of geophysics method. By analyzing the initial amplitude and relaxation time of the signal received by the MRS instrument, the content and the mean pore size of underground water can be calculated.The main distinction between MRS and other geophysical methods is that it measures the magnetic resonance signal directly from groundwater molecules, making it a selective tool sensitive to groundwater. As the signal generated by the protons is very small, the method is also sensitive to electromagnetic interference (noise) and this is one of the major limitations for practical application.During the study, it was found that in the investigated frequency range, the electromagnetic noise produced by electrical power lines was much less stable and regular than expected. The proportion of 50 Hz harmonics (regular part) in the noise energy is site-dependent and may vary between 20% and 50%. The amplitude and frequency of each harmonic may vary significantly from one record to another. Under these conditions, block subtraction scheme based on a high noise regularity cannot be used systematically. The sinusoid subtraction is generally more efficient than the block subtraction but it is difficult to carry out by software. Compared to the first two methods, the notch filtering technique does not need estimate the power-line harmonics and it is easy to realized with software, but we should study the influence to the signal that the notch filter do.In the north area that short of water, the larmor frequency of the test spots are close to power-line harmonic. Through the analysis to the noise and the test data on the spot, the 2350Hz power line interference is always a intractable problem and it influences the after data processing directly. During the removing of power-line harmonic, the lowpass, highpass, bandpass and bandstop will influence the signal's interest, and then have an effect on the parameter's precision. The paper carries out that to remove 2350Hz power line harmonic by notch filter and designs notch filter based on MATLAB and then validates the error of the notch filter. The result indicates that the notch filter is feasible and can be used to data processing. In addition, this method can select the frequency to be filtered based on actual instance, so it can be used proverbially.The paper totally includes five chapters. The contents of the paper are as follows.In the first chapter, the character of the MRS signal was analyzed, and to filter the high amplitude noise by trapped filter was brought forward to the question of filed experiment based on the summarization of the study actuality and developing direction of the filtering method.In the second chapter, a trapped filter was designed based on FIR, and was imitated in MATLAB. The error of the filter was validated and then the paper gave the applicability of it. At the last of this chapter some advice to the after part of the paper was put forward.In the third chapter, another trapped filter was designed based on IIR, and was imitated in MATLAB. The error of the filter was validated and then the paper gave the applicability of it. The notch filter based on IIR has a little data delay, so it can do notch processing many times. When doing this, something to be done is that to select noise frequency points based on the dialog box and frequency fig ,and then the program will filter the data of the selected frequency points.The adaptive notch filter has been designed in the fourth chapter , and notch filter's parameters were analyzed. The adaptive notch filter was influenced by the impact of the step. When the frequency deviation (Δf) was small, in ensuring that no attenuation or small signal attenuation circumstances, the steps can not be set too large, so notch filter's bandwidth became narrow, and notch filter's notch depth became smaller, and thus it can not be good for noise filtering. Compared to IIR method , the notch filter's effect is poor.In the fifth chapter, two problems(choice of sample frequency and frequency leak) meted during the signal processing were brought forward, and the resolvent of them were given also. And then the paper processed the date tested on the spot and gave the result of it. After notch filter processing, the data and frequency fig became better and the SNR was increased. The result indicated that the method mentioned in this chapter was simple, applied and effective. It should do well to the after fit and inverse data processing and enhance the effect of the after data processing ,also the accuracy of hydrological and geologic parameter.The sixth chapter summarizes all this paper, and puts up some questions which should be researched continuously in the future. |
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