Basic Theoretical Research On NMR For Coincident Loops Configuration

The surface nuclear magnetic resonance method(SNMR) uses artificial magnetic field at fixed frequency to directly stimulate the underground hydrogen nuclei, which causes the effect of nuclear magnetic resonance, then inversing the NMR signal produced the underground hydrogen nuclear transition to inferred underground water content information. Different from conventional geophysical methods which inferred groundwater information indirectly through the electrical conductivity or others, surface nuclear magnetic resonance method is the only geophysical method to detect groundwater directly.Currently, Yaramanci, U., Legchenko, A., Weichman, P.B. and other western scholars have been carrying out extensive theoretical and applied research in SNMR, especially the forward and inverse research in2-D and3-D surface nuclear magnetic resonance and applications in SNMR which has accumulated much actual probe experience, however, domestic scholars have not taken the elliptical polarization and phase lag into account in SNMR, failed to recognize the subsurface conductivity will cause the excitation field distortion, thus affecting the nuclear magnetic resonance response signal. Meanwhile, nearly all the SNMR applications in China are based on the one-dimensional inverse.This paper is divided into six chapters to summarize the work of the author which include theoretical study in SNMR based on the elliptical polarization and the forward and inverse research in1-D and2-D surface nuclear magnetic resonance based on coincidental loops configuration.The first chapter is the introduction. Discuss the current progress and existing problems at home and abroad in SNMR, the topics’purpose and significance of this article, and the main content, the method of choice, and research ideas in this paper. Finally, point out some innovation in the study.The second chapter briefly introduces the basic principles of surface nuclear magnetic resonance. Including the Larmor precession, the movement of M in oscillatory magnetic field and the free induction decay signal received in the ground receiving loop.In Chapter III we have derived in detail the surface nuclear magnetic resonance theory based on the effect of elliptical polarization, and choose the two aspects called the elliptical polarization of the vertical excited magnetic field and the surface nuclear magnetic resonance theory based on the effect of elliptical polarization. By using classical Chave algorithm to verify the existence of elliptical polarization and by introducing the ellipticity and semi-minor axis of the vertical excited magnetic field to study the properties of elliptical polarization, and the influences of elliptical polarization and phase lag in SNMR have been discussed by the parameters of tip angle of the proton, the kernel function and the response curves of the actual aquifer model in SNMR. The ellipticity reflects the degree of elliptical polarization of the vertical excited magnetic field, which reveals the strength of elliptical polarization in different underground location and conductivity condition. The semi-minor axis of the vertical excited magnetic field actually reflects the difference value of the co-rotating and counter-rotating parts. Different from the same co-rotating and counter-rotating parts in circular polarization, elliptical polarization causes the co-rotating and counter-rotating parts distortion, and the co-rotating and counter-rotating parts show the different influences on transmitter and receiver processes. By calculating the phase of the vertical excited magnetic field, the effect of phase lag has been found, which provides a theoretical foundation for inverting underground conductivity directly by use of phase information in surface nuclear magnetic resonance. In order to study the effects of elliptical polarization and phase lag in SNMR, we have calculated the tip angle, the kernel function and the initial amplitude and phase curves and pointed out the significant influence caused by elliptical polarization and phase lag in SNMR.The fourth chapter focuses on the one-dimensional forward modeling and inversion studies based on coincidental loops configuration in SNMR. This chapter uses the actual coincidental loops configuration, calculates the SNMR sensitivity matrix considered elliptical polarization and phase lag, this parameter reflects the NMR signal intensity at different underground deep aquifer in various pulse moments. Through the one-dimensional layered model, the initial amplitude and phase curves have been calculated. In the inversion process, through the introduction of the regularization factor to adjust the weights between data fitting and model smoothness, which enhances the stability of the inversion, while the introduction of the real part and imaginary part of the data to constrain the inversion, increase of data have been used to get more accurate inversion results.The fifth chapter focuses on the two-dimensional modeling and inversion studies based on coincidental loops configuration in SNMR. Actually, in SNMR detection, the data collected in ground is the summation of all the NMR response produced by underground proton. When the water content is not uniform layered distribution, if we use one-dimensional SNMR procedures to deal with, the errors or the unreasonable result will emerge. Therefore, it is essential to study the two-dimensional modeling and inversion studies. In this chapter, we derived two-dimensional SNMR forward and inverse theory based on coincidental loops configuration, calculate the two-dimensional SNMR response. In the inversion process, regularization factor has been used to enhance the stability of solution. By comparing the result from the real part, imaginary part and the joint inversion of the real part and imaginary part of the data, we find that the joint inversion improve the inversion accuracy and stability and can inverse the underground water content better. Meanwhile, I found, based on the theory of elliptical polarization and phase lag, the two-dimensional SNMR show different resolution in a North-South direction.In the sixth chapter, the author summarizes the results of the entire master’s thesis and provides some recommendations in SNMR. As a new geophysical method, surface nuclear magnetic resonance has numerous works for further theoretical studies, especially for further two-dimensional and three-dimensional SNMR research, improvement of detection depth and accuracy, and the relationship between SNMR parameters and hydrogeological parameters is need to be resolved.