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As a new groundwater exploration method, noninvasive surface nuclear magnetic resonance (SNMR) has the benefits of direct, quantitative and uniqueness estimation of water content and relaxation time (T2*) in the near surface groundwater exploration. In practice, the earth magnetic field is difficult to be determined accurately, due to its inhomogeneity, time-varying and susceptible to ambient noise, which results in off-resonance excitation and serious decrease in accuracy of the inversion result. In this paper, based on the model of surface nuclear magnetic off-resonance (SNMOR) and the expression for the kernel function, the influences of the frequency offset on the amplitude and phase of the free induced decay (FID) signal are discussed, and a complex envelop inversion (CEI) based on automatic matching system phase and involving both the real part and imaginary part of the signal is applied. By comparing the synthetic signals generated from the SNMR and SNMOR models, it can be concluded that the phase of the FID signal significantly changes with the increase of the frequency offset, and the amplitude of the signal can be increased by 65.9% for the synthetic model in this paper. Thus when the frequency offset is greater than 2 Hz, the distribution of water content and T2* from the inversion results using the SNMR kernel will have a serious deviation from the actual model. However, using the SNMOR kernel based on the frequency offset, the inversion results are more accurate, and the maximum error of the water content and T2* are 4.2% and 39.3 ms, respectively. Moreover, synthetic data with different noise levels are inverted by the CEI method and conventional amplitude envelop inversion method (or QTI). The results show CEI obtain better performances in stability and reliability at a high noise level. Finally, a field measurement of SNMOR is conducted in Taipingchi Reservoir near Changchun City, China. The off-resonance FID signals are obviously observed by utilizing the JLMRS instrument and can be used to estimate the frequency offset. The characteristics of the FID signal with the frequency offset confirm the correctness of the SNMOR model. And the inversion result of field data using SNMOR kernel show that the distribution of water content and T2* are consistent with the known geological data from the drillings and other geophysical methods, which is much better than that using the SNMR kernel or conventional amplitude envelop inversion method. Therefore, the validities and accuracies of the SNMOR model and CEI method proposed in this paper are verified, which provides a new idea and technique for groundwater exploration in the near surface.
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Keywords:
- nuclear magnetic resonance /
- frequency offset /
- groundwater /
- phase
[1] Legtchenko A 2013 Magnetic Resonance Imaging for Groundwater (Hoboken: John Wiley & Sons) p10
[2] Li X, Xiao L Z, Liu H B, Zhang Z F, Guo B X, Yu H J, Zong F R 2013 Acta Phys. Sin. 62 147602(in Chinese) [李新, 肖立志, 刘化冰, 张宗富, 郭葆鑫, 于慧俊, 宗芳荣 2013 62 147602]
[3] Behroozmand A A, Keating K, Auken E 2015 Surv. Geophys. 36 27
[4] Hertrich M 2008 Prog. Nucl. Mag. Res. Sp. 53 227
[5] Hrlimann M D 1998 J. Mag. Res. 131 232
[6] Chen Q, Marble A E, Colpitts B G, Balcom B J 2005 J. Mag. Res. 175 300
[7] Grunewald E, Knight R 2012 Geophysics 77 EN1
[8] Legchenko A, Vouillamoz J M, Lawson F M A, Alle C, Descloitres M, Boucher M 2016 Geophysics 81 WB23
[9] Legchenko A, Vouillamoz J M, Roy J 2010 Geophysics 75 L91
[10] Lin J, Jiang C D, Lin T T, Duan Q M, Wang Y J, Shang X L, Fan T H, Sun S Q, Tian B F, Zhao J, Qin S W 2013 Chin. J. Geophys. 56 3619(in Chinese) [林君, 蒋川东, 林婷婷, 段清明, 王应吉, 尚新磊, 范铁虎, 孙淑琴, 田宝凤, 赵静, 秦胜武 2013 地球 56 3619]
[11] Walbrecker J O, Hertrich M, Green A G 2011 Geophysics 76 G1
[12] Grombacher D, Walbrecker J O, Knight R 2014 Geophysics 79 E329
[13] Irons T P, Li Y 2014 Geophys. J. Int. 199 1372
[14] Grombacher D, Knight R 2015 Geophysics 80 E329
[15] Grombacher D, Mller-Petke M, Knight R 2016 Geophysics 81 WB33
[16] Mueller-Petke M, Yaramanci U 2010 Geophysics 75 WA199
[17] Roy J, Lubczynski M W 2014 Near Surf. Geophys. 12 309
[18] Chen B, Hu X, Li J, Liu Y 2016 Groundwater 55 171
[19] Weichman P B, Lavely E M, Ritzwoller M H 2000 Phys. Rev.. 62 1290
[20] Lehmann-Horn J A, Hertrich M, Greenhalgh S A, Green A G 2011 IEEE Trans. Geosci. Remote. Sens. 49 3878
[21] Bloch F 1946 Phys. Rev. 70 460
[22] Jiang C D, Lin J, Duan Q M, Tian B F, Hao H C 2011 Chin. J. Geophys. 54 2973(in Chinese) [蒋川东, 林君, 段清明, 田宝凤, 郝荟萃 2011 地球 54 2973]
[23] Walbrecker J O, Hertrich M, Green A G 2009 Geophysics 74 G27
[24] Gnther T, Rcker C, Spitzer K 2006 Geophys. J. Int. 166 506
[25] Jiang C, Liu J, Tian B, Sun S, Lin J, Mller-Petke M 2016 Geophysics 81 E363
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[1] Legtchenko A 2013 Magnetic Resonance Imaging for Groundwater (Hoboken: John Wiley & Sons) p10
[2] Li X, Xiao L Z, Liu H B, Zhang Z F, Guo B X, Yu H J, Zong F R 2013 Acta Phys. Sin. 62 147602(in Chinese) [李新, 肖立志, 刘化冰, 张宗富, 郭葆鑫, 于慧俊, 宗芳荣 2013 62 147602]
[3] Behroozmand A A, Keating K, Auken E 2015 Surv. Geophys. 36 27
[4] Hertrich M 2008 Prog. Nucl. Mag. Res. Sp. 53 227
[5] Hrlimann M D 1998 J. Mag. Res. 131 232
[6] Chen Q, Marble A E, Colpitts B G, Balcom B J 2005 J. Mag. Res. 175 300
[7] Grunewald E, Knight R 2012 Geophysics 77 EN1
[8] Legchenko A, Vouillamoz J M, Lawson F M A, Alle C, Descloitres M, Boucher M 2016 Geophysics 81 WB23
[9] Legchenko A, Vouillamoz J M, Roy J 2010 Geophysics 75 L91
[10] Lin J, Jiang C D, Lin T T, Duan Q M, Wang Y J, Shang X L, Fan T H, Sun S Q, Tian B F, Zhao J, Qin S W 2013 Chin. J. Geophys. 56 3619(in Chinese) [林君, 蒋川东, 林婷婷, 段清明, 王应吉, 尚新磊, 范铁虎, 孙淑琴, 田宝凤, 赵静, 秦胜武 2013 地球 56 3619]
[11] Walbrecker J O, Hertrich M, Green A G 2011 Geophysics 76 G1
[12] Grombacher D, Walbrecker J O, Knight R 2014 Geophysics 79 E329
[13] Irons T P, Li Y 2014 Geophys. J. Int. 199 1372
[14] Grombacher D, Knight R 2015 Geophysics 80 E329
[15] Grombacher D, Mller-Petke M, Knight R 2016 Geophysics 81 WB33
[16] Mueller-Petke M, Yaramanci U 2010 Geophysics 75 WA199
[17] Roy J, Lubczynski M W 2014 Near Surf. Geophys. 12 309
[18] Chen B, Hu X, Li J, Liu Y 2016 Groundwater 55 171
[19] Weichman P B, Lavely E M, Ritzwoller M H 2000 Phys. Rev.. 62 1290
[20] Lehmann-Horn J A, Hertrich M, Greenhalgh S A, Green A G 2011 IEEE Trans. Geosci. Remote. Sens. 49 3878
[21] Bloch F 1946 Phys. Rev. 70 460
[22] Jiang C D, Lin J, Duan Q M, Tian B F, Hao H C 2011 Chin. J. Geophys. 54 2973(in Chinese) [蒋川东, 林君, 段清明, 田宝凤, 郝荟萃 2011 地球 54 2973]
[23] Walbrecker J O, Hertrich M, Green A G 2009 Geophysics 74 G27
[24] Gnther T, Rcker C, Spitzer K 2006 Geophys. J. Int. 166 506
[25] Jiang C, Liu J, Tian B, Sun S, Lin J, Mller-Petke M 2016 Geophysics 81 E363
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