地面核磁偏共振响应特征与复包络反演方法  

Characteristics of surface nuclear magnetic off-resonance signal and complex envelope inversion

作　　者：蒋川东[1,2] 王琦 杜官峰 易晓峰[1,2] 田宝凤 

机构地区：[1]地球信息探测仪器教育部重点实验室(吉林大学),长春130061 [2]吉林大学仪器科学与电气工程学院,长春130061 [3]大学通信工程学院,长春130012

出　　处：《物理学报》2018年第1期53-66,共14页Acta Physica Sinica

基　　金：国家自然科学基金青年科学基金(批准号：41604083,41504086,41704103); 吉林省自然科学基金(批准号：20160101281JC)资助的课题

摘　　要：地面核磁共振(surface nuclear magnetic resonance,SNMR)方法在地下水探测领域具有直接、定量和解释唯一等优势,但是由于地磁场存在不均匀、随时间变化和易受噪声影响等难以确定的问题,导致偏共振激发,严重影响反演结果的准确性.本文基于地面核磁偏共振(surface nuclear magnetic off-resonance,SNMOR)模型和相应的核函数表达式,讨论了频率偏量对偏共振信号的影响,并提出了基于系统相位自动搜索和信号实部与虚部同时参与的复包络反演方法.通过仿真模型和反演结果对比得到:偏共振信号相位随频率的增加而增大,仿真模型中的信号幅度最大提高了65.9%;当频率偏量大于2 Hz时,利用SNMOR核函数的反演结果的准确度明显优于SNMR核函数的结果;当噪声较大时,复包络方法充分利用测量数据的有用信息,比常规幅度反演具有更高的稳定性和可靠性.最后,通过野外实测数据和反演结果,验证了本文提出的SNMOR模型和复包络反演方法的有效性和准确性,为地下水探测领域提供了新的技术手段.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 T-＊2 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

关 键 词：核磁共振 频率偏量 地下水 相位 

分 类 号：P641.7[天文地球—地质矿产勘探]