基于双通道参考技术的磁共振地下水探测电磁噪声分布规律研究

Research on the distribution of electromagnetic noise based on a two-channel reference technique for magnetic resonance sounding measurements

磁共振地下水探测(MRS)技术能直接测量水中氢质子的拉莫尔进动,可表征地下水含水量和介质孔隙度等信息,因而获得了广泛应用.然而,MRS信号微弱,实际探测中电磁噪声强度大,若噪声未被有效去除,会导致后续水文参数解释不准确,采用“8”字形线圈或运用带参考线圈的噪声抵消等方法可抑制噪声干扰,但这些方法消噪效果的好坏取决于电磁噪声的空间分布.另外,在强噪声或多噪声源环境下应用MRS方法,选择恰当的探测位置对提高探测效率以及增加探测结果的有效性具有重要意义.因此,本文使用双通道噪声采集装置在实验前记录环境中的电磁噪声,通过双通道参考技术推导同一时刻不同测点的噪声强度,再得到测区电磁噪声分布结果,该噪声分布情况对MRS方法探测方式的选取具有指导性的作用.同时,可根据噪声分布情况为探测线圈选择最佳测点以避开噪声强度大的区域,进而提高采集信号的信噪比.最后,通过采集室外环境下的电磁噪声,验证本文提出的基于双通道参考技术的电磁噪声分布规律研究方法的有效性,为可靠高效地开展磁共振地下水探测提供了新的技术手段.

The technique of magnetic resonance sounding(MRS)can measure the proton precession directly.It has the advantage of non-invasive and has been widely used to provide water content and hydrological properties of the subsurface.However,the level of MRS signal is low,the accuracy of the signal parameters are limited by electromagnetic noise.Some de-noising methods can be used such as figure-eight loop geometry and multichannel noise cancellation,but the effect of these methods depends on the spatial distribution of electromagnetic noise.Besides,the influence of noise is still the most important factor affecting the measurement time.Methods to get the noise distribution are therefore needed to determine the optimum placement of the MRS measurements and obtain high signal-to-noise ratios and shorter measurement time.In this paper,with a two-channel instrument,the electromagnetic noise is recorded at a given site prior to a MRS measurement.Although the two-channel instrument can only record noise for two points at the same time,and the noise for all points are definitely recorded not simultaneously.But the noise intensity of different measuring points at the same time can be deduced by a two-channel reference technique.It concluded that getting the electromagnetic noise distribution prior to MRS measurements may give a good instruction to the field experiments.So,the direction of figure-eight loop geometry and the position of the reference loop for multichannel instrument can be selected appropriately.In addition,the optimum site for a magnetic resonance sounding with the least distortion by noise can be identified based on this knowledge.Finally,the field measurement is conducted in urban area.And the validity of the two-channel reference technique used for getting the electromagnetic noise distribution proposed in this paper is proved,which provides a new idea and technique for the practical groundwater exploration.