用于土壤水分测量的磁共振射频线圈宽频匹配方法

A Wide-Band Matching Method for Radio Frequency Coils Used in Soil Moisture Measurement

准确检测土壤水分对于改善农业用水管理至关重要.为了实现土壤水分的准确测量,本文设计了一种微型inside-out核磁共振(NMR)传感器(测量区域位于传感器周围).其中磁体由三个轴向磁化的圆柱磁体组成,射频线圈由四个相同的弧面螺旋线圈串联组成.通过调整圆柱体之间的半径和高度,获得了一个圆周对称恒定梯度场.并利用宽频带匹配方法,增加射频线圈的激励区域,从而提高整体信噪比.将本文设计的传感器用于测量含水量不同的小颗粒沙土和大颗粒石英砂土壤模型样品的CPMG(Carr-Purcell-Meiboom-Gill)信号,通过逆拉普拉斯变换获得了T_(2)分布谱,并将T_(2)谱峰面积与称重法获得的含水量进行了相关性分析,探讨了该传感器用于测量土壤水分的可行性.

Accurate measurement of soil moisture is crucial in agricultural water management.A mini inside-out nuclear magnetic resonance(NMR)sensor has been designed to measure the moisture in model soils,consisting of three cylindrical magnets magnetized in the axial direction and four arc spiral coils with the same size in series.A circumferential symmetric constant gradient field can be obtained by adjusting the radius and height between the cylinders.To improve the signal-to-noise ratio of the sensor,this a wide-band matching method was proposed in this work to expand the excitation region of the radio frequency coils.The Carr-Purcell-Meiboom-Gill(CPMG)signal was analyzed through inverse Laplace transformation.With the results obtained,moisture content of the sample was found to be positively correlated with the integral area of T_(2)spectrum peak,verifying the feasibility of the sensor in soil moisture measurement.