二维阵列线圈核磁共振地下水探测理论研究

摘要

核磁共振法(Magnetic Resonance Sounding,MRS)是一种直接探测地下水的地球物理方法,目前只能对水平层状的含水层进行一维测深,对于尺寸小于线圈直径的二维或三维含水构造成像时,其灵敏度和横向分辨率很低.本文从研究二维阵列线圈核磁共振地下水探测方式的可行性出发,推导了地面发射线圈产生的椭圆极化激发场和阵列接收线圈的核磁共振响应核函数的表达式,数值计算了二维阵列线圈测量方式的MRS信号初始振幅图像和探测灵敏度,通过与二维发射/接收同一线圈测量方式对比,得到结论如下:二维阵列线圈测量方式可直接确定地下含水构造的水平位置,但需更大的激发电流强度和接收灵敏度;当中浅层(<45 m)地下含水构造的水平位置、深度和截面积变化时,二维阵列线圈测量方式的相对灵敏度均高于同一线圈方式,且水平相对灵敏度改善最明显;地下含水构造的深度增加时,其水平相对灵敏度逐渐降低;对于位于地下15 m处的浅层地下含水构造,其水平分辨率为5 m,垂直分辨率为4.5 m,横截面分辨率为8 m2,以上三种分辨率均由仪器噪声水平所决定.通过实施增加接收线圈的数量和匝数、提高激发脉冲矩和接收灵敏度等改进措施,现有的仪器系统可用二维阵列线圈测量方式.本文提出的理论和方法,将促使二维阵列线圈核磁共振方法在地下暗河、喀斯特溶洞和堤坝渗漏等二维或三维含水构造快速探测方面得到应用.%Magnetic Resonance Sounding (MRS) is a geophysical method designed for direct groundwater exploration. Initially, MRS was developed for characterizing horizontally stratified aquifers. However, for the 2D/3D target whose size may he much smaller than the loop, the accuracy and the lateral resolution may not he sufficient. In this paper, possibility of 2D MRS with an array loop for groundwater exploration is studied firstly. Then, the expressions of the elliptically polarized excitation field induced hy the transmitting loop and MRS response signal received hy the array loop are derived. The 2D imaging of MRS initial amplitude and detection sensitivity measured with the array loop are numerically simulated, and are compared with the result from using the coincident loop. Conclusions are as follows: (1) 2D MRS measurement with the array loop can directly detect the horizontal position of water conduit in the subsurface, butrequires higher excitation current and receiver sensitivity. (2) When the horizontal position, depth and section area of the water conduit vary, the detection sensitivity of 2D MRS measurement with the array loop is higher than that using the coincident loop, and the improvement of sensitivity is most obvious when horizontal position varying. (3) The sensitivity of 2D MRS measurement with the array loop detecting horizontal position varying decreases with increasing of the water conduit depth. (4) For shallow groundwater, the horizontal, vertical and section resolution determined hy the instrumental noise level is 5 m, 4. 5 m, 8 m , respectively. By increasing the number and turns of the receiver loop, improving excitation pulse moment and decreasing noise level, the existing instrument can be utilized to 2D MRS measurement. The theory and method presented in this paper will contribute to the application of 2D MRS with the array loop to fast detection of 2D/3D targets such as underground rivers, karstic conduits and dam seepages.