基于CPMG脉冲序列的地面磁共振探测玄武岩磁性含水层研究

Study on the detection of basalt magnetic aquifer using SNMR with CPMG pulse sequence

地面磁共振测深是目前唯一直接探测地下水的地球物理方法,具有灵敏、高效、无损等优点,在地下水探测中具有独特优势.玄武岩含水层常富含锶、偏硅酸等矿物质,为潜在矿泉水水源地,但其内部磁性矿物的存在限制了常规地面磁共振测深方法的应用.针对这一难题,笔者分别采用FID和CPMG两种脉冲序列开展了地面磁共振测深实验,结合钻孔资料验证了该方法的可行性.通过对两种脉冲序列反演弛豫时间和含水量的对比研究,发现FID脉冲序列反演获得平均横向弛豫时间T*2受非均匀磁场主导,并不能反映含水层孔隙度大小,但反演含水量结果优于CPMG脉冲序列;CPMG脉冲序列可以反演横向弛豫时间T2,但易导致含水量低估.因此,笔者认为在磁性含水层地面磁共振测深中,可采用FID脉冲序列反演含水量并结合CPMG脉冲序列反演T2,实现对含水层富水性和孔隙度大小进行探测.

Surface Nuclear Magnetic Resonance(SNMR) is the only geophysical method to directly detect groundwater, which is sensitive, efficient and nondestructive. The basalt aquifer is often rich in strontium, met silicic acid and other minerals, which is a potential source of mineral water. However, the presence of internal magnetic minerals limits the application of conventional SNMR. In order to solve this problem, two kinds of pulse sequences, FID and CPMG, are used to carry out the experiments, and the feasibility of this method is verified by combining the drilling data. The results show that T*2 measured by FID pulse sequence is dominated by inhomogeneous magnetic field, which cannot accurately reflect the porosity of aquifer, but the water content measured by FID pulse sequence is better than that measured by CPMG pulse sequence. CPMG pulse sequence provide an efficient way to inverse T2 while water content may be underestimated. Therefore, SNMR is advisable to be performed in detecting magnetic aquifer based on water content inversion of FID pulse sequence and T2 inversion of CPMG pulse sequence.